CODEWITHSUNDEEP
javascriptasynchronouspromiseasync-awaittypescript2.0
heathkit
heathkit

Reputation: 573

Method chaining with async/await in TypeScript

I have a situation where I need to call an async method on the result of an async method. 

class Parent {
  constructor(private child: Child) { }

  private getChild(): Promise<Child> {
    return Promise.resolve(this.child);
  }

  async getResult(): Promise<Child> {
     return await this.getChild()
  }
}

class Child {
  getText(): Promise<string> {
    return Promise.resolve('text');
  }
}

let child = new Child();
let container = new Parent(child);

let printText = async () => {
  await (await container.getResult()).getText();
}

printText();

Is there a good way to avoid the need to double await? I think I'd like to just do await container.getChild().getText();. What's the right way to make an API in TypeScript that will allow me to chain methods that return promises and then wait for the result with a single await?

EDIT: To clarify, this is more of an API design question. Is there a better pattern for doing what I'm trying to do (call an async method on an object returned by an async method)? Even if it means doing something totally different?

Upvotes: 8

Views: 7863

Answers (1)

Bergi
Bergi

Reputation: 665555

Is there a way to design my API better so that users won't need the double await? Even if it means changing the structure of my example drastically.

You must not make getResult asynchronous, it needs to return a Result instance right away so that further methods can be called on it. Your example is a bit weird since getChild doesn't really need to be asynchronous at all. But lets assume it is, and does some important.

You then could write

class Parent {
  private async getChild(): Promise<Child> {
    … // whatever
  }

  getResult(): Result {
     return new Result(this.getChild())
  }
}
class Result {
  constructor(p: Child) {
    this.promise = p;
  }
  async getText(): Promise<string> {
    return (await this.promise).getText();
  }
}

Now you can call parent.getResult().getText() directly. Basically, Result acts as a proxy wrapper around the Child class that does both the awaits. Maybe in your actual architecture you can even avoid the this.promise and do the child and text accesses in one step.

However, usually this is not worth it. Just let your caller await each step.

Upvotes: 8

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