Second Alert Box "Undefined"

Question

Possible Duplicate:
Referencing “this” inside setInterval/setTimeout within object prototype methods

The second alert box is saying "undefined"? Why is this?

<a id = "clickme">Click Me!</a>
<script>
var a = document.getElementById("clickme");
a.onclick = function(); {
    alert(this.innerHTML);
    setTimeout( function() {
        alert( this.innerHTML );
    }, 1000);
};
</script>

Answer 1

Because inside the function you're passing to setTimeout, this is not the a element anymore. It will either be the global object (window in browser-land) or undefined in strict mode.

Instead, store a reference to this;

var a = document.getElementById("clickme");
a.onclick = function(); {
    var that = this;

    alert(this.innerHTML);
    setTimeout( function() {
        alert( that.innerHTML );
    }, 1000);
};

Answer 2

https://developer.mozilla.org/en-US/docs/DOM/window.setTimeout#The_.22this.22_problem

this has a different value inside the setTimeout handler.