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.

El issue no nombra archivos, pruebas ni puntos de entrada. Empieza validando los ejemplos de generadores y revisando el caso de uso enlazado de Stack Overflow. Se considera terminado cuando los resultados sucesivos del generador exponen los tipos ordenados de yield y return sin cambiar el comportamiento del runtime de JavaScript.