Let’s say you are using some external components in your web page that renders a button as below:

<input type="button" value="Delete" onclick="delete();" />

The above code will call the delete() method when user clicks on the button.

But, what if you want to show a confirmation box before calling the delete()? Since the code is rendered by a external component, most probably you won’t be having control over how its code generated.

So, you have to intercept the onclick event and block it until your method is called and then invoke the original method.

Lets do it..

var btn = $("#btn");
btn.data("funcToCall", btn.attr("onclick"));
$("#btn").removeAttr("onclick");
$("#btn").bind("click", function(e){
    if(confirm("Are you sure?")){
        var func = $(this).data("funcToCall");
        eval(func);
    }
});

As you can see, the onclick function is first stored into that element and then the onclick attribute is removed. Finally when the button in clicked, our custom method is called first and based on the user decision, the delete() method is either called or not called.

From http://veerasundar.com/blog/2011/10/intercepting-onclick-event-in-javascript-using-jquery/