Remove From My Forums

Return a boolean value from a promise

Question

User-1418567014 posted

I want to get a boolean value from a promise.

Method One

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

Method Two

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

One and two are similar. The difference is in method Two I have the return statement either true or false. To get the value, I call the method as

if(this.tokenValid()) {
    console.log('return true');
} {
    console.log('return false');
}

Which method is right?

Friday, October 9, 2020 3:40 PM

Answers

0

Sign in to vote
User-474980206 posted

When your promise code calls resolve, it can pass the result for the promise, or a new promise that when resolved returns the value. So if to resolve, your code needs to maker another async call it can return a new promise.

so in example 1, your resolve returns the value.

in example 2, your resolve, resolves the current promise and returns a new pre-resolved promise. While typically the callback to a then return the value, if it’s a promise it supports returning the new promise. You just added extra fluff.

But as storage.get() returns a promise to begin with all the new promise code is extra fluff. It just needs to be

get tokenValid(): Promise<Boolean> { return this.storage.get('expiresAt') .then((expiresAt) => { return(Date.now() < expiresAt); }; }
- Marked as answer by Anonymous Thursday, October 7, 2021 12:00 AM
Saturday, October 10, 2020 3:59 PM

Answered by:

Return a boolean value from a promise

Question

Answers