Add support for asyncronous assertion functions and methods

[Nokel81]

Suggestion

🔍 Search Terms

• assertion
• method
• function
• async
• promise

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Allow functions and methods of the signature (...args: any[]) => Promise<asserts condition> which mean that condition is true if the promise resolves.

📃 Motivating Example

Sometimes it is necessary to call asynchronous functions to make sure that some field is assigned. For instance, a somewhat common pattern is to have an async ensureFieldX(): Promise<void> method and it would be nice if typescript knew that after that method has resolved (and not rejected).

class Foo {
  private bar: string | undefined;

  async getBar(): Promise<string> {
    await this.ensureBar();

    return this.bar;
  }

  private async ensureBar(): Promise<asserts this is (Foo & { bar: string })> {
     ...
  }
}

💻 Use Cases

See motivating example...

Could also be extended to asynchronous user defined type guards but I don't think that is as important.

[whzx5byb]

In this case you could use logical assignment as workaround:

class Foo {
  private bar: string | undefined;

  async getBar(): Promise<string> {
    return this.bar ||= await this.createBar();
  }

  private async createBar(): Promise<string> {
     //...
  }
}

[Nokel81]

You could indeed but that is a bit cumbersome.

[MartinJohns]

Duplicate of #37681.

[Nokel81]

Sorry for the noise then I tried looking but didn't find that one.