Javascript giving parameters to inner function

Question

So my basic setup is this:

for (var i = 0; i < 3; i++) {
    var indices = [-1, -1, -1];
    while (index == -1) {
        // Do Stuff
        index[i] = newIndex;  
    }       
    var press = function() { alert(i); };
    new control({press: press});                
}

Now when I press the each of the new controls instead of getting alert(0), alert(1) and alert(2) I get alert(3), alert(3) and alert(3). I can kind of understand whats going on. Now my question: how can i pass the different indexes to the function as I intended?

Arun P Johny · Accepted Answer · 2013-06-12 09:53:36Z

3

It is because closure variable i, the solution is to create a private closure for each loop.

for (var i = 0; i < 3; i++) {
    var indices = [-1, -1, -1];
    while (index == -1) {
        // Do Stuff
        index[i] = newIndex;  
    }       
    var press = (function(myvar){
        return function() { alert(myvar); };
    })(i);
    new control({press: press});                
}

edited Jun 12, 2013 at 9:53

answered Jun 12, 2013 at 9:34

Arun P Johny

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3 Comments

gdoron Over a year ago

For learning purpose it's better not calling the closure parameter i as it's confusing. IMHO.

eclipse Over a year ago

Thanks it works now :). Now another question what actually happens: (function(a))(b) means that b is passed as parameter .... oh wow now i got it myself: It's an anonymous function which is passed parameter b ?!

Arun P Johny Over a year ago

probably you should read about closures developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Closures and developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… should give you a start

gdoron · Accepted Answer · 2013-06-12 09:35:46Z

1

Use closure:

var press = (function (x) {
    return function () {
        alert(x);
    };
})(i);

This way the current i value is saved in a safe place, a private function.

Note that declaring variables (with var) inside loops are not standard, you should declare the press variable outside the loop.

answered Jun 12, 2013 at 9:35

gdoron

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