this | NN 2 IE 3 ECMA 1
Refers to the current object. For example, in a form control object event handler, you can pass the object as a parameter to the function:
    <input type="text" name="ZIP" onchange="validate(this);>"
Inside a custom object constructor, the keyword refers to the object itself, allowing you to assign values to its properties (even creating the properties at the same time):
    function CD(label, num, artist) {
        this.label = label;
        this.num = num;
        this.artist = artist;
    }
Inside a function, the this keyword refers to the function object. However, if the function is assigned as a method of a custom object constructor, this refers to the instance of the object in whose context the function executes.
Example
    <input type="text" name="phone" onchange="validate(this.value);">