Type narrowing for awaited values

Search Terms

promise, type narrowing, await

Suggestion

The type narrowing that occurs inside if statements, should work with awaited values

Though generally speaking a type is used for values that are already

Examples

type A = {
  kind: 'a'
  a: string
}

type B = {
  kind: 'b'
  b: string
}

let data: A | B

async function valueTest() {
  if ((await data.kind) === 'a') {
    // 'data' hasn't been narrowed to type 'A'
    data.a // Error
  }

  // Works fine without await
  if (data.kind === 'a') {
    data.a // Okay
  }
}

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

No implementation files or tests are named. Start by reproducing the TypeScript example with a discriminated union and an awaited property check, then trace how control-flow type narrowing handles the await expression. Done means the subsequent data.a access is accepted when the awaited kind is 'a', without changing emitted JavaScript.