Prefix Prefix - 1 month ago 5
Javascript Question

Repeat a Promise until it's not rejected or reach a timeout

I'm still a Promise noob and am trying to figure out how to have my Promise repeat itself.

have an ES6 promise that rejects if some global flag is not set. I need it to retry every 500ms until either:


  • the promise returns a resolve,

  • or a max number of attempts are reached (lets say 10).



Since Promises are async, I don't really want to use a
setInterval()
check, because I don't think that'd work correctly with asynchronous code. I need the checking to terminate as soon as the promise is resolved successfully ( or timeout reached ).

I'm using ES6 + React + ES6 Promises (so no Q or Bluebird-specific answers, please!)

http://jsfiddle.net/2k2kz9r9/8/

// CLASS
class Test extends React.Component {
constructor() {
this.state = {
status: 'setting up..',
}
}
componentDidMount() {
// TODO: how do I get this to loop with a timeout?
this.createSlot()
.then((slot) => {
this.setState({
status: slot
});
})
.catch((e) => {
this.setState({
status: e.message
});
})
}
createSlot() {
return new Promise((resolve, reject) => {
if (!this.checkIsReady()) {
reject(new Error('Global isnt ready yet'));
}
// more stuff here but going to resolve a string for simplicity sake
resolve('successful!');
});
}
checkIsReady() {
return window.globalThing && window.globalThing === true;
}
render() {
return ( <div>{this.state.status}</div> );
}
}





// RENDER OUT
React.render(< Test/> , document.getElementById('container'));


EDIT: function based on current feedback:

createSlot(tries) {
const _this = this;
return new Promise(function cb(resolve, reject) {
console.log(`${tries} remaining`);
if (--tries > 0) {
setTimeout(() => {
cb(resolve, reject);
}, 500);
} else {
const { divId, adUnitPath } = _this;
const { sizes } = _this.props;

// if it's not, reject
if (!_this.isPubadsReady()) {
reject(new Error('pubads not ready'));
}
// if it's there resolve
window.googletag.cmd.push(() => {
const slot = window.googletag
.defineSlot(adUnitPath, sizes, divId)
.addService(window.googletag.pubads());
resolve(slot);
});
}
});
}

Answer

As Mike McCaughan mentioned, you can use setTimeout to create a delay between attempts. Resolve or reject your promise once it succeeds or you run out of attempts.

function createPromise(tries, willFail) {
  return new Promise(function cb(resolve, reject) {
    console.log(tries + ' remaining');
    if (--tries > 0) {
      setTimeout(function() {
        cb(resolve, reject);
      }, 500);
    } else {
      if (willFail) {
        reject('Failure');
      } else {
        resolve('Success');
      }
    }
  });
}

// This one will fail after 3 attempts
createPromise(3, true)
  .then(msg => console.log('should not run'))
  .catch(msg => {
    console.log(msg);
    
    // This one will succeed after 5 attempts
    return createPromise(5, false);
  })
  .then(msg => console.log(msg))
  .catch(msg => console.log('should not run'));

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