Javascript infamous Loop issue?

We have 5 divs on the page, each with an ID ... div1, div2, div3, div4, div5

jQuery can do this ...

for (var i=1; i<=5; i++) {
    $("#div" + i).click ( function() { alert ($(this).index()) } )
}

But really addressing the problem (and building this up slowly) ...

STEP 1

for (var i=1; i<=5; i++) {
    $("#div" + i).click (
        // TODO: Write function to handle click event
    )
}

STEP 2

for (var i=1; i<=5; i++) {
    $("#div" + i).click (
        function(num) {
            // A functions variable values are set WHEN THE FUNCTION IS CALLED!
            // PLEASE UNDERSTAND THIS AND YOU ARE HOME AND DRY (took me 2 years)!
            // Now the click event is expecting a function as a handler so return it
            return function() { alert (num) }
        }(i) // We call the function here, passing in i
    )
}

SIMPLE TO UNDERSTAND ALTERNATIVE

If you can't get your head around that then this should be easier to understand and has the same effect ...

for (var i=1; i<=5; i++) {

    function clickHandler(num) {    
        $("#div" + i).click (
            function() { alert (num) }
        )
    }
    clickHandler(i);
    
}

This should be simple to understand if you remember that a functions variable values are set when the function is called (but this uses the exact same thought process as before)

Basically, in the first example you're binding the i inside the onclick handler directly to the i outside the onclick handler. So when the i outside the onclick handler changes, the i inside the onclick handler changes too.

In the second example, instead of binding it to the num in the onclick handler, you're passing it into a function, which then binds it to the num in the onclick handler. When you pass it into the function, the value of i is copied, not bound to num. So when i changes, num stays the same. The copy occurs because functions in JavaScript are "closures", meaning that once something is passed into the function, it's "closed" for outside modification.

Quoting myself for an explanation of the first example:

JavaScript's scopes are function-level, not block-level, and creating a closure just means that the enclosing scope gets added to the lexical environment of the enclosed function.

After the loop terminates, the function-level variable i has the value 5, and that's what the inner function 'sees'.

In the second example, for each iteration step the outer function literal will evaluate to a new function object with its own scope and local variable num, whose value is set to the current value of i. As num is never modified, it will stay constant over the lifetime of the closure: The next iteration step doesn't overwrite the old value as the function objects are independant.

Keep in mind that this approach is rather inefficient as two new function objects have to be created for each link. This is unnecessary, as they can easily be shared if you use the DOM node for information storage:

function linkListener() {
    alert(this.i);
}

function addLinks () {
    for(var i = 0; i < 5; ++i) {
        var link = document.createElement('a');
        link.appendChild(document.createTextNode('Link ' + i));
        link.i = i;
        link.onclick = linkListener;
        document.body.appendChild(link);
    }
}