JavaScript wait for backend response while iterating array

Question

I'm working with javascript and I want that wait for other function response before iterate next item.

Here is the desired behaviour:

This is my code:

let lista = [1,2,3,4]
console.log('Iteracion de la lista')
async function procesarLista(array){
    for(const item of array){
        console.log('-->START indice: ' + item)
        //Simulate delay (for each iteration) of backend response
        setTimeout(function(){
            console.log('....waiting.... for : ' + item );
        }, 2500);
        console.log('-->FINISH indice: ' + item)
    }
    console.log('Done');
 }
 //Execute:
 procesarLista(lista);

This is the WRONG result:

CertainPerformance · Accepted Answer · 2018-05-07 06:47:32Z

1

Try awaiting a Promise on each iteration inside the for loop:

let lista = [1, 2, 3, 4]
console.log('Iteracion de la lista')
async function procesarLista(array) {
  for (const item of array) {
    await new Promise((resolve) => {
      console.log('-->START indice: ' + item)
      //Simulate delay (for each iteration) of backend response
      setTimeout(function() {
        console.log('....waiting.... for : ' + item);
        resolve();
      }, 500);
    });
    console.log('-->FINISH indice: ' + item)
  }
  console.log('Done');
}
//Execute:
procesarLista(lista);

for..of requires regenerator-runtime. If you don't have that, then you can use reduce instead, but you'll have to await the last iteration's resolution inside the loop and await the final promise before logging Done:

let lista = [1, 2, 3, 4]
console.log('Iteracion de la lista')
async function procesarLista(array) {
  await new Promise ((outerResolve) => {
    array.reduce(async (lastPromise, item) => {
      await lastPromise;
      await new Promise((resolve) => {
        console.log('-->START indice: ' + item)
        //Simulate delay (for each iteration) of backend response
        setTimeout(function() {
          console.log('....waiting.... for : ' + item);
          resolve();
        }, 500);
      });
      console.log('-->FINISH indice: ' + item)
    }, Promise.resolve());
  });
  console.log('Done');
}
procesarLista(lista);

edited May 7, 2018 at 6:47

answered May 7, 2018 at 6:11

CertainPerformance

373k55 gold badges354 silver badges359 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Cesar Rodriguez Over a year ago

Thanks, but I have another error message: regeneratorRuntime is not defined

CertainPerformance Over a year ago

See edit - if you can't use for..of, then if you still want to use async/await, then use .reduce.

Santiago Ramirez · Accepted Answer · 2018-05-07 06:35:48Z

0

Maybe this would work for you

lista = [1, 2, 3, 4];

async function procesarItem(array, id) {
  if (id >= array.length) {
    console.log('Done');
    return;
  }
  console.log('-->START indice: ' + id);
  setTimeout(function () {
    console.log('-->FINISH indice: ' + id)
    procesarItem(array, id + 1);
  }, 2500);
  console.log('....waiting.... for : ' + id);
}

//Execute:
procesarItem(lista, 0);

answered May 7, 2018 at 6:35

Santiago Ramirez

5696 silver badges5 bronze badges

Comments

Emad Dehnavi · Accepted Answer · 2018-05-07 06:38:00Z

You can use callback as well :

let lista = [1,2,3,4]
console.log('Iteracion de la lista')
function procesarLista(array , callback){
  let isProcessDone = false;
  for(const item of array){

    //Simulate delay (for each iteration) of backend response
    setTimeout(function(){
            console.log('-->START indice: ' + item)
        console.log('....waiting.... for : ' + item );
        console.log('-->FINISH indice: ' + item)
        callback('Done')
    }, 2500);

  }
}
//Execute:
procesarLista(lista , function(result){
   // Do Something after each job is done
});

this is going to be the output :

Iteracion de la lista
-->START indice: 1
 ....waiting.... for : 1
-->FINISH indice: 1
-->START indice: 2
 ....waiting.... for : 2
-->FINISH indice: 2
-->START indice: 3
 ....waiting.... for : 3
-->FINISH indice: 3
-->START indice: 4
 ....waiting.... for : 4
-->FINISH indice: 4

here is an example :

https://jsfiddle.net/op98bpka/1/

Collectives™ on Stack Overflow

JavaScript wait for backend response while iterating array

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related