Asynchronous type guards and assertion signatures

## Search Terms async, asynchronous, promise, type guards, assertion signatures, asserts ## Suggestion TypeScript currently supports user-defined type guards and assertion signatures. ```typescript function isCustomType(value: unknown): value is CustomType { // ... } function assertIsCustomType(value: unknown): asserts value is CustomType { // ... } ``` I think it would be great if we could also define custom **asynchronous** type guards and assertions. ```typescript async function isCustomType(value: unknown): Promise<value is CustomType> { // ... } async function assertIsCustomType(value: unknown): Promise<asserts value is CustomType> { // ... } ``` ## Use Cases This feature would allow to check types and validate data asnychonously. Please look at the examples below. ## Examples Imagine the code like this: ```typescript interface User { name: string; } type Valid<T> = T & { readonly $valid: unique symbol; } async function createUser(user: User): Promise<void> { validateUser(user); await saveValidUserToDatabase(user); } function validateUser(user: User): asserts user is Valid<User> { if (user.name.length < 5) { throw new Error('User name must be at least 5 characters long'); } } async function saveValidUserToDatabase(user: Valid<User>): Promise<void> { // ... } ``` But sometimes, validation is done asynchronously, e.g. on server-side. Currently, you can achieve it this way: ```typescript async function createU

Posted by niko278 on Mar 30, 2020, 1:25 PM

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