Javascript get reference to parent object/class from event handler

You need to bind the function's context; otherwise this will be the global object:

$('element').click($.proxy(this.eventHandler, this));

In a modern browser you can also use Function.prototype.bind:

$('element').click(this.eventHandler.bind(this))

function Foo() {
    var _self = this;
    this.num=0;

    $('element').click(this.eventHandler);// jQuery to attach an onclick event to my element.

    this.eventHandler=function() {
        _self.num++;
    }
}

use a reference _self = this defined in the outer scope