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 se nombran archivos de implementación ni tests. Empieza reproduciendo el ejemplo de TypeScript con una unión discriminada y una comprobación de propiedad con await, y luego sigue cómo el estrechamiento de tipos del flujo de control gestiona la expresión await. Se considera terminado cuando se acepta el acceso posterior a data.a cuando el kind esperado es 'a', sin cambiar el JavaScript emitido.