Upgrading TypeScript to v4

I upgraded our project to TypeScript 4.0 to use newer TS/JS features like logical assignment operators. Here are the details.

Remember: TypeScript is about types—upgrading shouldn’t change runtime behavior.

Package Updates

{
  "typescript": "4.0.3",
  "@typescript-eslint/eslint-plugin": "^4.5.0",
  "@typescript-eslint/eslint-plugin-tslint": "^4.5.0",
  "@typescript-eslint/parser": "^4.5.0"
}

• Upgrade related @typescript-eslint packages because ESLint relies on the TS parser.
• Update other vendor types as needed when errors appear.

Issues Encountered

1. Parsing error: Cannot read property 'map' of undefined — fixed by upgrading the @typescript-eslint packages above.

2. Property 'canCostRelief' does not exist on type 'never' — TypeScript narrowed an intersection to never because selected had conflicting types (boolean vs string). Align the types.

3. This expression is not callable — quoteLines was typed as A | B, but the helper expected A. Narrow it (quoteLines as A[]) or adjust the function signature.

Final Thoughts

Version upgrades can be bumpy, but they improve type safety and unlock language features. Treat issues as learning opportunities and don’t postpone upgrades indefinitely.