How to Promise in loop?

Question

    var arr = ["node.js", "Java", "C#"];
var newArr = [];

let promise = new Promise((resolve, reject) => {

        for(let i = 0; i < arr.length; i++) {
            setTimeout(() => {
                newArr.push(arr[i])
                resolve(newArr);
            }, 1000);
        }

});

promise
        .then(result => {
             alert(result);
         }
);

Now it alerts data after first loop. How to display data from async operation with promises, after it passes whole loop?

Desired behavior: Add data to newArr in async mode, and display it after loop ends.

Well you can only resolve it once..... So you would have to wait until all the timeouts are done.... — epascarello, Sep 19 '17 at 01:28
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all — epascarello, Sep 19 '17 at 01:29
Possible duplicate of [How to return many Promises in a loop and wait for them all to do other stuff](https://stackoverflow.com/questions/31426740/how-to-return-many-promises-in-a-loop-and-wait-for-them-all-to-do-other-stuff) — alexanderbird, Sep 19 '17 at 01:40

score 2 · Accepted Answer · answered Sep 19 '17 at 01:37

You need to have multiple promises with promise all

const myPromises = [];
for (let i=0; i<5; i++) {
  myPromises.push(new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("done", i);
        resolve(i)
      } , i*500);
  })); 
}

Promise.all(myPromises).then(values => { 
  console.log("All: ", values)
});

How to Promise in loop?

1 Answers1