How to call a function after completion of another function using callback?

Question

function one(){
    console.log(1);
}
function two(callback){
    setTimeout(()=>{
        console.log(2);
    },2000);
    callback();
}
two(one);

While I am running this code, 1 is displaying first and then 2 because 2 is taking 2 second time to display. Suppose if there is an api instead of console.log(2) in function two which is taking 2 second to respond, so How I can call function one after completion of function two using callback(); which I can do in this case if I use callback() inside setTimeout function but what if there is an api where I am not using setTimeout and it's taking 2-3 second?

You can call api with async - await, so that 2nd function will be called after api call is completed :) — Akhil Aravind
– Akhil Aravind, Commented Jan 31, 2020 at 10:57
I want to call 2nd function first which is taking longer time then function 1 using callback. Is that possible? — Ayaz
– Ayaz, Commented Jan 31, 2020 at 11:02
call your second function first with async and then call the first function on completing second. Simple. Bro think simple it will resolve all questions — Akhil Aravind
– Akhil Aravind, Commented Jan 31, 2020 at 11:05

Liad Yogev · Accepted Answer · 2020-01-31 11:05:48Z

4

Even if there is an api call which takes time to resolve, you can still use a callback at the same way you just did with the setTimeout example.

Alternatively, use a Promise:

function two(callback) {
    new Promise(res => {
        setTimeout(() => {
            res();
        }, 2000);
    }).then(callback)
}

or using async/await syntax:

async function two(callback) {
    await new Promise(res => {
        setTimeout(() => {
            res();
        }, 2000);
    })
    callback();
}

edited Jan 31, 2020 at 11:05

answered Jan 31, 2020 at 10:59

Liad Yogev

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2 Comments

Ayaz Over a year ago

Earlier I was thinking we can do it without using promise or async but we have to use it. This is helpful though. Thank you.

Liad Yogev Over a year ago

@MohdAyazurRahmanShaikh You can simply use a callback like in your example if you want to avoid using Promises.

Zulqarnain Jalil · Accepted Answer · 2020-01-31 11:02:37Z

1

you can use sync function like this

function two() {//this function takes 2 seconds
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('resolved');
    }, 2000);
  });
}
function one(){
    console.log("one");
}
async function asyncCall() {
  console.log('calling');
  const result = await two();
 one()
  // expected output: 'resolved'
}

asyncCall();

here is reference link

answered Jan 31, 2020 at 11:02

Zulqarnain Jalil

1,69117 silver badges27 bronze badges

Comments

KingGary · Accepted Answer · 2020-02-03 07:42:22Z

1

You can use Promises:

function one(){
    console.log(1);
}

function two(){
    return new Promise((resolve, reject) => {
       setTimeout(()=>{
         console.log(2);
         resolve()
      },2000);
    })
}

two().then(() => one())

edited Feb 3, 2020 at 7:42

answered Jan 31, 2020 at 10:59

KingGary

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Comments

GrafiCode · Accepted Answer · 2020-01-31 10:59:28Z

0

Put callback(); inside setTimeout:

function one(){
    console.log(1);
}
function two(callback){
    setTimeout(()=>{
        console.log(2);
        callback();
    },2000);
    
}
two(one);

answered Jan 31, 2020 at 10:59

GrafiCode

3,3653 gold badges31 silver badges33 bronze badges

2 Comments

Ayaz Over a year ago

I have mentioned it in question that I can do this if I put callback() inside setTimeout but what if I don't use setTimeout and still that particular code is taking 2-3 second like while we call api, it might take longer time.

GrafiCode Over a year ago

@MohdAyazurRahmanShaikh let's say you will end up using request, you will have a success callback wherein you can perform any action AFTER the execution

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