Typescript returning boolean after promise resolved

Question

I'm trying to return a boolean after a promise resolves but typescript gives an error saying

A 'get' accessor must return a value.

my code looks like.

get tokenValid(): boolean {
    // Check if current time is past access token's expiration
    this.storage.get('expires_at').then((expiresAt) => {
      return Date.now() < expiresAt;
    }).catch((err) => { return false });
}

This code is for Ionic 3 Application and the storage is Ionic Storage instance.

Fasces

You can't do this... You can return a Promise<boolean> by just returning this.storage.get... though. – Disquieting

@Fasces I've not usually seen asynchronous code inside of a get, rather the async process sets a property, that the get retrieves... – Ketron

And you probably shouldn't use a getter for a thing that has side effects. – Ceiba

@bergi It does not look to me like the code inside tokenValid is causing side effects. Can you provide more details. – Weakfish

@ShaunLuttin it queries the storage (which takes time) and can change at any time – Ceiba

@Ceiba The vocabulary here is unclear to me. I was under the impression that side effects refer to causing mutations outside the function's body. What you're referring to seems more like a violation of referential transparency, unless a query to the storage is causing the storage to change. – Weakfish

@ShaunLuttin Yes, that's a more accurate description. I used "side effect" because a) time is often considered as one b) we indeed don't know what a query does (and "sending a request" could be an action that mutates all kinds of stuff, at least on the lower levels). – Ceiba

@Ceiba Thank you for the additional details. I appreciate it. – Weakfish

@Ceiba Is there way to use this code, not as a getter and making it function with this async behavior and return boolean? – Fasces

@Fasces No, when it's asynchronous it has to return a promise for the boolean. To make it a method, just omit the get. – Ceiba

@Ceiba Can I use async/await and make this call synchronous? – Fasces

@Fasces No, it's impossible to make it synchronous. Using async/await syntax only is sugar for then calls - it's still asynchronous and returns a promise. – Ceiba

Answer 1

You can return a Promise that resolves to a boolean like this:

get tokenValid(): Promise<boolean> {
  // |
  // |----- Note this additional return statement. 
  // v
  return this.storage.get('expires_at')
    .then((expiresAt) => {
      return Date.now() < expiresAt;
    })
    .catch((err) => {
      return false;
    });
}

The code in your question only has two return statements: one inside the Promise's then handler and one inside its catch handler. We added a third return statement inside the tokenValid() accessor, because the accessor needs to return something too.

Here is a working example in the TypeScript playground:

class StorageManager { 

  // stub out storage for the demo
  private storage = {
    get: (prop: string): Promise<any> => { 
      return Promise.resolve(Date.now() + 86400000);
    }
  };

  get tokenValid(): Promise<boolean> {
    return this.storage.get('expires_at')
      .then((expiresAt) => {
        return Date.now() < expiresAt;
      })
      .catch((err) => {
        return false;
      });
  }
}

const manager = new StorageManager();
manager.tokenValid.then((result) => { 
  window.alert(result); // true
});

Weakfish

@Fasces I'm afraid I don't know what additional information to provide in this answer. – Weakfish

This won't pass a tslint test considering Promise<boolean> is not the only possible return type. Better to use Promise<any> – Spatiotemporal

Answer 2

Your function should be:

get tokenValid(): Promise<Boolean> {
    return new Promise((resolve, reject) => {
      this.storage.get('expires_at')
        .then((expiresAt) => {
          resolve(Date.now() < expiresAt);
        })
        .catch((err) => {
          reject(false);
      });
 });
}

Scandic

Hello, if I change the code reject(false); to reject(err), can I get the false value? – Bertrando

No, the parameter of reject function must be the same as the return type of your function. – Scandic

Okay, if we don't use resolve or reject, is that okay? I mean we just return a bool value and not pass resolve or reject. If so, what is the propose of resolve? – Bertrando

As you can see i return new Promise class, so i need to call resolve or reject. If you want to return only value true or false, you just write: this.storage.get('expires_at') .then((expiresAt) => { return Date.now() < expiresAt; }) .catch((err) => { return false; }); – Scandic