Need third async function to execute after the execution of first two in js?

Question

Condition

first two async functions start at the same time
don't know which one finish first

Need to execute a third function on the completion of both of these.

Trying to use promise. Please help achieving it using promise and some other way also if its possible.

something i tried not sure if it's right...

function foop(a){
 return new Promise(function(resolve, reject){
   window.setTimeout(function(){
   console.log("first promise executed");
   }, 2000);  
   window.setTimeout(function(){
   console.log("second promise executed");
   if(a>5) resolve(a+10);

   }, 2000);

  if(a<5) reject("error");
 });
}

foop(12).then(function(val){
  window.setTimeout(function(){
   console.log("first then executed -"+ val);
  }, 1000);
}).then(function(val){
  window.setTimeout(function(){
  console.log("second then executed -"+ val);
  }, 500);
}).catch(function(err){
   console.log("error occured");
});

Expected output order

first promise executed      //or second promise executed
second promise executed     // or first promise executed
first then executed -22         //only executed after first two async completes in any order
second then executed -22        // execute after the third async completes (first then completes)
error occured                // in case of reject

Freyja · Accepted Answer · 2016-08-26 17:05:57Z

Use Promise.all:

console.log('starting...');

// example function: sleep for 'delay' ms then resolve to 'value'
const wait = (delay, value) => new Promise((resolve, reject) => setTimeout(() => { console.log('done:', value); resolve(value); }, delay));

// start two async functions at the same time
// (in this case, they complete in random order)
const firstPromise = wait(900 + 200 * Math.random(), 'first');
const secondPromise = wait(900 + 200 * Math.random(), 'second');

// Promise.all will wait until all promises given are resolved:
Promise.all([ firstPromise, secondPromise ])
  .then((results) => {
    // if all promises are resolved, results is an array of the
    // result values from each, in the order they are given
  
    console.log(results); // = [ 'first', 'second' ]

    // run some other function again...
    return wait(500, 'third');
  })
  .then((result) => {
    // 'result' is the result from the third function
    
    // more functions, etc...
    return wait(800, 'fourth'); 
  })
  .then((result) => {
   
    // you can even use Promise.all again to run two functions simultatenously, inside a .then
    return Promise.all([
       wait(1100, 'fifth'),
       wait(900, 'sixth')
    ]);
  })
  // etc...
  .catch((error) => {
    // if any promise fail, error will be the error from the first to fail
    
    console.error('Error:', error);
  });

first and second will start together as expected and then the third in then BUT then i need to execute the forth after this third function in then.
If you need to execute a fourth function, then you should chain another .then call to Promise.all. As long as each .then-function in the chain returns a promise, they will always be run sequentially, e.g., each one after the previous promise was resolved.
can you please :) add what you said in the comment to the code ? REQUEST
Can you please tell when to use "defer" , as i couldn't find anything over here - developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

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