typeof return

Suggestion

When return type of a function is known, introduce a special keyword which gives access to that return type.

✅ Viability Checklist

My suggestion meets these guidelines:

• 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

Introduce a mechanism to use the return type of the current function, when it is known in advance.

📃 Motivating Example

Consider the example below where return type string of function type F is known in advance, but the type F might be unavailable, at least easily. In that case, the return type of the F is known, but there is no alias to make use of it.

There could be a magic alias, say typeof return, which holds the return type of the current function, if that type is defined in advance.

type F = () => string;

const implementF = (implementation: F) => {};

implementF(() => {
  const result: typeof return = 'abc'; // string
  return result;
});

Here return is a reserved keyword in JavaScript, so there should be no identifier named "return".

Basically typeof return would return the type (if known), that would be expected, if there was a ReturnStatement at the place where typeof return was written.

Beginne mit der Prüfung der im Issue verlinkten TypeScript Design Goals und analysiere anschließend, wie sich der vorgeschlagene typeof-Rückgabewert für kontextuell typisierte Funktionsausdrücke verhalten sollte. Als abgeschlossen gilt die Aufgabe, wenn ein geklärtes Design für Syntax und Semantik vorliegt, einschließlich der Fälle, in denen der Rückgabetyp bekannt ist; es werden keine Implementierungsdateien oder Tests genannt.