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

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 pletion 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?

• javascript
• callback

Share Improve this question Follow edited Jan 31, 2020 at 13:52 GrafiCode 3,37433 gold badges2929 silver badges3232 bronze badges asked Jan 31, 2020 at 10:55 AyazAyaz 4111 silver badge44 bronze badges 3

• You can call api with async - await, so that 2nd function will be called after api call is pleted :) – 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 Commented Jan 31, 2020 at 11:02
• call your second function first with async and then call the first function on pleting second. Simple. Bro think simple it will resolve all questions – Akhil Aravind Commented Jan 31, 2020 at 11:05

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4 Answers 4

Sorted by: Reset to default 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();
}

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

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())

Put callback(); inside setTimeout:

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