There’s a nice clean way to to this with [].reduce. var chain = itemsToProcess.reduce(function (previous, item) { return previous.then(function (previousValue) { // do what you want with previous value // return your async operation return Q.delay(100); }) }, Q.resolve(/* set the first “previousValue” here */)); chain.then(function (lastResult) { // … }); reduce iterates through the … Read more
You use reduce over the array to chain the promises together. There is no need make this recursive. // for angularjs: var Q = $q.when; var p = a.reduce(function(prev, el) { return prev.then(function(arr) { return makeRequest(el).then(function(res) { return arr.concat([res]); }); }); }, Q([]));
All the answers here are really complicated in my opinion. Kos has the right idea but you can shorten the code by writing more idiomatic promise code: function retry(operation, delay) { return operation().catch(function(reason) { return Q.delay(delay).then(retry.bind(null, operation, delay * 2)); }); } And with comments: function retry(operation, delay) { return operation(). // run the operation … Read more
Q promises can be resolved with only one argument – a promise stands for one single value, not for a collection of them. Put them in an array explicitly if you need multiple values. For the multiple-parameter-callbacks, you can use .spread().
Since promises can only resolve once (to either fulfilled or rejected), the first resolution wins and any further calls will be ignored. From the docs: In all cases where a promise is resolved (i.e. either fulfilled or rejected), the resolution is permanent and cannot be reset. Attempting to call resolve, reject, or notify if promise … Read more
The old adage goes that you should pick the right tool for the job. ES6 promises provide the basics. If all you ever want or need is the basics, then that should/could work just fine for you. But, there are more tools in the tool bin than just the basics and there are situations where … Read more
render() method should render UI from this.props and this.state, so to asynchronously load data, you can use this.state to store imageId: imageUrl mapping. Then in your componentDidMount() method, you can populate imageUrl from imageId. Then the render() method should be pure and simple by rendering the this.state object Note that the this.state.imageUrls is populated asynchronously, … Read more
In simple terms, inside a then handler function: A) When x is a value (number, string, etc): return x is equivalent to return Promise.resolve(x) throw x is equivalent to return Promise.reject(x) B) When x is a Promise that is already settled (not pending anymore): return x is equivalent to return Promise.resolve(x), if the Promise was … Read more
How a promise library decides If it has a .then function – that’s the only standard promise libraries use. The Promises/A+ specification has a notion called thenable which is basically “an object with a then method”. Promises will and should assimilate anything with a then method. All of the promise implementation you’ve mentioned do this. … Read more
this is always the object the method is called on. However, when passing the method to then(), you are not calling it! The method will be stored somewhere and called from there later. If you want to preserve this, you will have to do it like this: .then(() => this.method2()) or if you have to … Read more