javascript callback, how to include additional parameter

Question

I have a situation where I'm using a package/plugin that has a bind function.

I call the bind with a code which is the filter for when myevents fires callbacks. In this senario, three calls will be made to bind, one with parameter 't1', the second for 't2' and the third for 't3'.

When an event is fired from the myevents object, the callback is called (depending on the code) with the data parameter.

My issue is when the callback function is called, I need to know the index number of the loop when it was originally bound. For example, I have the i variable in the alert statement but I dont' know how to get the value there to save for when the callback is eventually fired.

Seems like there would be some combination of "function(data,i)" to accomplish this.

bindList = array('t1','t2','t3');

for (var i = 0; i < bindList.length; i++) {
        myevents.bind(bindList[i], function(data) {
            alert('data arrive on item ' + i + ': ' + data);
        });
    }

I would appreciate any help. Thanks.

Joseph · Accepted Answer · 2013-04-24 20:42:44Z

5

Use a closure. Since JS variables are "function scoped", the i at 0 refers to the same i at any point in the iteration. We need to create a scope for each iteration so that the binding code refers to the closure's i rather than the loop's i.

for (var i = 0; i < bindList.length; i++) {
  (function(i){
    myevents.bind(bindList[i], function(data) {
      alert('data arrive on item ' + i + ': ' + data);
    });
  }(i));
}

answered Apr 24, 2013 at 20:42

Joseph

120k31 gold badges186 silver badges238 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Karoly Horvath Over a year ago

Just a small explanation: in the original code i refers to the outside variable, so by the time the callback gets executed the loop finished and all callbacks will use the same value bindList.length.

Scott Over a year ago

Thank you! Joseph's and Wahl's were great answers. Tried this one and it works. So easy... so slick! THanks!

Collectives™ on Stack Overflow

javascript callback, how to include additional parameter

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related