Pass a value to callback in nodejs

Question

Possible Duplicate:
Javascript closure inside loops - simple practical example

for (i = 0; i < 100; i++) {
    setTimeout(function() {
        console.log(i);
    }, 500)
}

In the above code, it will print only 100. I know the reason. But how do i send the current i value to the callback in setTimeout?

Guffa · Answer 1 · 2012-09-22T20:12:54.373

2

Use an anonymous function inside the loop to create a closure:

for (i = 0; i < 100; i++) {

  (function(i){

    window.setTimeout(function() {
      console.log(i);
    }, 500)

  })(i);

}

As you are passing the value to the function when calling, the value will be copied and each instance of the function will have its own copy that doesn't change when the loop counter changes.

edited Sep 22 '12 at 20:12

answered Sep 22 '12 at 19:33

Guffa

687,336
108
737
1,005

1

Nit: The original callback the OP uses (`function() { console.log(i); }`) is already a closure. You are basically saying "Use a closure instead of a closure". The important point is that you are executing the function immediately, thus creating a new scope. – Felix Kling Sep 22 '12 at 19:44
1

@FelixKling: That capture captures the loop variable instead of copying it, so you need another closure, not a closure instead of the closure. – Guffa Sep 22 '12 at 19:53
OK, you are not saying this, but to me it sounded like this. But no, technically you don't another closure, you need a new scope (in JS a function invocation) to capture the current value of the variable. As I said, this is all nitpick, but it is not correct to refer to immediately executed functions as "closures" only. That's not the main characteristic of that construct, creating a new scope is. If JavaScript had a different syntax for closures (like PHP, as opposed to "normal" functions), the difference would be more apparent. – Felix Kling Sep 22 '12 at 22:50

Denys Séguret · Answer 2 · 2012-09-22T19:50:03.533

1

setTimeout accepts parameters that are passed to the callback :

for (i=0;i<100;i++){
    setTimeout(function(i){
        console.log(i);
    },500, i)
}

edited Sep 22 '12 at 19:50

answered Sep 22 '12 at 19:32

Denys Séguret

372,613
87
782
758

bt what if its some other callback. (setTimeout was juz an example). Any generic callback which already have some parameters by default. – shahalpk Sep 22 '12 at 19:35
1

If you can't do this, use the old closure solution (see all other answers). – Denys Séguret Sep 22 '12 at 19:35
2

@Guffa This is node.js, not IE – Denys Séguret Sep 22 '12 at 19:36
1

Since `setTimeout` is not part of ES and node.js has nothing to do with Mozilla, better link to the node.js documentation: http://nodejs.org/api/timers.html. – Felix Kling Sep 22 '12 at 19:43
@dystroy: Right. Slipped my mind. – Guffa Sep 22 '12 at 19:49
@FelixKling You're right. I'll edit my answer, that will be cleaner. – Denys Séguret Sep 22 '12 at 19:49

score 0 · Answer 3 · answered Sep 22 '12 at 19:33

0

This works:

for (var i = 0; i < 100; i++) {
    (function(i) {
        setTimeout(function() {
            console.log(i);
        }, 500);
    })(i);
}

answered Sep 22 '12 at 19:33

David G

94,763
41
167
253

score 0 · Answer 4 · answered Sep 22 '12 at 19:33

0

for (i = 0; i < 100; i++) {
    setTimeout((function(i) {
        return function(){
            console.log(i);
        }
    })(i), 500)
}

answered Sep 22 '12 at 19:33

Musa

96,336
17
118
137

Pass a value to callback in nodejs

4 Answers4