JavaScript PromiseJavaScript Promise

JavaScript Promises are used to track the state of an asynchronous task.

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new Promise(/* executor */ function(resolve, reject){...});

Promise states

  • fulfilled - the action relating to the promise successed
  • rejected - the action relating to the promise failed
  • pending - hasn’t fulfilled or rejected yet
  • settled - has fulfilled or rejected

A promise can be resolved to either a promise or thenable, in which case it will store the promise or thenable for later unwrapping.

how to create a promise

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var promise = new Promise(function(resolve, reject){
    //process a task, possibly async, then...
    if(/* task successed */){
        resolve("it worked!");
    }
    else{
        reject(Error("it  failed!"));
    }
})

how to use promise

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promise.then(function(result){
    console.log(result); // "it worked!"
}, function(err){
    console.log(err); //"it failed!"
});

then() takes two arguments, callback for a success case, and another for the failure case. Both are optional, so you are able to add a callback for the success or failure case only.

Resources:

Guide to JavaScript Promise， Promise API Reference

Update (2017/06/11)

The purpose of async/await function is to simplify the behavior of using promises synchronously and to perform some behavior on a group of Promises. Just like Promises are similar to structured callbacks, async/await is similar to combining generators and promises. -MDN

Also read Why Async/Await is better than Promise