checkJs: x.js should use x.d.ts for type inference

[Conaclos]

Suggestion

🔍 Search Terms

checkjs, declaration files

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

If a declaration file with the same name than a JavaScript file is available (in the same location), then the declaration file should be used to check the js file.

📃 Motivating Example

Assume that "checkJs" and "strict" options are enabled.

Given the following files:

// @filename a.d.ts
export function f(n: number): number;

// @filename a.js
function f(a) {
    // Parameter 'a' implicitly has an 'any' type
    return a + 1
}

the type of f should be inferred and should thus type-check. TSC does not currently use a.d.ts to infer types of a.js.
Note that importing f in another js file, already infer f's type

// @filename b.js
import { f } from "./a.js"
// type of f is inferred