Why promise.then() behaves asynchronously?

Question

In the below code why promise.then() behaves asynchronously. In other words , why browser is not waiting for the code written inside promise.then() method to be executed?.What tell the browser engine so that promise.then() makes an asynchronous call?

const money = 500;

let promise = new Promise(function(resolve,reject){
    if(money > 400){
        resolve('You have a car!');
    }else{
        reject('Yo do not have enough money to buy the Car!');
    }
});
console.log('Before');
promise.then(function(data){
    console.log('Success '+data);
});
console.log('After');

The above code prints the output in the below order, which means promise.then() works asynchronously.

Before
After
Success You have a car!

JavaScript is single threaded, and the callback function in `then()` is queued first. — , Sep 27 '18 at 15:17
Promises are supposed to be asynchronous. You can force them to wait by means of the `await` command. — HoldOffHunger, Sep 27 '18 at 15:18
but there is no event loop like how it is being used for setTimeout function. — user3742125, Sep 27 '18 at 15:27

score 2 · Answer 1 · answered Sep 27 '18 at 15:24

2

Promise is asynchronous

You can use await like this to make synchronous

const money = 500;

let promise = new Promise(function(resolve,reject){
    if(money > 400){
        resolve('You have a car!');
    }else{
        reject('Yo do not have enough money to buy the Car!');
    }
});
console.log('Before');

let data = await promise
console.log('Success '+data);
console.log('After');

To get result

Before
Success You have a car!
After

answered Sep 27 '18 at 15:24

Tung Duong

1,156
7
19

but there is no event loop like how it is being used for setTimeout function. If you execute the code here - latentflip.com/loupe/ you could see no event loop is running. I was under the assumption that there is an event loop behind every asynchronous call. – user3742125 Sep 27 '18 at 15:31

Why promise.then() behaves asynchronously?

1 Answers1