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.

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.

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:

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:

async function createUser(user: User): Promise<void> {
    await validateUser(user);
    await saveValidUserToDatabase(user as Valid<User>);
}

async function validateUser(user: User): Promise<void> {
    // ...
}

But if TypeScript supported asynchronous assertions, you could skip as Valid<User> type assertion and let the TS do the job:

async function createUser(user: User): Promise<void> {
    await validateUser(user);
    await saveValidUserToDatabase(user);
}

async function validateUser(user: User): Promise<asserts user is Valid<User>> {
    // ...
}

Exactly the same issue could be presented for user-defined type guards.

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.

Beginne damit, TypeScripts bestehende Behandlung von benutzerdefinierten Typwächtern und Assertion-Signaturen im Compiler und Type Checker zu überprüfen. Definiere das Verhalten asynchroner Wächter und Assertionen, einschließlich der Auswirkungen von await auf die Eingrenzung, und füge anschließend Compiler-Tests hinzu, die die vorgeschlagenen Promise-basierten Signaturen und die daraus resultierenden Control-Flow-Typen abdecken.