diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 28a68801..98669128 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -25,30 +25,6 @@
 
 ## Guidelines
 
-### Extend scripts
-
-- Create a [pull request](#Pull-Request-Process) for [application.yaml](./src/application/application.yaml)
-- 🙏 For any new script, please add `revertCode` and `docs` values if possible.
-- Structure of `script` object:
-  - `name`: *`string`* (**required**)
-    - Name of the script
-    - E.g. `Disable targeted ads`
-  - `code`: *`string`* (**required**)
-    - Batch file commands that will be executed
-  - `docs`: *`string`* | `[ string, ... ]`
-    - Documentation URL or list of URLs for those who wants to learn more about the script
-    - E.g. `https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_telemetry`
-  - `revertCode`: `string`
-    - Code that'll undo the change done by `code` property.
-    - E.g. let's say `code` sets an environment variable as `setx POWERSHELL_TELEMETRY_OPTOUT 1`
-      - then `revertCode` should be doing `setx POWERSHELL_TELEMETRY_OPTOUT 0`
-  - `recommend`: `"standard"` | `"strict"` | `undefined` (default)
-    - If not defined then the script will not be recommended
-    - If defined it can be either
-      - `standard`: Will be recommended for general users
-      - `strict`: Will only be recommended with a warning
-- See [typings](./src/application/application.yaml.d.ts) for documentation as code.
-
 ### Handle the state in presentation layer
 
 - There are two types of components:
diff --git a/README.md b/README.md
index 1eaa73d2..37484d48 100644
--- a/README.md
+++ b/README.md
@@ -27,13 +27,14 @@
 - No need to run any compiled software that has access to your system, just run the generated scripts
 - Have full visibility into what the tweaks do as you enable them
 - Ability to revert (undo) applied scripts
+- Everything is transparent: both application and its infrastructure are open-source and automated
 - Easily extendable
-- Everything is open-source and automated (both application and its infrastructure)
 
 ## Extend scripts
 
 - Fork it & add more scripts in [application.yaml](src/application/application.yaml) and send a pull request 👌
-- 📖 More: [extend scripts | CONTRIBUTING.md](./CONTRIBUTING.md#extend-scripts)
+- 📖 If you're unsure about the syntax you can refer to the [application file | documentation](docs/application-file.md).
+- 🙏 For any new script, please add `revertCode` and `docs` values if possible.
 
 ## Commands
 
diff --git a/docs/application-file.md b/docs/application-file.md
new file mode 100644
index 00000000..39df51b4
--- /dev/null
+++ b/docs/application-file.md
@@ -0,0 +1,139 @@
+# Application file
+
+- privacy.sexy is a data-driven application where it reads the necessary OS-specific logic from [`application.yaml`](./../src/application/application.yaml)
+- 💡 Best practices
+  - If you repeat yourself, try to utilize [YAML-defined functions](#function)
+  - Always try to add documentation and a way to revert a tweak in [scripts](#script)
+- 📖 Types in code: [`application.d.ts`](./../src/application/application.yaml.d.ts)
+
+## Objects
+
+### `Application`
+
+- Application file simply defines different categories and their scripts in a tree structure.
+- Application file also allows defining common [function](#function)s to be used throughout the application if you'd like different scripts to share same code.
+
+#### `Application` syntax
+
+- `actions: [` ***[`Category`](#Category)*** `, ... ]` **(required)**
+  - Each [category](#category) is rendered as different cards in card presentation.
+  - ❗ Application must consist of at least one category.
+- `functions: [` ***[`Function`](#Function)*** `, ... ]`
+  - Functions are optionally defined to re-use the same code throughout different scripts.
+
+### `Category`
+
+- Category has a parent that has tree-like structure where it can have subcategories or subscripts.
+- It's a logical grouping of different scripts and other categories.
+
+#### `Category` syntax
+
+- `category:` *`string`*  (**required**)
+  - Name of the category
+  - ❗ Must be unique throughout the application
+- `children: [` ***[`Category`](#category)*** `|` [***`Script`***](#Script) `, ... ]`  (**required**)
+  - ❗ Category must consist of at least one subcategory or script.
+  - Children can be combination of scripts and subcategories.
+
+### `Script`
+
+- Script represents a single tweak.
+- A script must include either:
+  - A `code` and `revertCode`
+  - Or `call` to call YAML-defined functions
+- 🙏 For any new script, please add `revertCode` and `docs` values if possible.
+
+#### `Script` syntax
+
+- `name`: *`string`* (**required**)
+  - Name of the script
+  - ❗ Must be unique throughout the application
+  - E.g. `Disable targeted ads`
+- `code`: *`string`* (may be **required**)
+  - Batch file commands that will be executed
+  - 💡 If defined, best practice to also define `revertCode`
+  - ❗ If not defined `call` must be defined, do not define if `call` is defined.
+- `revertCode`: `string`
+  - Code that'll undo the change done by `code` property.
+  - E.g. let's say `code` sets an environment variable as `setx POWERSHELL_TELEMETRY_OPTOUT 1`
+    - then `revertCode` should be doing `setx POWERSHELL_TELEMETRY_OPTOUT 0`
+  - ❗ Do not define if `call` is defined.
+- `call`: ***[`FunctionCall`](#FunctionCall)*** | `[` ***[`FunctionCall`](#FunctionCall)*** `, ... ]` (may be **required**)
+  - A shared function or sequence of functions to call (called in order)
+  - ❗ If not defined `code` must be defined
+- `docs`: *`string`* | `[`*`string`*`, ... ]`
+  - Single documentation URL or list of URLs for those who wants to learn more about the script
+  - E.g. `https://docs.microsoft.com/en-us/windows-server/`
+- `recommend`: `"standard"` | `"strict"` | `undefined` (default)
+  - If not defined then the script will not be recommended
+  - If defined it can be either
+    - `standard`: Only non-breaking scripts without limiting OS functionality
+    - `strict`: Scripts that can break certain functionality in favor of privacy and security
+
+### `FunctionCall`
+
+- Describes a single call to a function by optionally providing values to its parameters.
+- 👀 See [parameter substitution](#parameter-substitution) for an example usage
+
+#### `FunctionCall` syntax
+
+- `function`: *`string`* (**required**)
+  - Name of the function to call.
+  - ❗ Function with same name must defined in `functions` property of [Application](#application)
+- `parameters`: `[ parameterName:` *`parameterValue`*`, ... ]`
+  - Defines key value dictionary for each parameter and its value
+  - E.g.
+
+    ```yaml
+      parameters:
+        userDefinedParameterName: parameterValue
+        # ...
+        appName: Microsoft.WindowsFeedbackHub
+    ```
+
+### `Function`
+
+- Functions allow re-usable code throughout the defined scripts.
+- Functions are templates compiled by privacy.sexy and uses special expressions.
+- Expressions are defined inside mustaches (double brackets, `{{` and `}}`)
+- 👀 See [parameter substitution](#parameter-substitution) for an example usage
+
+#### Parameter substitution
+
+A simple function example
+
+```yaml
+  function: EchoArgument
+  parameters: [ 'argument' ]
+  code: Hello {{ $argument }} !
+```
+
+It would print "Hello world" if it's called in a [script](#script) as following:
+
+```yaml
+  script: Echo script
+  call:
+    function: EchoArgument
+    parameters:
+      argument: World
+```
+
+#### `Function` syntax
+
+- `name`: *`string`* (**required**)
+  - Name of the function that scripts will use.
+  - Convention is to use camelCase, and be verbs.
+  - E.g. `uninstallStoreApp`
+  - ❗ Function names must be unique
+- `parameters`: `[` *`string`* `, ... ]`
+  - Name of the parameters that the function has.
+  - Parameter values are provided by a [Script](#script) through a [FunctionCall](#functioncall)
+  - Parameter names must be defined to be used in expressions such as [parameter substitution](#parameter-substitution)
+  - ❗ Parameter names must be unique
+ `code`: *`string`* (**required**)
+  - Batch file commands that will be executed
+  - 💡 If defined, best practice to also define `revertCode`
+- `revertCode`: *`string`*
+  - Code that'll undo the change done by `code` property.
+  - E.g. let's say `code` sets an environment variable as `setx POWERSHELL_TELEMETRY_OPTOUT 1`
+    - then `revertCode` should be doing `setx POWERSHELL_TELEMETRY_OPTOUT 0`
diff --git a/src/application/Parser/ApplicationParser.ts b/src/application/Parser/ApplicationParser.ts
index c4d499ea..35c3eb0d 100644
--- a/src/application/Parser/ApplicationParser.ts
+++ b/src/application/Parser/ApplicationParser.ts
@@ -4,14 +4,16 @@ import { IApplication } from '@/domain/IApplication';
 import { IProjectInformation } from '@/domain/IProjectInformation';
 import { ApplicationYaml } from 'js-yaml-loader!./../application.yaml';
 import { parseCategory } from './CategoryParser';
-import { ProjectInformation } from '../../domain/ProjectInformation';
+import { ProjectInformation } from '@/domain/ProjectInformation';
+import { ScriptCompiler } from './Compiler/ScriptCompiler';
 
 
 export function parseApplication(content: ApplicationYaml, env: NodeJS.ProcessEnv = process.env): IApplication {
     validate(content);
+    const compiler = new ScriptCompiler(content.functions);
     const categories = new Array<Category>();
     for (const action of content.actions) {
-        const category = parseCategory(action);
+        const category = parseCategory(action, compiler);
         categories.push(category);
     }
     const info = readAppInformation(env);
@@ -21,7 +23,7 @@ export function parseApplication(content: ApplicationYaml, env: NodeJS.ProcessEn
     return app;
 }
 
-function readAppInformation(environment): IProjectInformation {
+function readAppInformation(environment: NodeJS.ProcessEnv): IProjectInformation {
     return new ProjectInformation(
         environment.VUE_APP_NAME,
         environment.VUE_APP_VERSION,
diff --git a/src/application/Parser/CategoryParser.ts b/src/application/Parser/CategoryParser.ts
index 734cc430..d806c39d 100644
--- a/src/application/Parser/CategoryParser.ts
+++ b/src/application/Parser/CategoryParser.ts
@@ -3,6 +3,7 @@ import { Script } from '@/domain/Script';
 import { Category } from '@/domain/Category';
 import { parseDocUrls } from './DocumentationParser';
 import { parseScript } from './ScriptParser';
+import { IScriptCompiler } from './Compiler/IScriptCompiler';
 
 let categoryIdCounter: number = 0;
 
@@ -11,14 +12,17 @@ interface ICategoryChildren {
     subScripts: Script[];
 }
 
-export function parseCategory(category: YamlCategory): Category {
+export function parseCategory(category: YamlCategory, compiler: IScriptCompiler): Category {
+    if (!compiler) {
+        throw new Error('undefined compiler');
+    }
     ensureValid(category);
     const children: ICategoryChildren = {
         subCategories: new Array<Category>(),
         subScripts: new Array<Script>(),
     };
     for (const categoryOrScript of category.children) {
-        parseCategoryChild(categoryOrScript, children, category);
+        parseCategoryChild(categoryOrScript, children, category, compiler);
     }
     return new Category(
         /*id*/ categoryIdCounter++,
@@ -42,13 +46,16 @@ function ensureValid(category: YamlCategory) {
 }
 
 function parseCategoryChild(
-    categoryOrScript: any, children: ICategoryChildren, parent: YamlCategory) {
+    categoryOrScript: any,
+    children: ICategoryChildren,
+    parent: YamlCategory,
+    compiler: IScriptCompiler) {
     if (isCategory(categoryOrScript)) {
-        const subCategory = parseCategory(categoryOrScript as YamlCategory);
+        const subCategory = parseCategory(categoryOrScript as YamlCategory, compiler);
         children.subCategories.push(subCategory);
     } else if (isScript(categoryOrScript)) {
         const yamlScript = categoryOrScript as YamlScript;
-        const script = parseScript(yamlScript);
+        const script = parseScript(yamlScript, compiler);
         children.subScripts.push(script);
     } else {
         throw new Error(`Child element is neither a category or a script.
@@ -57,7 +64,8 @@ function parseCategoryChild(
 }
 
 function isScript(categoryOrScript: any): boolean {
-    return categoryOrScript.code && categoryOrScript.code.length > 0;
+    return (categoryOrScript.code && categoryOrScript.code.length > 0)
+        || categoryOrScript.call;
 }
 
 function isCategory(categoryOrScript: any): boolean {
diff --git a/src/application/Parser/Compiler/IScriptCompiler.ts b/src/application/Parser/Compiler/IScriptCompiler.ts
new file mode 100644
index 00000000..747a98fe
--- /dev/null
+++ b/src/application/Parser/Compiler/IScriptCompiler.ts
@@ -0,0 +1,7 @@
+import { IScriptCode } from '@/domain/IScriptCode';
+import { YamlScript } from 'js-yaml-loader!./application.yaml';
+
+export interface IScriptCompiler {
+    canCompile(script: YamlScript): boolean;
+    compile(script: YamlScript): IScriptCode;
+}
diff --git a/src/application/Parser/Compiler/ScriptCompiler.ts b/src/application/Parser/Compiler/ScriptCompiler.ts
new file mode 100644
index 00000000..aa009e89
--- /dev/null
+++ b/src/application/Parser/Compiler/ScriptCompiler.ts
@@ -0,0 +1,200 @@
+import { IScriptCode } from '@/domain/IScriptCode';
+import { ScriptCode } from '@/domain/ScriptCode';
+import { YamlScript, YamlFunction, FunctionCall, ScriptFunctionCall, FunctionCallParameters } from 'js-yaml-loader!./application.yaml';
+import { IScriptCompiler } from './IScriptCompiler';
+
+interface ICompiledCode {
+    readonly code: string;
+    readonly revertCode: string;
+}
+
+export class ScriptCompiler implements IScriptCompiler {
+    constructor(private readonly functions: readonly YamlFunction[]) {
+        ensureValidFunctions(functions);
+    }
+    public canCompile(script: YamlScript): boolean {
+        if (!script.call) {
+            return false;
+        }
+        return true;
+    }
+    public compile(script: YamlScript): IScriptCode {
+        this.ensureCompilable(script.call);
+        const compiledCodes = new Array<ICompiledCode>();
+        const calls = getCallSequence(script.call);
+        calls.forEach((currentCall, currentCallIndex) => {
+            ensureValidCall(currentCall, script.name);
+            const commonFunction = this.getFunctionByName(currentCall.function);
+            let functionCode = compileCode(commonFunction, currentCall.parameters);
+            if (currentCallIndex !== calls.length - 1) {
+                functionCode = appendLine(functionCode);
+            }
+            compiledCodes.push(functionCode);
+        });
+        const scriptCode = merge(compiledCodes);
+        return new ScriptCode(script.name, scriptCode.code, scriptCode.revertCode);
+    }
+
+    private getFunctionByName(name: string): YamlFunction {
+        const func = this.functions.find((f) => f.name === name);
+        if (!func) {
+            throw new Error(`called function is not defined "${name}"`);
+        }
+        return func;
+    }
+
+    private ensureCompilable(call: ScriptFunctionCall) {
+        if (!this.functions || this.functions.length === 0) {
+            throw new Error('cannot compile without shared functions');
+        }
+        if (typeof call !== 'object') {
+            throw new Error('called function(s) must be an object');
+        }
+    }
+}
+
+function getDuplicates(texts: readonly string[]): string[] {
+    return texts.filter((item, index) => texts.indexOf(item) !== index);
+}
+
+function printList(list: readonly string[]): string {
+    return `"${list.join('","')}"`;
+}
+
+function ensureNoDuplicatesInFunctionNames(functions: readonly YamlFunction[]) {
+    const duplicateFunctionNames = getDuplicates(functions
+        .map((func) => func.name.toLowerCase()));
+    if (duplicateFunctionNames.length) {
+        throw new Error(`duplicate function name: ${printList(duplicateFunctionNames)}`);
+    }
+}
+
+function ensureNoDuplicatesInParameterNames(functions: readonly YamlFunction[]) {
+    const functionsWithParameters = functions
+        .filter((func) => func.parameters && func.parameters.length > 0);
+    for (const func of functionsWithParameters) {
+        const duplicateParameterNames = getDuplicates(func.parameters);
+        if (duplicateParameterNames.length) {
+            throw new Error(`"${func.name}": duplicate parameter name: ${printList(duplicateParameterNames)}`);
+        }
+    }
+}
+
+function ensureNoDuplicateCode(functions: readonly YamlFunction[]) {
+    const duplicateCodes = getDuplicates(functions.map((func) => func.code));
+    if (duplicateCodes.length > 0) {
+        throw new Error(`duplicate "code" in functions: ${printList(duplicateCodes)}`);
+    }
+    const duplicateRevertCodes = getDuplicates(functions
+        .filter((func) => func.revertCode)
+        .map((func) => func.revertCode));
+    if (duplicateRevertCodes.length > 0) {
+        throw new Error(`duplicate "revertCode" in functions: ${printList(duplicateRevertCodes)}`);
+    }
+}
+
+function ensureValidFunctions(functions: readonly YamlFunction[]) {
+    if (!functions) {
+        return;
+    }
+    ensureNoDuplicatesInFunctionNames(functions);
+    ensureNoDuplicatesInParameterNames(functions);
+    ensureNoDuplicateCode(functions);
+}
+
+function appendLine(code: ICompiledCode): ICompiledCode {
+    const appendLineIfNotEmpty = (str: string) => str ? `${str}\n` : str;
+    return {
+        code: appendLineIfNotEmpty(code.code),
+        revertCode: appendLineIfNotEmpty(code.revertCode),
+    };
+}
+
+function merge(codes: readonly ICompiledCode[]): ICompiledCode {
+    return {
+        code: codes.map((code) => code.code).join(''),
+        revertCode: codes.map((code) => code.revertCode).join(''),
+    };
+}
+
+function compileCode(func: YamlFunction, parameters: FunctionCallParameters): ICompiledCode {
+    return {
+        code: compileExpressions(func.code, parameters),
+        revertCode: compileExpressions(func.revertCode, parameters),
+    };
+}
+
+function compileExpressions(code: string, parameters: FunctionCallParameters): string {
+    let intermediateCode = compileToIL(code);
+    intermediateCode = substituteParameters(intermediateCode, parameters);
+    ensureNoExpressionLeft(intermediateCode);
+    return intermediateCode;
+}
+
+function substituteParameters(intermediateCode: string, parameters: FunctionCallParameters): string {
+    const parameterNames = getUniqueParameterNamesFromIL(intermediateCode);
+    if (parameterNames.length && !parameters) {
+        throw new Error(`no parameters defined, expected: ${printList(parameterNames)}`);
+    }
+    for (const parameterName of parameterNames) {
+        const parameterValue = parameters[parameterName];
+        intermediateCode = substituteParameter(intermediateCode, parameterName, parameterValue);
+    }
+    return intermediateCode;
+}
+
+function ensureValidCall(call: FunctionCall, scriptName: string) {
+    if (!call) {
+        throw new Error(`undefined function call in script "${scriptName}"`);
+    }
+    if (!call.function) {
+        throw new Error(`empty function name called in script "${scriptName}"`);
+    }
+}
+
+function getCallSequence(call: ScriptFunctionCall): FunctionCall[] {
+    if (call instanceof Array) {
+        return call as FunctionCall[];
+    }
+    return [ call as FunctionCall ];
+}
+
+function getDistinctValues(values: readonly string[]): string[] {
+    return values.filter((value, index, self) => {
+        return self.indexOf(value) === index;
+      });
+}
+
+// Trim each expression and put them inside "{{exp|}}" e.g. "{{ $hello }}" becomes "{{exp|$hello}}"
+function compileToIL(code: string) {
+    return code.replace(/\{\{([\s]*[^;\s\{]+[\s]*)\}\}/g, (_, match) => {
+        return `\{\{exp|${match.trim()}\}\}`;
+    });
+}
+
+// Parses all distinct usages of {{exp|$parameterName}}
+function getUniqueParameterNamesFromIL(ilCode: string) {
+    const allSubstitutions = ilCode.matchAll(/\{\{exp\|\$([^;\s\{]+[\s]*)\}\}/g);
+    const allParameters = Array.from(allSubstitutions, (match) => match[1]);
+    const uniqueParameterNames = getDistinctValues(allParameters);
+    return uniqueParameterNames;
+}
+
+// substitutes {{exp|$parameterName}} to value of the parameter
+function substituteParameter(ilCode: string, parameterName: string, parameterValue: string) {
+    if (!parameterValue) {
+        throw Error(`parameter value is not provided for "${parameterName}" in function call`);
+    }
+    const pattern = `{{exp|$${parameterName}}}`;
+    return ilCode.split(pattern).join(parameterValue); // as .replaceAll() is not yet supported by TS
+}
+
+// finds all "{{exp|..}} left"
+function ensureNoExpressionLeft(ilCode: string) {
+    const allSubstitutions = ilCode.matchAll(/\{\{exp\|(.*?)\}\}/g);
+    const allMatches = Array.from(allSubstitutions, (match) => match[1]);
+    const uniqueExpressions = getDistinctValues(allMatches);
+    if (uniqueExpressions.length > 0) {
+        throw new Error(`unknown expression: ${printList(uniqueExpressions)}`);
+    }
+}
diff --git a/src/application/Parser/ScriptParser.ts b/src/application/Parser/ScriptParser.ts
index 8190585d..6b96f48c 100644
--- a/src/application/Parser/ScriptParser.ts
+++ b/src/application/Parser/ScriptParser.ts
@@ -2,15 +2,18 @@ import { Script } from '@/domain/Script';
 import { YamlScript } from 'js-yaml-loader!./application.yaml';
 import { parseDocUrls } from './DocumentationParser';
 import { RecommendationLevelNames, RecommendationLevel } from '@/domain/RecommendationLevel';
+import { IScriptCompiler } from './Compiler/IScriptCompiler';
+import { IScriptCode } from '@/domain/IScriptCode';
+import { ScriptCode } from '@/domain/ScriptCode';
 
-export function parseScript(yamlScript: YamlScript): Script {
-    if (!yamlScript) {
-        throw new Error('script is null or undefined');
+export function parseScript(yamlScript: YamlScript, compiler: IScriptCompiler): Script {
+    validateScript(yamlScript);
+    if (!compiler) {
+        throw new Error('undefined compiler');
     }
     const script = new Script(
         /* name */              yamlScript.name,
-        /* code */              yamlScript.code,
-        /* revertCode */        yamlScript.revertCode,
+        /* code */              parseCode(yamlScript, compiler),
         /* docs */              parseDocUrls(yamlScript),
         /* level */             getLevel(yamlScript.recommend));
     return script;
@@ -30,3 +33,29 @@ function getLevel(level: string): RecommendationLevel | undefined {
     }
     return RecommendationLevel[typedLevel as keyof typeof RecommendationLevel];
 }
+
+function parseCode(yamlScript: YamlScript, compiler: IScriptCompiler): IScriptCode {
+    if (compiler.canCompile(yamlScript)) {
+        return compiler.compile(yamlScript);
+    }
+    return new ScriptCode(yamlScript.name, yamlScript.code, yamlScript.revertCode);
+}
+
+function ensureNotBothCallAndCode(yamlScript: YamlScript) {
+    if (yamlScript.code && yamlScript.call) {
+        throw new Error('cannot define both "call" and "code"');
+    }
+    if (yamlScript.revertCode && yamlScript.call) {
+        throw new Error('cannot define "revertCode" if "call" is defined');
+    }
+}
+
+function validateScript(yamlScript: YamlScript) {
+    if (!yamlScript) {
+        throw new Error('undefined script');
+    }
+    if (!yamlScript.code && !yamlScript.call) {
+        throw new Error('must define either "call" or "code"');
+    }
+    ensureNotBothCallAndCode(yamlScript);
+}
diff --git a/src/application/State/Code/Generation/UserScriptGenerator.ts b/src/application/State/Code/Generation/UserScriptGenerator.ts
index 9ccf48cf..595a0251 100644
--- a/src/application/State/Code/Generation/UserScriptGenerator.ts
+++ b/src/application/State/Code/Generation/UserScriptGenerator.ts
@@ -63,6 +63,6 @@ function appendSelection(
 
 function appendCode(selection: SelectedScript, builder: CodeBuilder) {
     const name = selection.revert ? `${selection.script.name} (revert)` : selection.script.name;
-    const scriptCode = selection.revert ? selection.script.revertCode : selection.script.code;
+    const scriptCode = selection.revert ? selection.script.code.revert : selection.script.code.execute;
     builder.appendFunction(name, scriptCode);
 }
diff --git a/src/application/State/Filter/UserFilter.ts b/src/application/State/Filter/UserFilter.ts
index ba1757d7..452ed36f 100644
--- a/src/application/State/Filter/UserFilter.ts
+++ b/src/application/State/Filter/UserFilter.ts
@@ -23,7 +23,6 @@ export class UserFilter implements IUserFilter {
             (script) => isScriptAMatch(script, filterLowercase));
         const filteredCategories = this.application.getAllCategories().filter(
             (category) => category.name.toLowerCase().includes(filterLowercase));
-
         const matches = new FilterResult(
             filteredScripts,
             filteredCategories,
@@ -43,11 +42,11 @@ function isScriptAMatch(script: IScript, filterLowercase: string) {
     if (script.name.toLowerCase().includes(filterLowercase)) {
         return true;
     }
-    if (script.code.toLowerCase().includes(filterLowercase)) {
+    if (script.code.execute.toLowerCase().includes(filterLowercase)) {
         return true;
     }
-    if (script.revertCode) {
-        return script.revertCode.toLowerCase().includes(filterLowercase);
+    if (script.code.revert) {
+        return script.code.revert.toLowerCase().includes(filterLowercase);
     }
     return false;
 }
diff --git a/src/application/application.yaml.d.ts b/src/application/application.yaml.d.ts
index 7bc71c9b..b10c80a6 100644
--- a/src/application/application.yaml.d.ts
+++ b/src/application/application.yaml.d.ts
@@ -1,16 +1,7 @@
 declare module 'js-yaml-loader!*' {
-    export type CategoryOrScript = YamlCategory | YamlScript;
-    export type DocumentationUrls = ReadonlyArray<string> | string;
-
-    export interface YamlDocumentable {
-        docs?: DocumentationUrls;
-    }
-
-    export interface YamlScript extends YamlDocumentable {
-        name: string;
-        code: string;
-        revertCode: string;
-        recommend: string | undefined;
+    export interface ApplicationYaml {
+        actions: ReadonlyArray<YamlCategory>;
+        functions: ReadonlyArray<YamlFunction> | undefined;
     }
 
     export interface YamlCategory extends YamlDocumentable {
@@ -18,8 +9,37 @@ declare module 'js-yaml-loader!*' {
         category: string;
     }
 
-    export interface ApplicationYaml {
-        actions: ReadonlyArray<YamlCategory>;
+    export type CategoryOrScript = YamlCategory | YamlScript;
+    export type DocumentationUrls = ReadonlyArray<string> | string;
+
+    export interface YamlDocumentable {
+        docs?: DocumentationUrls;
+    }
+
+    export interface YamlFunction {
+        name: string;
+        code: string;
+        revertCode?: string;
+        parameters?: readonly string[];
+    }
+
+    export interface FunctionCallParameters {
+        [index: string]: string;
+    }
+
+    export interface FunctionCall {
+        function: string;
+        parameters?: FunctionCallParameters;
+    }
+
+    export type ScriptFunctionCall = readonly FunctionCall[] | FunctionCall | undefined;
+
+    export interface YamlScript extends YamlDocumentable {
+        name: string;
+        code: string | undefined;
+        revertCode: string | undefined;
+        call: ScriptFunctionCall;
+        recommend: string | undefined;
     }
 
     const content: ApplicationYaml;
diff --git a/src/domain/IScript.ts b/src/domain/IScript.ts
index 9c4b3b83..1c7dcb6e 100644
--- a/src/domain/IScript.ts
+++ b/src/domain/IScript.ts
@@ -1,12 +1,12 @@
 import { IEntity } from '../infrastructure/Entity/IEntity';
 import { IDocumentable } from './IDocumentable';
 import { RecommendationLevel } from './RecommendationLevel';
+import { IScriptCode } from './IScriptCode';
 
 export interface IScript extends IEntity<string>, IDocumentable {
     readonly name: string;
     readonly level?: RecommendationLevel;
     readonly documentationUrls: ReadonlyArray<string>;
-    readonly code: string;
-    readonly revertCode: string;
+    readonly code: IScriptCode;
     canRevert(): boolean;
 }
diff --git a/src/domain/IScriptCode.ts b/src/domain/IScriptCode.ts
new file mode 100644
index 00000000..dddb637e
--- /dev/null
+++ b/src/domain/IScriptCode.ts
@@ -0,0 +1,4 @@
+export interface IScriptCode {
+    readonly execute: string;
+    readonly revert: string;
+}
diff --git a/src/domain/Script.ts b/src/domain/Script.ts
index bf56f8ff..918f34b5 100644
--- a/src/domain/Script.ts
+++ b/src/domain/Script.ts
@@ -1,26 +1,22 @@
 import { BaseEntity } from '@/infrastructure/Entity/BaseEntity';
 import { IScript } from './IScript';
 import { RecommendationLevel } from './RecommendationLevel';
+import { IScriptCode } from './IScriptCode';
 
 export class Script extends BaseEntity<string> implements IScript {
     constructor(
         public readonly name: string,
-        public readonly code: string,
-        public readonly revertCode: string,
+        public readonly code: IScriptCode,
         public readonly documentationUrls: ReadonlyArray<string>,
         public readonly level?: RecommendationLevel) {
         super(name);
-        validateCode(name, code);
-        validateLevel(level);
-        if (revertCode) {
-            validateCode(name, revertCode);
-            if (code === revertCode) {
-                throw new Error(`${name}: Code itself and its reverting code cannot be the same`);
-            }
+        if (!code) {
+            throw new Error(`undefined code (script: ${name})`);
         }
+        validateLevel(level);
     }
     public canRevert(): boolean {
-        return Boolean(this.revertCode);
+        return Boolean(this.code.revert);
     }
 }
 
@@ -29,40 +25,3 @@ function validateLevel(level?: RecommendationLevel) {
         throw new Error(`invalid level: ${level}`);
     }
 }
-
-function validateCode(name: string, code: string): void {
-    if (!code || code.length === 0) {
-        throw new Error(`Code of ${name} is empty or null`);
-    }
-    ensureCodeHasUniqueLines(name, code);
-    ensureNoEmptyLines(name, code);
-}
-
-function ensureNoEmptyLines(name: string, code: string): void {
-    if (code.split('\n').some((line) => line.trim().length === 0)) {
-        throw Error(`Script has empty lines "${name}"`);
-    }
-}
-
-function mayBeUniqueLine(codeLine: string): boolean {
-    const trimmed = codeLine.trim();
-    if (trimmed === ')' || trimmed === '(') { // "(" and ")" are used often in batch code
-        return false;
-    }
-    if (codeLine.startsWith(':: ') || codeLine.startsWith('REM ')) { // Is comment?
-        return false;
-    }
-    return true;
-}
-
-function ensureCodeHasUniqueLines(name: string, code: string): void {
-    const lines = code.split('\n')
-        .filter((line) => mayBeUniqueLine(line));
-    if (lines.length === 0) {
-        return;
-    }
-    const duplicateLines = lines.filter((e, i, a) => a.indexOf(e) !== i);
-    if (duplicateLines.length !== 0) {
-        throw Error(`Duplicates detected in script "${name}":\n ${duplicateLines.join('\n')}`);
-    }
-}
diff --git a/src/domain/ScriptCode.ts b/src/domain/ScriptCode.ts
new file mode 100644
index 00000000..37c703dd
--- /dev/null
+++ b/src/domain/ScriptCode.ts
@@ -0,0 +1,57 @@
+import { IScriptCode } from './IScriptCode';
+
+export class ScriptCode implements IScriptCode {
+    constructor(
+        scriptName: string,
+        public readonly execute: string,
+        public readonly revert: string) {
+        if (!scriptName) {
+            throw new Error('script name is undefined');
+        }
+        validateCode(scriptName, execute);
+        if (revert) {
+            scriptName = `${scriptName} (revert)`;
+            validateCode(scriptName, revert);
+            if (execute === revert) {
+                throw new Error(`${scriptName}: Code itself and its reverting code cannot be the same`);
+            }
+        }
+    }
+}
+
+function validateCode(name: string, code: string): void {
+    if (!code || code.length === 0) {
+        throw new Error(`code of ${name} is empty or undefined`);
+    }
+    ensureCodeHasUniqueLines(name, code);
+    ensureNoEmptyLines(name, code);
+}
+
+function ensureNoEmptyLines(name: string, code: string): void {
+    if (code.split('\n').some((line) => line.trim().length === 0)) {
+        throw Error(`Script has empty lines "${name}"`);
+    }
+}
+
+function ensureCodeHasUniqueLines(name: string, code: string): void {
+    const lines = code.split('\n')
+        .filter((line) => mayBeUniqueLine(line));
+    if (lines.length === 0) {
+        return;
+    }
+    const duplicateLines = lines.filter((e, i, a) => a.indexOf(e) !== i);
+    if (duplicateLines.length !== 0) {
+        throw Error(`Duplicates detected in script "${name}":\n ${duplicateLines.join('\n')}`);
+    }
+}
+
+function mayBeUniqueLine(codeLine: string): boolean {
+    const trimmed = codeLine.trim();
+    if (trimmed === ')' || trimmed === '(') { // "(" and ")" are used often in batch code
+        return false;
+    }
+    if (codeLine.startsWith(':: ') || codeLine.startsWith('REM ')) { // Is comment?
+        return false;
+    }
+    return true;
+}
diff --git a/tests/unit/application/Parser/ApplicationParser.spec.ts b/tests/unit/application/Parser/ApplicationParser.spec.ts
index fb83fd82..26620eae 100644
--- a/tests/unit/application/Parser/ApplicationParser.spec.ts
+++ b/tests/unit/application/Parser/ApplicationParser.spec.ts
@@ -5,22 +5,41 @@ import 'mocha';
 import { expect } from 'chai';
 import { parseCategory } from '@/application/Parser/CategoryParser';
 import { RecommendationLevel } from '@/domain/RecommendationLevel';
+import { ScriptCompilerStub } from '../../stubs/ScriptCompilerStub';
 
 describe('ApplicationParser', () => {
     describe('parseApplication', () => {
         it('can parse current application file', () => {
-            expect(() => parseApplication(applicationFile)).to.not.throw();
+            // act
+            const act = () => parseApplication(applicationFile);
+            // assert
+            expect(act).to.not.throw();
         });
         it('throws when undefined', () => {
-            expect(() => parseApplication(undefined)).to.throw('application is null or undefined');
+            // arrange
+            const expectedError = 'application is null or undefined';
+            // act
+            const act = () => parseApplication(undefined);
+            // assert
+            expect(act).to.throw(expectedError);
         });
         it('throws when undefined actions', () => {
-            const sut: ApplicationYaml = { actions: undefined };
-            expect(() => parseApplication(sut)).to.throw('application does not define any action');
+            // arrange
+            const sut: ApplicationYaml = { actions: undefined, functions: undefined };
+            const expectedError = 'application does not define any action';
+            // act
+            const act = () => parseApplication(sut);
+            // assert
+            expect(act).to.throw(expectedError);
         });
         it('throws when has no actions', () => {
-            const sut: ApplicationYaml = { actions: [] };
-            expect(() => parseApplication(sut)).to.throw('application does not define any action');
+            // arrange
+            const sut: ApplicationYaml = { actions: [], functions: undefined };
+            const expectedError = 'application does not define any action';
+            // act
+            const act = () => parseApplication(sut);
+            // assert
+            expect(act).to.throw(expectedError);
         });
         describe('information', () => {
             it('returns expected repository version', () => {
@@ -28,7 +47,7 @@ describe('ApplicationParser', () => {
                 const expected = 'expected-version';
                 const env = getProcessEnvironmentStub();
                 env.VUE_APP_VERSION = expected;
-                const sut: ApplicationYaml = { actions: [ getTestCategory() ] };
+                const sut: ApplicationYaml = { actions: [ getTestCategory() ], functions: undefined };
                 // act
                 const actual = parseApplication(sut, env).info.version;
                 // assert
@@ -39,7 +58,7 @@ describe('ApplicationParser', () => {
                 const expected = 'https://expected-repository.url';
                 const env = getProcessEnvironmentStub();
                 env.VUE_APP_REPOSITORY_URL = expected;
-                const sut: ApplicationYaml = { actions: [ getTestCategory() ] };
+                const sut: ApplicationYaml = { actions: [ getTestCategory() ], functions: undefined };
                 // act
                 const actual = parseApplication(sut, env).info.repositoryUrl;
                 // assert
@@ -50,7 +69,7 @@ describe('ApplicationParser', () => {
                 const expected = 'expected-app-name';
                 const env = getProcessEnvironmentStub();
                 env.VUE_APP_NAME = expected;
-                const sut: ApplicationYaml = { actions: [ getTestCategory() ] };
+                const sut: ApplicationYaml = { actions: [ getTestCategory() ], functions: undefined };
                 // act
                 const actual = parseApplication(sut, env).info.name;
                 // assert
@@ -61,7 +80,7 @@ describe('ApplicationParser', () => {
                 const expected = 'https://expected.sexy';
                 const env = getProcessEnvironmentStub();
                 env.VUE_APP_HOMEPAGE_URL = expected;
-                const sut: ApplicationYaml = { actions: [ getTestCategory() ] };
+                const sut: ApplicationYaml = { actions: [ getTestCategory() ], functions: undefined };
                 // act
                 const actual = parseApplication(sut, env).info.homepage;
                 // assert
@@ -71,8 +90,9 @@ describe('ApplicationParser', () => {
         it('parses actions', () => {
             // arrange
             const actions = [ getTestCategory('test1'), getTestCategory('test2') ];
-            const expected = [ parseCategory(actions[0]), parseCategory(actions[1]) ];
-            const sut: ApplicationYaml = { actions };
+            const compiler = new ScriptCompilerStub();
+            const expected = [ parseCategory(actions[0], compiler), parseCategory(actions[1], compiler) ];
+            const sut: ApplicationYaml = { actions, functions: undefined };
             // act
             const actual = parseApplication(sut).actions;
             // assert
@@ -103,6 +123,7 @@ function getTestScript(scriptName: string, level: RecommendationLevel = Recommen
         code: 'script code',
         revertCode: 'revert code',
         recommend: RecommendationLevel[level].toLowerCase(),
+        call: undefined,
     };
 }
 
diff --git a/tests/unit/application/Parser/CategoryParser.spec.ts b/tests/unit/application/Parser/CategoryParser.spec.ts
index c7d7e395..8aa3b832 100644
--- a/tests/unit/application/Parser/CategoryParser.spec.ts
+++ b/tests/unit/application/Parser/CategoryParser.spec.ts
@@ -4,88 +4,150 @@ import { parseCategory } from '@/application/Parser/CategoryParser';
 import { YamlCategory, CategoryOrScript, YamlScript } from 'js-yaml-loader!./application.yaml';
 import { parseScript } from '@/application/Parser/ScriptParser';
 import { parseDocUrls } from '@/application/Parser/DocumentationParser';
-import { RecommendationLevel } from '@/domain/RecommendationLevel';
+import { ScriptCompilerStub } from '../../stubs/ScriptCompilerStub';
+import { YamlScriptStub } from '../../stubs/YamlScriptStub';
 
 describe('CategoryParser', () => {
     describe('parseCategory', () => {
-
-        it('throws when undefined', () => {
-            expect(() => parseCategory(undefined)).to.throw('category is null or undefined');
+        describe('invalid category', () => {
+            it('throws when undefined', () => {
+                // arrange
+                const expectedMessage = 'category is null or undefined';
+                const category = undefined;
+                const compiler = new ScriptCompilerStub();
+                // act
+                const act = () => parseCategory(category, compiler);
+                // assert
+                expect(act).to.throw(expectedMessage);
+            });
+            it('throws when children are empty', () => {
+                // arrange
+                const expectedMessage = 'category has no children';
+                const category: YamlCategory = {
+                    category: 'test',
+                    children: [],
+                };
+                const compiler = new ScriptCompilerStub();
+                // act
+                const act = () => parseCategory(category, compiler);
+                // assert
+                expect(act).to.throw(expectedMessage);
+            });
+            it('throws when children are undefined', () => {
+                // arrange
+                const expectedMessage = 'category has no children';
+                const category: YamlCategory = {
+                    category: 'test',
+                    children: undefined,
+                };
+                const compiler = new ScriptCompilerStub();
+                // act
+                const act = () => parseCategory(category, compiler);
+                // assert
+                expect(act).to.throw(expectedMessage);
+            });
+            it('throws when name is empty or undefined', () => {
+                // arrange
+                const expectedMessage = 'category has no name';
+                const invalidNames = ['', undefined];
+                invalidNames.forEach((invalidName) => {
+                    const category: YamlCategory = {
+                        category: invalidName,
+                        children: getTestChildren(),
+                    };
+                    const compiler = new ScriptCompilerStub();
+                    // act
+                    const act = () => parseCategory(category, compiler);
+                    // assert
+                    expect(act).to.throw(expectedMessage);
+                });
+            });
         });
-
-        it('throws when children is empty', () => {
-            const category: YamlCategory = {
-                category: 'test',
-                children: [],
-            };
-            expect(() => parseCategory(category)).to.throw('category has no children');
+        it('throws when compiler is undefined', () => {
+            // arrange
+            const expectedError = 'undefined compiler';
+            const compiler = undefined;
+            const category = getValidCategory();
+            // act
+            const act = () => parseCategory(category, compiler);
+            // assert
+            expect(act).to.throw(expectedError);
         });
-
-        it('throws when children is undefined', () => {
-            const category: YamlCategory = {
-                category: 'test',
-                children: undefined,
-            };
-            expect(() => parseCategory(category)).to.throw('category has no children');
-        });
-
-        it('throws when name is empty', () => {
-            const category: YamlCategory = {
-                category: '',
-                children: getTestChildren(),
-            };
-            expect(() => parseCategory(category)).to.throw('category has no name');
-        });
-
-        it('throws when name is undefined', () => {
-            const category: YamlCategory = {
-                category: undefined,
-                children: getTestChildren(),
-            };
-            expect(() => parseCategory(category)).to.throw('category has no name');
-        });
-
         it('returns expected docs', () => {
             // arrange
             const url = 'https://privacy.sexy';
             const expected = parseDocUrls({ docs: url });
+            const compiler = new ScriptCompilerStub();
             const category: YamlCategory = {
                 category: 'category name',
                 children: getTestChildren(),
                 docs: url,
             };
             // act
-            const actual = parseCategory(category).documentationUrls;
+            const actual = parseCategory(category, compiler).documentationUrls;
             // assert
             expect(actual).to.deep.equal(expected);
         });
-
-        it('returns expected scripts', () => {
-            // arrange
-            const script = getTestScript();
-            const expected = [ parseScript(script) ];
-            const category: YamlCategory = {
-                category: 'category name',
-                children: [ script ],
-            };
-            // act
-            const actual = parseCategory(category).scripts;
-            // assert
-            expect(actual).to.deep.equal(expected);
+        describe('parses expected subscript', () => {
+            it('single script with code', () => {
+                // arrange
+                const script = YamlScriptStub.createWithCode();
+                const compiler = new ScriptCompilerStub();
+                const expected = [ parseScript(script, compiler) ];
+                const category: YamlCategory = {
+                    category: 'category name',
+                    children: [ script ],
+                };
+                // act
+                const actual = parseCategory(category, compiler).scripts;
+                // assert
+                expect(actual).to.deep.equal(expected);
+            });
+            it('single script with function call', () => {
+                // arrange
+                const script = YamlScriptStub.createWithCall();
+                const compiler = new ScriptCompilerStub()
+                    .withCompileAbility(script);
+                const expected = [ parseScript(script, compiler) ];
+                const category: YamlCategory = {
+                    category: 'category name',
+                    children: [ script ],
+                };
+                // act
+                const actual = parseCategory(category, compiler).scripts;
+                // assert
+                expect(actual).to.deep.equal(expected);
+            });
+            it('multiple scripts with function call and code', () => {
+                // arrange
+                const callableScript = YamlScriptStub.createWithCall();
+                const scripts = [ callableScript, YamlScriptStub.createWithCode() ];
+                const compiler = new ScriptCompilerStub()
+                    .withCompileAbility(callableScript);
+                const expected = scripts.map((script) => parseScript(script, compiler));
+                const category: YamlCategory = {
+                    category: 'category name',
+                    children: scripts,
+                };
+                // act
+                const actual = parseCategory(category, compiler).scripts;
+                // assert
+                expect(actual).to.deep.equal(expected);
+            });
         });
-
         it('returns expected subcategories', () => {
             // arrange
             const expected: YamlCategory[]  = [ {
                 category: 'test category',
-                children: [ getTestScript() ],
+                children: [ YamlScriptStub.createWithCode() ],
             }];
             const category: YamlCategory = {
                 category: 'category name',
                 children: expected,
             };
+            const compiler = new ScriptCompilerStub();
             // act
-            const actual = parseCategory(category).subCategories;
+            const actual = parseCategory(category, compiler).subCategories;
             // assert
             expect(actual).to.have.lengthOf(1);
             expect(actual[0].name).to.equal(expected[0].category);
@@ -94,17 +156,16 @@ describe('CategoryParser', () => {
     });
 });
 
-function getTestChildren(): ReadonlyArray<CategoryOrScript> {
-    return [
-        getTestScript(),
-    ];
-}
-
-function getTestScript(): YamlScript {
+function getValidCategory(): YamlCategory {
     return {
-        name: 'script name',
-        code: 'script code',
-        revertCode: 'revert code',
-        recommend: RecommendationLevel[RecommendationLevel.Standard],
+        category: 'category name',
+        children: getTestChildren(),
+        docs: undefined,
     };
 }
+
+function getTestChildren(): ReadonlyArray<CategoryOrScript> {
+    return [
+        YamlScriptStub.createWithCode(),
+    ];
+}
diff --git a/tests/unit/application/Parser/Compiler/ScriptCompiler.spec.ts b/tests/unit/application/Parser/Compiler/ScriptCompiler.spec.ts
new file mode 100644
index 00000000..5fcefe1f
--- /dev/null
+++ b/tests/unit/application/Parser/Compiler/ScriptCompiler.spec.ts
@@ -0,0 +1,363 @@
+import 'mocha';
+import { expect } from 'chai';
+import { ScriptCompiler } from '@/application/Parser/Compiler/ScriptCompiler';
+import { YamlScriptStub } from '../../../stubs/YamlScriptStub';
+import { YamlFunction, YamlScript, FunctionCall, ScriptFunctionCall, FunctionCallParameters } from 'js-yaml-loader!./application.yaml';
+import { IScriptCode } from '@/domain/IScriptCode';
+import { IScriptCompiler } from '@/application/Parser/Compiler/IScriptCompiler';
+
+describe('ScriptCompiler', () => {
+    describe('ctor', () => {
+        it('throws when functions have same names', () => {
+            // arrange
+            const expectedError = `duplicate function name: "same-func-name"`;
+            const functions: YamlFunction[] = [ {
+                name: 'same-func-name',
+                code: 'non-empty-code',
+            }, {
+                name: 'same-func-name',
+                code: 'non-empty-code-2',
+            }];
+            // act
+            const act = () => new ScriptCompiler(functions);
+            // assert
+            expect(act).to.throw(expectedError);
+        });
+        it('throws when function parameters have same names', () => {
+            // arrange
+            const func: YamlFunction = {
+                name: 'function-name',
+                code: 'non-empty-code',
+                parameters: [ 'duplicate', 'duplicate' ],
+            };
+            const expectedError = `"${func.name}": duplicate parameter name: "duplicate"`;
+            // act
+            const act = () => new ScriptCompiler([func]);
+            // assert
+            expect(act).to.throw(expectedError);
+        });
+        describe('throws when when function have duplicate code', () => {
+            it('code', () => {
+                // arrange
+                const expectedError = `duplicate "code" in functions: "duplicate-code"`;
+                const functions: YamlFunction[] = [ {
+                    name: 'func-1',
+                    code: 'duplicate-code',
+                }, {
+                    name: 'func-2',
+                    code: 'duplicate-code',
+                }];
+                // act
+                const act = () => new ScriptCompiler(functions);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+            it('revertCode', () => {
+                // arrange
+                const expectedError = `duplicate "revertCode" in functions: "duplicate-revert-code"`;
+                const functions: YamlFunction[] = [ {
+                    name: 'func-1',
+                    code: 'code-1',
+                    revertCode: 'duplicate-revert-code',
+                }, {
+                    name: 'func-2',
+                    code: 'code-2',
+                    revertCode: 'duplicate-revert-code',
+                }];
+                // act
+                const act = () => new ScriptCompiler(functions);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+        });
+    });
+    describe('canCompile', () => {
+        it('returns true if "call" is defined', () => {
+            // arrange
+            const sut = new ScriptCompiler([]);
+            const script = YamlScriptStub.createWithCall();
+            // act
+            const actual = sut.canCompile(script);
+            // assert
+            expect(actual).to.equal(true);
+        });
+        it('returns false if "call" is undefined', () => {
+            // arrange
+            const sut = new ScriptCompiler([]);
+            const script = YamlScriptStub.createWithCode();
+            // act
+            const actual = sut.canCompile(script);
+            // assert
+            expect(actual).to.equal(false);
+        });
+    });
+    describe('compile', () => {
+        describe('invalid state', () => {
+            it('throws if functions are empty', () => {
+                // arrange
+                const expectedError = 'cannot compile without shared functions';
+                const functions = [];
+                const sut = new ScriptCompiler(functions);
+                const script = YamlScriptStub.createWithCall();
+                // act
+                const act = () => sut.compile(script);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+            it('throws if call is not an object', () => {
+                // arrange
+                const expectedError = 'called function(s) must be an object';
+                const invalidValues = [undefined, 'string', 33];
+                const sut = new ScriptCompiler(createFunctions());
+                invalidValues.forEach((invalidValue) => {
+                    const script = YamlScriptStub.createWithoutCallOrCodes() // because call ctor overwrites "undefined"
+                        .withCall(invalidValue as any);
+                    // act
+                    const act = () => sut.compile(script);
+                    // assert
+                    expect(act).to.throw(expectedError);
+                });
+            });
+            describe('invalid function reference', () => {
+                it('throws if function does not exist', () => {
+                    // arrange
+                    const sut = new ScriptCompiler(createFunctions());
+                    const nonExistingFunctionName = 'non-existing-func';
+                    const expectedError = `called function is not defined "${nonExistingFunctionName}"`;
+                    const call: ScriptFunctionCall = { function: nonExistingFunctionName };
+                    const script = YamlScriptStub.createWithCall(call);
+                    // act
+                    const act = () => sut.compile(script);
+                    // assert
+                    expect(act).to.throw(expectedError);
+                });
+                it('throws if function is undefined', () => {
+                    // arrange
+                    const existingFunctionName = 'existing-func';
+                    const sut = new ScriptCompiler(createFunctions(existingFunctionName));
+                    const call: ScriptFunctionCall = [
+                        { function: existingFunctionName },
+                        undefined,
+                    ];
+                    const script = YamlScriptStub.createWithCall(call);
+                    const expectedError = `undefined function call in script "${script.name}"`;
+                    // act
+                    const act = () => sut.compile(script);
+                    // assert
+                    expect(act).to.throw(expectedError);
+                });
+                it('throws if function name is not given', () => {
+                    // arrange
+                    const existingFunctionName = 'existing-func';
+                    const sut = new ScriptCompiler(createFunctions(existingFunctionName));
+                    const call: FunctionCall[] = [
+                        { function: existingFunctionName },
+                        { function: undefined }];
+                    const script = YamlScriptStub.createWithCall(call);
+                    const expectedError = `empty function name called in script "${script.name}"`;
+                    // act
+                    const act = () => sut.compile(script);
+                    // assert
+                    expect(act).to.throw(expectedError);
+                });
+            });
+        });
+        describe('builds code as expected', () => {
+            it('builds single call as expected', () => {
+                // arrange
+                const functionName = 'testSharedFunction';
+                const expected: IScriptCode = {
+                    execute: 'expected-code',
+                    revert: 'expected-revert-code',
+                };
+                const func: YamlFunction = {
+                    name: functionName,
+                    parameters: [],
+                    code: expected.execute,
+                    revertCode: expected.revert,
+                };
+                const sut = new ScriptCompiler([func]);
+                const call: FunctionCall = { function: functionName };
+                const script = YamlScriptStub.createWithCall(call);
+                // act
+                const actual = sut.compile(script);
+                // assert
+                expect(actual).to.deep.equal(expected);
+            });
+            it('builds call sequence as expected', () => {
+                // arrange
+                const firstFunction: YamlFunction = {
+                    name: 'first-function-name',
+                    parameters: [],
+                    code: 'first-function-code',
+                    revertCode: 'first-function-revert-code',
+                };
+                const secondFunction: YamlFunction = {
+                    name: 'second-function-name',
+                    parameters: [],
+                    code: 'second-function-code',
+                    revertCode: 'second-function-revert-code',
+                };
+                const expected: IScriptCode = {
+                    execute: 'first-function-code\nsecond-function-code',
+                    revert: 'first-function-revert-code\nsecond-function-revert-code',
+                };
+                const sut = new ScriptCompiler([firstFunction, secondFunction]);
+                const call: FunctionCall[] = [
+                    { function: firstFunction.name },
+                    { function: secondFunction.name },
+                ];
+                const script = YamlScriptStub.createWithCall(call);
+                // act
+                const actual = sut.compile(script);
+                // assert
+                expect(actual).to.deep.equal(expected);
+            });
+        });
+        describe('parameter substitution', () => {
+            describe('substitutes by ignoring whitespaces inside mustaches', () => {
+                // arrange
+                const mustacheVariations = [
+                    'Hello {{ $test }}!',
+                    'Hello {{$test }}!',
+                    'Hello {{ $test}}!',
+                    'Hello {{$test}}!'];
+                mustacheVariations.forEach((variation) => {
+                    it(variation, () => {
+                        // arrange
+                        const env = new TestEnvironment({
+                            code: variation,
+                            parameters: {
+                                test: 'world',
+                            },
+                        });
+                        const expected = env.expect('Hello world!');
+                        // act
+                        const actual = env.sut.compile(env.script);
+                        // assert
+                        expect(actual).to.deep.equal(expected);
+                    });
+                });
+            });
+            describe('substitutes as expected', () => {
+                it('with different parameters', () => {
+                    // arrange
+                    const env = new TestEnvironment({
+                        code: 'He{{ $firstParameter }} {{ $secondParameter }}!',
+                        parameters: {
+                            firstParameter: 'llo',
+                            secondParameter: 'world',
+                        },
+                    });
+                    const expected = env.expect('Hello world!');
+                    // act
+                    const actual = env.sut.compile(env.script);
+                    // assert
+                    expect(actual).to.deep.equal(expected);
+                });
+                it('with same parameter repeated', () => {
+                    // arrange
+                    const env = new TestEnvironment({
+                        code: '{{ $parameter }} {{ $parameter }}!',
+                        parameters: {
+                            parameter: 'Hodor',
+                        },
+                    });
+                    const expected = env.expect('Hodor Hodor!');
+                    // act
+                    const actual = env.sut.compile(env.script);
+                    // assert
+                    expect(actual).to.deep.equal(expected);
+                });
+            });
+            it('throws when parameters is undefined', () => {
+                // arrange
+                const env = new TestEnvironment({
+                    code: '{{ $parameter }} {{ $parameter }}!',
+                });
+                const expectedError = 'no parameters defined, expected: "parameter"';
+                // act
+                const act = () => env.sut.compile(env.script);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+            it('throws when parameter value is not provided', () => {
+                // arrange
+                const env = new TestEnvironment({
+                    code: '{{ $parameter }} {{ $parameter }}!',
+                    parameters: {
+                        parameter: undefined,
+                    },
+                });
+                const expectedError = 'parameter value is not provided for "parameter" in function call';
+                // act
+                const act = () => env.sut.compile(env.script);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+            it('throws on unknown expressions', () => {
+                // arrange
+                const env = new TestEnvironment({
+                    code: '{{ each }}',
+                    parameters: {
+                        parameter: undefined,
+                    },
+                });
+                const expectedError = 'unknown expression: "each"';
+                // act
+                const act = () => env.sut.compile(env.script);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+        });
+    });
+});
+
+interface ITestCase {
+    code: string;
+    parameters?: FunctionCallParameters;
+}
+
+class TestEnvironment {
+    public readonly sut: IScriptCompiler;
+    public readonly script: YamlScript;
+    constructor(testCase: ITestCase) {
+        const functionName = 'testFunction';
+        const func: YamlFunction = {
+            name: functionName,
+            parameters: testCase.parameters ? Object.keys(testCase.parameters) : undefined,
+            code: this.getCode(testCase.code, 'execute'),
+            revertCode: this.getCode(testCase.code, 'revert'),
+        };
+        this.sut = new ScriptCompiler([func]);
+        const call: FunctionCall = {
+            function: functionName,
+            parameters: testCase.parameters,
+        };
+        this.script = YamlScriptStub.createWithCall(call);
+    }
+    public expect(code: string): IScriptCode {
+        return {
+            execute: this.getCode(code, 'execute'),
+            revert: this.getCode(code, 'revert'),
+        };
+    }
+    private getCode(text: string, type: 'execute' | 'revert'): string {
+        return `${text} (${type})`;
+    }
+}
+
+function createFunctions(...names: string[]): YamlFunction[] {
+    if (!names || names.length === 0) {
+        names = ['test-function'];
+    }
+    return names.map((functionName) => {
+        const func: YamlFunction = {
+            name: functionName,
+            parameters: [],
+            code: `REM test-code (${functionName})`,
+            revertCode: `REM test-revert-code (${functionName})`,
+        };
+        return func;
+    });
+}
diff --git a/tests/unit/application/Parser/ScriptParser.spec.ts b/tests/unit/application/Parser/ScriptParser.spec.ts
index c038e7bc..6369c3e4 100644
--- a/tests/unit/application/Parser/ScriptParser.spec.ts
+++ b/tests/unit/application/Parser/ScriptParser.spec.ts
@@ -1,58 +1,93 @@
-import { YamlScript } from 'js-yaml-loader!./application.yaml';
 import 'mocha';
 import { expect } from 'chai';
 import { parseScript } from '@/application/Parser/ScriptParser';
 import { parseDocUrls } from '@/application/Parser/DocumentationParser';
 import { RecommendationLevelNames, RecommendationLevel } from '@/domain/RecommendationLevel';
+import { ScriptCode } from '@/domain/ScriptCode';
+import { ScriptCompilerStub } from '../../stubs/ScriptCompilerStub';
+import { YamlScriptStub } from '../../stubs/YamlScriptStub';
 
 describe('ScriptParser', () => {
     describe('parseScript', () => {
         it('parses name as expected', () => {
             // arrange
-            const script = getValidScript();
-            script.name = 'expected-name';
+            const expected = 'test-expected-name';
+            const script = YamlScriptStub.createWithCode()
+                .withName(expected);
+            const compiler = new ScriptCompilerStub();
             // act
-            const actual = parseScript(script);
+            const actual = parseScript(script, compiler);
             // assert
-            expect(actual.name).to.equal(script.name);
-        });
-        it('parses code as expected', () => {
-            // arrange
-            const script = getValidScript();
-            script.code = 'expected-code';
-            // act
-            const actual = parseScript(script);
-            // assert
-            expect(actual.code).to.equal(script.code);
-        });
-        it('parses revertCode as expected', () => {
-            // arrange
-            const script = getValidScript();
-            script.code = 'expected-code';
-            // act
-            const actual = parseScript(script);
-            // assert
-            expect(actual.revertCode).to.equal(script.revertCode);
+            expect(actual.name).to.equal(expected);
         });
         it('parses docs as expected', () => {
             // arrange
-            const script = getValidScript();
-            script.docs = [ 'https://expected-doc1.com', 'https://expected-doc2.com' ];
+            const docs = [ 'https://expected-doc1.com', 'https://expected-doc2.com' ];
+            const script = YamlScriptStub.createWithCode()
+                .withDocs(docs);
+            const compiler = new ScriptCompilerStub();
             const expected = parseDocUrls(script);
             // act
-            const actual = parseScript(script);
+            const actual = parseScript(script, compiler);
             // assert
             expect(actual.documentationUrls).to.deep.equal(expected);
         });
+        describe('invalid script', () => {
+            it('throws when script is undefined', () => {
+                // arrange
+                const expectedError = 'undefined script';
+                const compiler = new ScriptCompilerStub();
+                const script = undefined;
+                // act
+                const act = () => parseScript(script, compiler);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+            it('throws when both function call and code are defined', () => {
+                // arrange
+                const expectedError = 'cannot define both "call" and "code"';
+                const compiler = new ScriptCompilerStub();
+                const script = YamlScriptStub
+                    .createWithCall()
+                    .withCode('code');
+                // act
+                const act = () => parseScript(script, compiler);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+            it('throws when both function call and revertCode are defined', () => {
+                // arrange
+                const expectedError = 'cannot define "revertCode" if "call" is defined';
+                const compiler = new ScriptCompilerStub();
+                const script = YamlScriptStub
+                    .createWithCall()
+                    .withRevertCode('revert-code');
+                // act
+                const act = () => parseScript(script, compiler);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+            it('throws when neither call or revertCode are defined', () => {
+                // arrange
+                const expectedError = 'must define either "call" or "code"';
+                const compiler = new ScriptCompilerStub();
+                const script = YamlScriptStub.createWithoutCallOrCodes();
+                // act
+                const act = () => parseScript(script, compiler);
+                // assert
+                expect(act).to.throw(expectedError);
+            });
+        });
         describe('level', () => {
             it('accepts undefined level', () => {
                 const undefinedLevels: string[] = [ '', undefined ];
                 undefinedLevels.forEach((undefinedLevel) => {
                     // arrange
-                    const script = getValidScript();
+                    const compiler = new ScriptCompilerStub();
+                    const script = YamlScriptStub.createWithCode();
                     script.recommend = undefinedLevel;
                     // act
-                    const actual = parseScript(script);
+                    const actual = parseScript(script, compiler);
                     // assert
                     expect(actual.level).to.equal(undefined);
                 });
@@ -60,10 +95,11 @@ describe('ScriptParser', () => {
             it('throws on unknown level', () => {
                 // arrange
                 const unknownLevel = 'boi';
-                const script = getValidScript();
+                const compiler = new ScriptCompilerStub();
+                const script = YamlScriptStub.createWithCode();
                 script.recommend = unknownLevel;
                 // act
-                const act = () => parseScript(script);
+                const act = () => parseScript(script, compiler);
                 // assert
                 expect(act).to.throw(`unknown level: "${unknownLevel}"`);
             });
@@ -71,10 +107,11 @@ describe('ScriptParser', () => {
                 const nonStringTypes: any[] = [ 5, true ];
                 nonStringTypes.forEach((nonStringType) => {
                     // arrange
-                    const script = getValidScript();
+                    const script = YamlScriptStub.createWithCode();
+                    const compiler = new ScriptCompilerStub();
                     script.recommend = nonStringType;
                     // act
-                    const act = () => parseScript(script);
+                    const act = () => parseScript(script, compiler);
                     // assert
                     expect(act).to.throw(`level must be a string but it was ${typeof nonStringType}`);
                 });
@@ -84,10 +121,11 @@ describe('ScriptParser', () => {
                     it(levelText, () => {
                         // arrange
                         const expectedLevel = RecommendationLevel[levelText];
-                        const script = getValidScript();
+                        const script = YamlScriptStub.createWithCode();
+                        const compiler = new ScriptCompilerStub();
                         script.recommend = levelText;
                         // act
-                        const actual = parseScript(script);
+                        const actual = parseScript(script, compiler);
                         // assert
                         expect(actual.level).to.equal(expectedLevel);
                     });
@@ -95,24 +133,66 @@ describe('ScriptParser', () => {
             });
             it('parses level case insensitive', () => {
                 // arrange
-                const script = getValidScript();
+                const script = YamlScriptStub.createWithCode();
+                const compiler = new ScriptCompilerStub();
                 const expected = RecommendationLevel.Standard;
                 script.recommend = RecommendationLevel[expected].toUpperCase();
                 // act
-                const actual = parseScript(script);
+                const actual = parseScript(script, compiler);
                 // assert
                 expect(actual.level).to.equal(expected);
             });
         });
+        describe('code', () => {
+            it('parses code as expected', () => {
+                // arrange
+                const expected = 'expected-code';
+                const script = YamlScriptStub
+                    .createWithCode()
+                    .withCode(expected);
+                const compiler = new ScriptCompilerStub();
+                // act
+                const parsed = parseScript(script, compiler);
+                // assert
+                const actual = parsed.code.execute;
+                expect(actual).to.equal(expected);
+            });
+            it('parses revertCode as expected', () => {
+                // arrange
+                const expected = 'expected-revert-code';
+                const script = YamlScriptStub
+                    .createWithCode()
+                    .withRevertCode(expected);
+                const compiler = new ScriptCompilerStub();
+                // act
+                const parsed = parseScript(script, compiler);
+                // assert
+                const actual = parsed.code.revert;
+                expect(actual).to.equal(expected);
+            });
+            describe('compiler', () => {
+                it('throws when compiler is not defined', () => {
+                    // arrange
+                    const script = YamlScriptStub.createWithCode();
+                    const compiler = undefined;
+                    // act
+                    const act = () => parseScript(script, compiler);
+                    // assert
+                    expect(act).to.throw('undefined compiler');
+                });
+                it('gets code from compiler', () => {
+                    // arrange
+                    const expected = new ScriptCode('test-script', 'code', 'revert-code');
+                    const script = YamlScriptStub.createWithCode();
+                    const compiler = new ScriptCompilerStub()
+                        .withCompileAbility(script, expected);
+                    // act
+                    const parsed = parseScript(script, compiler);
+                    // assert
+                    const actual = parsed.code;
+                    expect(actual).to.equal(expected);
+                });
+            });
+        });
     });
 });
-
-function getValidScript(): YamlScript {
-    return {
-        name: 'valid-name',
-        code: 'valid-code',
-        revertCode: 'expected revert code',
-        docs: ['hello.com'],
-        recommend: RecommendationLevel[RecommendationLevel.Standard].toLowerCase(),
-    };
-}
diff --git a/tests/unit/application/State/Filter/UserFilter.spec.ts b/tests/unit/application/State/Filter/UserFilter.spec.ts
index 07a6b42c..2ec92edc 100644
--- a/tests/unit/application/State/Filter/UserFilter.spec.ts
+++ b/tests/unit/application/State/Filter/UserFilter.spec.ts
@@ -18,10 +18,11 @@ describe('UserFilter', () => {
             // assert
             expect(isCalled).to.be.equal(true);
         });
-        it('currentFilter is undefined', () => {
+        it('sets currentFilter to undefined', () => {
             // arrange
             const sut = new UserFilter(new ApplicationStub());
             // act
+            sut.setFilter('non-important');
             sut.removeFilter();
             // assert
             expect(sut.currentFilter).to.be.equal(undefined);
@@ -51,68 +52,69 @@ describe('UserFilter', () => {
             expect(actual.hasAnyMatches()).be.equal(false);
             expect(actual.query).to.equal(nonMatchingFilter);
         });
-        describe('signals when script matches', () => {
-            it('code matches', () => {
-                // arrange
-                const code = 'HELLO world';
-                const filter = 'Hello WoRLD';
-                let actual: IFilterResult;
-                const script = new ScriptStub('id').withCode(code);
-                const category = new CategoryStub(33).withScript(script);
-                const sut = new UserFilter(new ApplicationStub()
-                    .withAction(category));
-                sut.filtered.on((filterResult) => actual = filterResult);
-                // act
-                sut.setFilter(filter);
-                // assert
-                expect(actual.hasAnyMatches()).be.equal(true);
-                expect(actual.categoryMatches).to.have.lengthOf(0);
-                expect(actual.scriptMatches).to.have.lengthOf(1);
-                expect(actual.scriptMatches[0]).to.deep.equal(script);
-                expect(actual.query).to.equal(filter);
-                expect(sut.currentFilter).to.deep.equal(actual);
+        describe('signals when matches', () => {
+            describe('signals when script matches', () => {
+                it('code matches', () => {
+                    // arrange
+                    const code = 'HELLO world';
+                    const filter = 'Hello WoRLD';
+                    let actual: IFilterResult;
+                    const script = new ScriptStub('id').withCode(code);
+                    const category = new CategoryStub(33).withScript(script);
+                    const sut = new UserFilter(new ApplicationStub()
+                        .withAction(category));
+                    sut.filtered.on((filterResult) => actual = filterResult);
+                    // act
+                    sut.setFilter(filter);
+                    // assert
+                    expect(actual.hasAnyMatches()).be.equal(true);
+                    expect(actual.categoryMatches).to.have.lengthOf(0);
+                    expect(actual.scriptMatches).to.have.lengthOf(1);
+                    expect(actual.scriptMatches[0]).to.deep.equal(script);
+                    expect(actual.query).to.equal(filter);
+                    expect(sut.currentFilter).to.deep.equal(actual);
+                });
+                it('revertCode matches', () => {
+                    // arrange
+                    const revertCode = 'HELLO world';
+                    const filter = 'Hello WoRLD';
+                    let actual: IFilterResult;
+                    const script = new ScriptStub('id').withRevertCode(revertCode);
+                    const category = new CategoryStub(33).withScript(script);
+                    const sut = new UserFilter(new ApplicationStub()
+                        .withAction(category));
+                    sut.filtered.on((filterResult) => actual = filterResult);
+                    // act
+                    sut.setFilter(filter);
+                    // assert
+                    expect(actual.hasAnyMatches()).be.equal(true);
+                    expect(actual.categoryMatches).to.have.lengthOf(0);
+                    expect(actual.scriptMatches).to.have.lengthOf(1);
+                    expect(actual.scriptMatches[0]).to.deep.equal(script);
+                    expect(actual.query).to.equal(filter);
+                    expect(sut.currentFilter).to.deep.equal(actual);
+                });
+                it('name matches', () => {
+                    // arrange
+                    const name = 'HELLO world';
+                    const filter = 'Hello WoRLD';
+                    let actual: IFilterResult;
+                    const script = new ScriptStub('id').withName(name);
+                    const category = new CategoryStub(33).withScript(script);
+                    const sut = new UserFilter(new ApplicationStub()
+                        .withAction(category));
+                    sut.filtered.on((filterResult) => actual = filterResult);
+                    // act
+                    sut.setFilter(filter);
+                    // assert
+                    expect(actual.hasAnyMatches()).be.equal(true);
+                    expect(actual.categoryMatches).to.have.lengthOf(0);
+                    expect(actual.scriptMatches).to.have.lengthOf(1);
+                    expect(actual.scriptMatches[0]).to.deep.equal(script);
+                    expect(actual.query).to.equal(filter);
+                    expect(sut.currentFilter).to.deep.equal(actual);
+                });
             });
-            it('revertCode matches', () => {
-                // arrange
-                const revertCode = 'HELLO world';
-                const filter = 'Hello WoRLD';
-                let actual: IFilterResult;
-                const script = new ScriptStub('id').withRevertCode(revertCode);
-                const category = new CategoryStub(33).withScript(script);
-                const sut = new UserFilter(new ApplicationStub()
-                    .withAction(category));
-                sut.filtered.on((filterResult) => actual = filterResult);
-                // act
-                sut.setFilter(filter);
-                // assert
-                expect(actual.hasAnyMatches()).be.equal(true);
-                expect(actual.categoryMatches).to.have.lengthOf(0);
-                expect(actual.scriptMatches).to.have.lengthOf(1);
-                expect(actual.scriptMatches[0]).to.deep.equal(script);
-                expect(actual.query).to.equal(filter);
-                expect(sut.currentFilter).to.deep.equal(actual);
-            });
-            it('name matches', () => {
-                // arrange
-                const name = 'HELLO world';
-                const filter = 'Hello WoRLD';
-                let actual: IFilterResult;
-                const script = new ScriptStub('id').withName(name);
-                const category = new CategoryStub(33).withScript(script);
-                const sut = new UserFilter(new ApplicationStub()
-                    .withAction(category));
-                sut.filtered.on((filterResult) => actual = filterResult);
-                // act
-                sut.setFilter(filter);
-                // assert
-                expect(actual.hasAnyMatches()).be.equal(true);
-                expect(actual.categoryMatches).to.have.lengthOf(0);
-                expect(actual.scriptMatches).to.have.lengthOf(1);
-                expect(actual.scriptMatches[0]).to.deep.equal(script);
-                expect(actual.query).to.equal(filter);
-                expect(sut.currentFilter).to.deep.equal(actual);
-            });
-
             it('signals when category matches', () => {
                 // arrange
                 const categoryName = 'HELLO world';
diff --git a/tests/unit/domain/Script.spec.ts b/tests/unit/domain/Script.spec.ts
index ca2c7064..826c9ee6 100644
--- a/tests/unit/domain/Script.spec.ts
+++ b/tests/unit/domain/Script.spec.ts
@@ -1,76 +1,149 @@
 import 'mocha';
 import { expect } from 'chai';
 import { Script } from '@/domain/Script';
-import { RecommendationLevelNames, RecommendationLevel } from '@/domain/RecommendationLevel';
+import { RecommendationLevel, RecommendationLevels } from '@/domain/RecommendationLevel';
+import { ScriptCode } from '@/domain/ScriptCode';
+import { IScriptCode } from '@/domain/IScriptCode';
 
 describe('Script', () => {
     describe('ctor', () => {
-        describe('code', () => {
-            it('cannot construct with duplicate lines', () => {
-                const code = 'duplicate\nduplicate\ntest\nduplicate';
-                expect(() => createWithCode(code)).to.throw();
-            });
-            it('cannot construct with empty lines', () => {
-                const code = 'duplicate\n\n\ntest\nduplicate';
-                expect(() => createWithCode(code)).to.throw();
-            });
+        describe('scriptCode', () => {
             it('sets as expected', () => {
-                const expected = 'expected-revert';
-                const sut = createWithCode(expected);
-                expect(sut.code).to.equal(expected);
+                // arrange
+                const name = 'test-script';
+                const expected = new ScriptCode(name, 'expected-execute', 'expected-revert');
+                const sut = new ScriptBuilder()
+                    .withCode(expected)
+                    .build();
+                // act
+                const actual = sut.code;
+                // assert
+                expect(actual).to.deep.equal(expected);
             });
-        });
-        describe('revertCode', () => {
-            it('cannot construct with duplicate lines', () => {
-                const code = 'duplicate\nduplicate\ntest\nduplicate';
-                expect(() => createWithCode('REM', code)).to.throw();
-            });
-            it('cannot construct with empty lines', () => {
-                const code = 'duplicate\n\n\ntest\nduplicate';
-                expect(() => createWithCode('REM', code)).to.throw();
-            });
-            it('cannot construct with when same as code', () => {
-                const code = 'REM';
-                expect(() => createWithCode(code, code)).to.throw();
-            });
-            it('sets as expected', () => {
-                const expected = 'expected-revert';
-                const sut = createWithCode('abc', expected);
-                expect(sut.revertCode).to.equal(expected);
+            it('throws if undefined', () => {
+                // arrange
+                const name = 'script-name';
+                const expectedError = `undefined code (script: ${name})`;
+                const code: IScriptCode = undefined;
+                // act
+                const construct = () => new ScriptBuilder()
+                    .withName(name)
+                    .withCode(code)
+                    .build();
+                // assert
+                expect(construct).to.throw(expectedError);
             });
         });
         describe('canRevert', () => {
             it('returns false without revert code', () => {
-                const sut = createWithCode('code');
-                expect(sut.canRevert()).to.equal(false);
+                // arrange
+                const sut = new ScriptBuilder()
+                    .withCodes('code')
+                    .build();
+                // act
+                const actual = sut.canRevert();
+                // assert
+                expect(actual).to.equal(false);
             });
             it('returns true with revert code', () => {
-                const sut = createWithCode('code', 'non empty revert code');
-                expect(sut.canRevert()).to.equal(true);
+                // arrange
+                const sut = new ScriptBuilder()
+                    .withCodes('code', 'non empty revert code')
+                    .build();
+                // act
+                const actual = sut.canRevert();
+                // assert
+                expect(actual).to.equal(true);
             });
         });
         describe('level', () => {
             it('cannot construct with invalid wrong value', () => {
-                expect(() => createWithLevel(55)).to.throw('invalid level');
+                // arrange
+                const invalidValue: RecommendationLevel = 55;
+                const expectedError = 'invalid level';
+                // act
+                const construct = () => new ScriptBuilder()
+                    .withRecommendationLevel(invalidValue)
+                    .build();
+                // assert
+                expect(construct).to.throw(expectedError);
             });
             it('sets undefined as expected', () => {
-                const sut = createWithLevel(undefined);
-                expect(sut.level).to.equal(undefined);
+                // arrange
+                const expected = undefined;
+                // act
+                const sut = new ScriptBuilder()
+                    .withRecommendationLevel(expected)
+                    .build();
+                // assert
+                expect(sut.level).to.equal(expected);
             });
             it('sets as expected', () => {
-                for (const expected of RecommendationLevelNames) {
-                    const sut = createWithLevel(RecommendationLevel[expected]);
-                    const actual = RecommendationLevel[sut.level];
+                // arrange
+                for (const expected of RecommendationLevels) {
+                    // act
+                    const sut = new ScriptBuilder()
+                        .withRecommendationLevel(expected)
+                        .build();
+                    // assert
+                    const actual = sut.level;
                     expect(actual).to.equal(expected);
                 }
             });
         });
+        describe('documentationUrls', () => {
+            it('sets as expected', () => {
+                // arrange
+                const expected = [ 'doc1', 'doc2 '];
+                // act
+                const sut = new ScriptBuilder()
+                    .withDocumentationUrls(expected)
+                    .build();
+                const actual = sut.documentationUrls;
+                // assert
+                expect(actual).to.equal(expected);
+            });
+        });
     });
 });
 
-function createWithCode(code: string, revertCode?: string): Script {
-    return new Script('name', code, revertCode, [], RecommendationLevel.Standard);
-}
-function createWithLevel(level: RecommendationLevel): Script {
-    return new Script('name', 'code', 'revertCode', [], level);
+class ScriptBuilder {
+    private name = 'test-script';
+    private code: IScriptCode = new ScriptCode(this.name, 'code', 'revert-code');
+    private level = RecommendationLevel.Standard;
+    private documentationUrls: readonly string[] = undefined;
+
+    public withCodes(code: string, revertCode = ''): ScriptBuilder {
+        this.code = new ScriptCode(this.name, code, revertCode);
+        return this;
+    }
+
+    public withCode(code: IScriptCode): ScriptBuilder {
+        this.code = code;
+        return this;
+    }
+
+    public withName(name: string): ScriptBuilder {
+        this.name = name;
+        return this;
+    }
+
+    public withRecommendationLevel(level: RecommendationLevel): ScriptBuilder {
+        this.level = level;
+        return this;
+    }
+
+    public withDocumentationUrls(urls: readonly string[]): ScriptBuilder {
+        this.documentationUrls = urls;
+        return this;
+    }
+
+    public build(): Script {
+        return new Script(
+            this.name,
+            this.code,
+            this.documentationUrls,
+            this.level,
+        );
+    }
 }
diff --git a/tests/unit/domain/ScriptCode.spec.ts b/tests/unit/domain/ScriptCode.spec.ts
new file mode 100644
index 00000000..5b4e02fd
--- /dev/null
+++ b/tests/unit/domain/ScriptCode.spec.ts
@@ -0,0 +1,93 @@
+import 'mocha';
+import { expect } from 'chai';
+import { ScriptCode } from '@/domain/ScriptCode';
+
+describe('ScriptCode', () => {
+    describe('scriptName', () => {
+        it('throws if undefined', () => {
+            // arrange
+            const expectedError = 'name is undefined';
+            const name = undefined;
+            // act
+            const act = () => new ScriptCode(name, 'non-empty-code', '');
+            // assert
+            expect(act).to.throw(expectedError);
+        });
+    });
+    describe('code', () => {
+        it('cannot construct with duplicate lines', () => {
+            // arrange
+            const code = 'duplicate\nduplicate\ntest\nduplicate';
+            // act
+            const act = () => createSut(code);
+            // assert
+            expect(act).to.throw();
+        });
+        it('cannot construct with empty lines', () => {
+            // arrange
+            const code = 'line1\n\n\nline2';
+            // act
+            const act = () => createSut(code);
+            // assert
+            expect(act).to.throw();
+        });
+        it('cannot construct with empty or undefined values', () => {
+            // arrange
+            const name = 'test-code';
+            const errorMessage = `code of ${name} is empty or undefined`;
+            const invalidValues = [ '', undefined ];
+            invalidValues.forEach((invalidValue) => {
+                // act
+                const act = () => new ScriptCode(name, invalidValue, '');
+                // assert
+                expect(act).to.throw(errorMessage);
+            });
+        });
+        it('sets as expected', () => {
+            // arrange
+            const expected = 'expected-revert';
+            // act
+            const sut = createSut(expected);
+            // assert
+            expect(sut.execute).to.equal(expected);
+        });
+    });
+    describe('revert', () => {
+        it('cannot construct with duplicate lines', () => {
+            // arrange
+            const code = 'duplicate\nduplicate\ntest\nduplicate';
+            // act
+            const act = () => createSut('REM', code);
+            // assert
+            expect(act).to.throw();
+        });
+        it('cannot construct with empty lines', () => {
+            // arrange
+            const code = 'line1\n\n\nline2';
+            // act
+            const act = () => createSut('REM', code);
+            // assert
+            expect(act).to.throw();
+        });
+        it('cannot construct with when same as code', () => {
+            // arrange
+            const code = 'REM';
+            // act
+            const act = () => createSut(code, code);
+            // assert
+            expect(act).to.throw();
+        });
+        it('sets as expected', () => {
+            // arrange
+            const expected = 'expected-revert';
+            // act
+            const sut = createSut('abc', expected);
+            // assert
+            expect(sut.revert).to.equal(expected);
+        });
+    });
+});
+
+function createSut(code: string, revert = ''): ScriptCode {
+    return new ScriptCode('test-code', code, revert);
+}
diff --git a/tests/unit/presentation/Scripts/ScriptsTree/ScriptNodeParser.spec.ts b/tests/unit/presentation/Scripts/ScriptsTree/ScriptNodeParser.spec.ts
index ee566184..5c2920cb 100644
--- a/tests/unit/presentation/Scripts/ScriptsTree/ScriptNodeParser.spec.ts
+++ b/tests/unit/presentation/Scripts/ScriptsTree/ScriptNodeParser.spec.ts
@@ -3,11 +3,11 @@ import { expect } from 'chai';
 import { getScriptNodeId, getScriptId, getCategoryNodeId, getCategoryId } from '@/presentation/Scripts/ScriptsTree/ScriptNodeParser';
 import { CategoryStub } from '../../../stubs/CategoryStub';
 import { ScriptStub } from '../../../stubs/ScriptStub';
-import { parseSingleCategory, parseAllCategories } from '../../../../../src/presentation/Scripts/ScriptsTree/ScriptNodeParser';
+import { parseSingleCategory, parseAllCategories } from '@/presentation/Scripts/ScriptsTree/ScriptNodeParser';
 import { ApplicationStub } from '../../../stubs/ApplicationStub';
-import { INode, NodeType } from '../../../../../src/presentation/Scripts/ScriptsTree/SelectableTree/Node/INode';
-import { IScript } from '../../../../../src/domain/IScript';
-import { ICategory } from '../../../../../src/domain/ICategory';
+import { INode, NodeType } from '@/presentation/Scripts/ScriptsTree/SelectableTree/Node/INode';
+import { IScript } from '@/domain/IScript';
+import { ICategory } from '@/domain/ICategory';
 
 describe('ScriptNodeParser', () => {
     it('can convert script id and back', () => {
@@ -80,7 +80,7 @@ describe('ScriptNodeParser', () => {
 
 function isReversible(category: ICategory): boolean {
     if (category.scripts) {
-        return category.scripts.every((s) => s.revertCode);
+        return category.scripts.every((s) => s.canRevert());
     }
     return category.subCategories.every((c) => isReversible(c));
 }
@@ -100,8 +100,8 @@ function expectSameCategory(node: INode, category: ICategory): void {
     }
     function getErrorMessage(field: string) {
         return  `Unexpected node field: ${field}.\n` +
-                `\nActual node:\n${JSON.stringify(node, null, 2)}` +
-                `\nExpected category:\n${JSON.stringify(category, null, 2)}`;
+                `\nActual node:\n${print(node)}` +
+                `\nExpected category:\n${print(category)}`;
     }
 }
 
@@ -110,11 +110,15 @@ function expectSameScript(node: INode, script: IScript): void {
     expect(node.id).to.equal(getScriptNodeId(script), getErrorMessage('id'));
     expect(node.documentationUrls).to.equal(script.documentationUrls, getErrorMessage('documentationUrls'));
     expect(node.text).to.equal(script.name, getErrorMessage('name'));
-    expect(node.isReversible).to.equal(!!script.revertCode, getErrorMessage('revertCode'));
+    expect(node.isReversible).to.equal(script.canRevert(), getErrorMessage('canRevert'));
     expect(node.children).to.equal(undefined);
     function getErrorMessage(field: string) {
         return  `Unexpected node field: ${field}.` +
-                `\nActual node:\n${JSON.stringify(node, null, 2)}\n` +
-                `\nExpected script:\n${JSON.stringify(script, null, 2)}`;
+                `\nActual node:\n${print(node)}\n` +
+                `\nExpected script:\n${print(script)}`;
     }
 }
+
+function print(object: any) {
+    return JSON.stringify(object, null, 2);
+}
diff --git a/tests/unit/presentation/Scripts/ScriptsTree/SelectableTree/Node/Reverter/ScriptReverter.spec.ts b/tests/unit/presentation/Scripts/ScriptsTree/SelectableTree/Node/Reverter/ScriptReverter.spec.ts
index 42d0422c..0501ff52 100644
--- a/tests/unit/presentation/Scripts/ScriptsTree/SelectableTree/Node/Reverter/ScriptReverter.spec.ts
+++ b/tests/unit/presentation/Scripts/ScriptsTree/SelectableTree/Node/Reverter/ScriptReverter.spec.ts
@@ -4,7 +4,7 @@ import { ScriptReverter } from '@/presentation/Scripts/ScriptsTree/SelectableTre
 import { SelectedScriptStub } from '../../../../../../stubs/SelectedScriptStub';
 import { getScriptNodeId } from '@/presentation/Scripts/ScriptsTree/ScriptNodeParser';
 import { ScriptStub } from '../../../../../../stubs/ScriptStub';
-import { UserSelection } from '../../../../../../../../src/application/State/Selection/UserSelection';
+import { UserSelection } from '@/application/State/Selection/UserSelection';
 import { SelectedScript } from '@/application/State/Selection/SelectedScript';
 import { ApplicationStub } from '../../../../../../stubs/ApplicationStub';
 import { CategoryStub } from '../../../../../../stubs/CategoryStub';
diff --git a/tests/unit/stubs/ScriptCompilerStub.ts b/tests/unit/stubs/ScriptCompilerStub.ts
new file mode 100644
index 00000000..366551b5
--- /dev/null
+++ b/tests/unit/stubs/ScriptCompilerStub.ts
@@ -0,0 +1,18 @@
+import { IScriptCompiler } from '@/application/Parser/Compiler/IScriptCompiler';
+import { IScriptCode } from '@/domain/IScriptCode';
+import { YamlScript } from 'js-yaml-loader!./application.yaml';
+
+export class ScriptCompilerStub implements IScriptCompiler {
+    public compilables = new Map<YamlScript, IScriptCode>();
+    public canCompile(script: YamlScript): boolean {
+        return this.compilables.has(script);
+    }
+    public compile(script: YamlScript): IScriptCode {
+        return this.compilables.get(script);
+    }
+    public withCompileAbility(script: YamlScript, result?: IScriptCode): ScriptCompilerStub {
+        this.compilables.set(script, result ||
+            { execute: `compiled code of ${script.name}`, revert: `compiled revert code of ${script.name}` });
+        return this;
+    }
+}
diff --git a/tests/unit/stubs/ScriptStub.ts b/tests/unit/stubs/ScriptStub.ts
index c64c572e..1912e25d 100644
--- a/tests/unit/stubs/ScriptStub.ts
+++ b/tests/unit/stubs/ScriptStub.ts
@@ -4,8 +4,10 @@ import { RecommendationLevel } from '@/domain/RecommendationLevel';
 
 export class ScriptStub extends BaseEntity<string> implements IScript {
     public name = `name${this.id}`;
-    public code = `REM code${this.id}`;
-    public revertCode = `REM revertCode${this.id}`;
+    public code = {
+        execute: `REM execute-code (${this.id})`,
+        revert: `REM revert-code (${this.id})`,
+    };
     public readonly documentationUrls = new Array<string>();
     public level = RecommendationLevel.Standard;
 
@@ -14,7 +16,7 @@ export class ScriptStub extends BaseEntity<string> implements IScript {
     }
 
     public canRevert(): boolean {
-        return Boolean(this.revertCode);
+        return Boolean(this.code.revert);
     }
 
     public withLevel(value: RecommendationLevel): ScriptStub {
@@ -23,7 +25,7 @@ export class ScriptStub extends BaseEntity<string> implements IScript {
     }
 
     public withCode(value: string): ScriptStub {
-        this.code = value;
+        this.code.execute = value;
         return this;
     }
 
@@ -33,7 +35,7 @@ export class ScriptStub extends BaseEntity<string> implements IScript {
     }
 
     public withRevertCode(revertCode: string): ScriptStub {
-        this.revertCode = revertCode;
+        this.code.revert = revertCode;
         return this;
     }
 }
diff --git a/tests/unit/stubs/YamlScriptStub.ts b/tests/unit/stubs/YamlScriptStub.ts
new file mode 100644
index 00000000..eb35844b
--- /dev/null
+++ b/tests/unit/stubs/YamlScriptStub.ts
@@ -0,0 +1,61 @@
+import { RecommendationLevel } from '@/domain/RecommendationLevel';
+import { ScriptFunctionCall, YamlScript } from 'js-yaml-loader!./application.yaml';
+
+export class YamlScriptStub implements YamlScript {
+    public static createWithCode(): YamlScriptStub {
+        return new YamlScriptStub()
+            .withCode('stub-code')
+            .withRevertCode('stub-revert-code');
+    }
+    public static createWithCall(call?: ScriptFunctionCall): YamlScriptStub {
+        let instance = new YamlScriptStub();
+        if (call) {
+            instance = instance.withCall(call);
+        } else {
+            instance = instance.withMockCall();
+        }
+        return instance;
+    }
+    public static createWithoutCallOrCodes(): YamlScriptStub {
+        return new YamlScriptStub();
+    }
+
+    public name = 'valid-name';
+    public code = undefined;
+    public revertCode = undefined;
+    public call = undefined;
+    public recommend = RecommendationLevel[RecommendationLevel.Standard].toLowerCase();
+    public docs = ['hello.com'];
+
+    private constructor() { }
+
+    public withName(name: string): YamlScriptStub {
+        this.name = name;
+        return this;
+    }
+
+    public withDocs(docs: string[]): YamlScriptStub {
+        this.docs = docs;
+        return this;
+    }
+
+    public withCode(code: string): YamlScriptStub {
+        this.code = code;
+        return this;
+    }
+
+    public withRevertCode(revertCode: string): YamlScriptStub {
+        this.revertCode = revertCode;
+        return this;
+    }
+
+    public withMockCall(): YamlScriptStub {
+        this.call = { function: 'func', parameters: [] };
+        return this;
+    }
+
+    public withCall(call: ScriptFunctionCall): YamlScriptStub {
+        this.call = call;
+        return this;
+    }
+}