Seth Livingston

How to declare the TypeScript Promise rejection type

Spoiler alert: It isn't possible.

Consider a utility function that returns a rejected Promise.

export function rejectWithNumber(n: number): Promise<number> {
  return Promise.reject(n);
}

This compiles, but the return type declaration isn’t correct.

In Promise<T>, T is the fulfillment type. The rejection type is always any.

So what’s the correct return type declaration for the function?

export function rejectWithNumber(n: number): Promise<any> {
  return Promise.reject(n);                  ^^^^^^^^^^^^
}

This is a little confusing, though.

This function doesn’t return a Promise<any> because the rejection type is always any. It returns a Promise<any> because it doesn’t know what the Promise’s fulfilled type is. Only the calling function knows.

It’s not possible to declare the TypeScript Promise rejection type. It’s always any.

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Seth Livingston