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.

Start by reviewing the TypeScript Design Goals linked in the issue, then analyze how the proposed typeof return should behave for contextually typed function expressions. Done would require a resolved design for the syntax and semantics, including when the return type is known; no implementation files or tests are named.