addEventListener, for(), index. how to use closure

Question

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JavaScript closure inside loops – simple practical example (44 answers)

Javascript infamous Loop issue? [duplicate] (5 answers)

Closed 6 years ago.

I have this code:

var items = this.llistat.getElementsByTagName('a');

for( var i = 0; i < items.length; i++ ){    
  items[i].addEventListener('click', function(event) {
    alert( i );
  }, items[i]);
}

where the event is listened, but there are 3 items and the alert allways print 3 on any of the elements (it doesn't respect the index),

Dosen't items[i] shouldn't do the job as closure?

thanks!

Answer 1

That's a classical closure issue : you must create a new function bound, not to the 'i' variable, but to its value at the time of binding :

var items = this.llistat.getElementsByTagName('a');

for( var i = 0; i < items.length; i++ ) {
        items[i].addEventListener('click', listener.bind( null, i) );
}

function listener(index) {
         alert(index);
}

Answer 2

No, the third argument of addEventListener is the useCapture one. See MDN for more information.

But you can use:

for( var i = 0; i < items.length; i++ ){
    (function(i){
        items[i].addEventListener('click', function(event) {
            alert( i );
        }, false);
    })(i);
}

or

var handler = function(event) {
    var i = items.indexOf(this);
    alert( i );
};
for( var i = 0; i < items.length; i++ ){
    items[i].addEventListener('click', handler, false);
}

The first one creates a new event handler for each element, so it needs more memory. The second one reuses the same event listener, but uses indexOf, so it's more slow.