Get confused by var scope for JavaScript

Asked Mar 20 '20 at 14:27

Active Mar 20 '20 at 15:30

Viewed 39 times

I am confused by the output of the following code:

for (var i = 0; i < 5; i++) { 

    setTimeout(function() { 
        console.log(i); 
    }, i * 1000 );

}

The output is 5 5 5 5 5, there is a 1-sec delay between each 5.

I know var is function scoped, but the function inside setTimeout is also in the for loop. why the output isn't 0 1 2 3 4, there is a 1-sec delay between 0 and 1, a 2-sec delay between 1 and 2, a 3-sec delay between 2 and 3, a 4-sec delay between 3 and 4?

edited Mar 20 '20 at 15:30

rhys_stubbs

asked Mar 20 '20 at 14:27

Kyle Zhang

1

Because `i` has changed before the time has passed – Quentin Mar 20 '20 at 14:28
`setTimeout` doesn't block, it lets the loop to continue while waiting for launching the delayed function. – Teemu Mar 20 '20 at 14:30
@MisterJojo — No. There's only one process and one event loop. – Quentin Mar 20 '20 at 15:32
because there is 6 differents task running at 6 differents times – Mister Jojo Mar 20 '20 at 16:51
@Quentin what is that process and how the event loop works behind it? – Kyle Zhang Mar 21 '20 at 22:11

Get confused by var scope for JavaScript

0 Answers0