Type Alias Asyncify<Fn>

Asyncify: SetReturnType<Fn, Promise<Awaited<ReturnType<Fn>>>>

Create an async version of the given function type, by boxing the return type in Promise while keeping the same parameter types.

Use-case: You have two functions, one synchronous and one asynchronous that do the same thing. Instead of having to duplicate the type definition, you can use Asyncify to reuse the synchronous type.

Type Parameters

  • Fn extends (...arguments_: any[]) => any

Example

import type {Asyncify} from 'type-fest';

// Synchronous function.
function getFooSync(someArg: SomeType): Foo {
	// …
}

type AsyncifiedFooGetter = Asyncify<typeof getFooSync>;
//=> type AsyncifiedFooGetter = (someArg: SomeType) => Promise<Foo>;

// Same as `getFooSync` but asynchronous.
const getFooAsync: AsyncifiedFooGetter = (someArg) => {
	// TypeScript now knows that `someArg` is `SomeType` automatically.
	// It also knows that this function must return `Promise<Foo>`.
	// If you have `@typescript-eslint/promise-function-async` linter rule enabled, it will even report that "Functions that return promises must be async.".

	// …
}

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