Preserve ordered yield types when no logic is involved

Suggestion

🔍 Search Terms

yield ordered return types

✅ 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

Make types more definite for each yielded return value in a generator function

See also https://stackoverflow.com/questions/66538431/function-chain-factory-in-typescript-preserving-input-output-types

📃 Motivating Example

Right now, if I do this:

function* work() {
  yield 1;
  yield 'test';
  return [123];
}

Right now this gives:

const g = work();
g.next().value  // string | number | number[]
g.next().value  // string | number | number[]
g.next().value  // string | number | number[]

But since there's no logic/branching/etc, it would be more helpful to me to have:

const g = work();
g.next().value  // number
g.next().value  // string
g.next().value  // number[]

💻 Use Cases

Sometimes I want to use a function generator to represent a series of steps that I want to examine (whether at runtime or during testing, etc) before deciding whether to continue. In other words, I want to break up a function into a series of "pauseable" steps.

L’issue ne nomme aucun fichier, test ni point d’entrée. Commencez par valider les exemples de générateurs et examiner le cas d’utilisation Stack Overflow associé. C’est terminé lorsque les résultats successifs du générateur exposent les types yield et return ordonnés sans modifier le comportement du runtime JavaScript.