what is event loop in javascript

1const first = () => console.log('Hi,i am first');
2const second = () => console.log('Hi,i am second');
3const third = () => console.log('Hi,i am third');
4const fourth = () => {
5    first();
6    setTimeout(second, 4000);
7    //store in queue & it will execute after 4 seconds
8    setTimeout(third, 2000);
9    //store in queue & it will execute after 2 seconds
10    console.log('Hi,i am fourth');
11};
12fourth();
13
14//Expected output:
15/*Hi,i am first
16  Hi,i am fourth
17  Hi,i am third
18  Hi,i am second 
19*/

1The Event Loop has one simple job — to monitor the Call Stack 
2and the Callback Queue. If the Call Stack is empty, it will 
3take the first event from the queue and will push it to the 
4Call Stack, which effectively runs it. Such an iteration is 
5called a tick in the Event Loop. Each event is just a function 
6callback.
7
8#bpn

1
2            
3                
4            
5         console.log('Hi!');
6
7setTimeout(() => {
8    console.log('Execute immediately.');
9}, 0);
10
11console.log('Bye!');
12Code language: JavaScript (javascript)

1The Event Loop has one simple job — to monitor the Call Stack 
2and the Callback Queue

1         function task(message) {
2    // emulate time consuming task
3    let n = 10000000000;
4    while (n > 0){
5        n--;
6    }
7    console.log(message);
8}
9
10console.log('Start script...');
11task('Download a file.');
12console.log('Done!');
13Code language: JavaScript (javascript)

1console.log('Hi!');
2
3setTimeout(() => {
4    console.log('Execute immediately.');
5}, 0);
6
7console.log('Bye!');
8
9// print order
10// 'Hi!'
11// 'Bye!'
12// 'Execute immediately.'