Refactor to improve iterations

- Use function abstractions (such as map, reduce, filter etc.) over
  for-of loops to gain benefits of having less side effects and easier
  readability.
- Enable `downLevelIterations` for writing modern code with lazy evaluation.
- Refactor for of loops to named abstractions to clearly express their
  intentions without needing to analyse the loop itself.
- Add missing cases for changes that had no tests.

tsconfig.json

@@ -10,6 +10,7 @@
     "module": "esnext",
     "jsx": "preserve",
     "importHelpers": true,
+    "downlevelIteration": true,
     "moduleResolution": "node",
     "experimentalDecorators": true,
     "esModuleInterop": true,