(TS) Removing Promise from a Type

Handling Promise types in TypeScript can be cumbersome when dealing with asynchronous code. In such cases, using the Awaited type can be quite useful.

Awaited

Awaited is one of the new utility types introduced in TypeScript version 4.5.

// A = string
type A = Awaited<Promise<string>>;

This allows you to easily extract the value returned by a Promise.

Handling Nested Promises

The Awaited type is also useful for handling multiple nested Promises.

// B = number
type B = Awaited<Promise<Promise<number>>>;

It helps to prevent unnecessary nesting when dealing with asynchronous code. Here’s a practical example:

async function fetchData(): Promise<Data> {
  // Fetch data asynchronously
}
 
async function processData(): Promise<Result> {
  const data = await fetchData();
  // Process data and return the result
}
 
type Result = Awaited<ReturnType<typeof processData>>;

Dealing with Ambiguous Promise Types

Awaited is useful even in situations where different types are mixed.

// C = boolean | number
type C = Awaited<boolean | Promise<number>>;

This allows for detailed automatic type inference related to Promise.all.

declare function MaybePromise<T>(value: T): T | Promise<T>;
 
async function doSomething(): Promise<[number, number]> {
  const result = await Promise.all([MaybePromise(100), MaybePromise(200)]);
 
  // Below error occurs in versions below TypeScript 4.5.
  // Error!
  //
  // [number | Promise<100>, number | Promise<200>]
  //
  // is not assignable to type
  //
  // [number, number]
  return result;
}

For more details, refer to this GitHub issue.

References

https://stackoverflow.com/questions/48944552/typescript-how-to-unwrap-remove-promise-from-a-type