Gradually migrating from Javascript to Typescript

1. Add TSC

In tsconfig.json:

{
	"compilerOptions": {
		"allowJs": true
	}
}

Now we can use TSC to compile JavaScript. TypeScript won’t typecheck the existing JavaScript code, but it will transpile it.

2a. Enable typechecking for JavaScript(optional)

Enable this in tsconfig.json:

{
	"compilerOptions": {
		"allowJs": true,
		"checkJS": true
	}
}

Now whenever TS compiles a JS file it’ll do its best to infer types and typecheck as it goes.

enable checkJS may report too many type errors at once, we can:

1. turn checkJS off, and instead enable checking for an individual Javascript file at a time by add the // @ts-check directive (a regular comment at the top of the file).
2. keep checkJS on, add the // @ts-nocheck directive to those files that we don’t want to fix now.

2b. Add JSDoc Annotations (optional)

JSDoc are funny looking comments above some Javascript and Typescript code with @-annotations like @param, @returns, and so do. TS understands JSDoc and uses it as input to its typechecker the same way that it uses explicit type annotations in TS code:

/**
 * @param word {string} An input string to convert
 * @returns {string} The string in PAscalCase
 */
export function toPascalCase(word){
  return ...;
}

3. Rename files to .ts

4. Make it strict

Fix all type-related errors and disable TSC’s JS interoperability flags:

{
	"compilerOptions": {
		"allowJs": false,
		"checkJS": false
	}
}