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*/

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.'