for loop with setTimeout prints number to console

Question

I read about closures and find this code.

for(i=0;i<5;i++){
    setTimeout(function(){
        console.log(i);
    },2000)
}

This outputs 5 number 5 times after 2 second. I understand this but before five appears there is number above him. And when I execute again this code it changes it adds 5 to his old value. What is it? Write code on console, see what it outputs then explain me what is it?

A screenshot or at least paste from your console would help to clarify your question a lot. — Bergi, Apr 01 '17 at 16:10
@Bergi - It's a single for loop... just paste it in your own console. — Andrew Hill, Apr 01 '17 at 16:12
@Bergi - That's the very nature of his question. There is, in fact, a random number that appears above the output from the setTimeout. His question is effectively "what is this number?". How is he supposed to be more descriptive if he doesn't know what the number is? — Andrew Hill, Apr 01 '17 at 16:19

Bergi · Answer 1 · 2017-04-01T16:33:06.063

4

It's the "result" value of the evaluated statement, which for a loop is the result value of the last statement in the loop body, which is the return value of the setTimeout call. And that returns a timer id (which allows you to cancel the timeout).

You also can see this behaviour with more simple statements:

> 0;
< 0
> console.log(1);
  1  // the output of the log()
< undefined
> var i=2;
< undefined // a declaration has no result
> var i=3; i;
< 3
> for (;i<3;i++) 4;
< undefined // the body was not evaluated
> for (;i<4;i++) 5;
< 5

Notice how the leading arrow represents input and result output.

edited Apr 01 '17 at 16:33

answered Apr 01 '17 at 16:09

Bergi

630,263
148
957
1,375

I never paid attention to those tiny tiny *>* *<* good explanation :) – exexzian Apr 01 '17 at 16:35
I find it curious that `var a = 3;` outputs `undefined`, `var b = 3; b;` outputs `3`, but `var c = 3; c; var d = 3;` outputs `3` not `undefined`. So the eventual output isn't simply the result of the last statement in a multi-statement line. – Andrew Willems Mar 25 '18 at 00:15

score 1 · Answer 2 · answered Nov 08 '17 at 08:07

That number is the intervalID

Code: when you run the below code in browser debugger

setTimeout(() => console.log('hello'), 2000);

Response would be like

> 1822
> hello

You can use that intervalID to remove clear the timeout

Just to show you the confirmation of that number as intervalID

> var test = setTimeout(() => console.log('hello'), 2000); test;
< 11272
  hello
> console.log(test);
  11272
< undefined

for loop with setTimeout prints number to console

2 Answers2

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