Overriding a JavaScript function while referencing the original

I have a function, a(), that I want to override, but also have the original a() be performed in an order depending on the context. For example, sometimes when I'm generating a page I'll want to override like this:

function a() {
    new_code();
    original_a();
}

and sometimes like this:

function a() {
    original_a();
    other_new_code();
}

How do I get that original_a() from within the over-riding a()? Is it even possible?

Please don't suggest alternatives to over-riding in this way, I know of many. I'm asking about this way specifically.

Solution 1:

You could do something like this:

var a = (function() {
    var original_a = a;

    if (condition) {
        return function() {
            new_code();
            original_a();
        }
    } else {
        return function() {
            original_a();
            other_new_code();
        }
    }
})();

Declaring original_a inside an anonymous function keeps it from cluttering the global namespace, but it's available in the inner functions.

Like Nerdmaster mentioned in the comments, be sure to include the () at the end. You want to call the outer function and store the result (one of the two inner functions) in a, not store the outer function itself in a.

Solution 2:

The Proxy pattern might help you:

(function() {
    // log all calls to setArray
    var proxied = jQuery.fn.setArray;
    jQuery.fn.setArray = function() {
        console.log( this, arguments );
        return proxied.apply( this, arguments );
    };
})();

The above wraps its code in a function to hide the "proxied"-variable. It saves jQuery's setArray-method in a closure and overwrites it. The proxy then logs all calls to the method and delegates the call to the original. Using apply(this, arguments) guarantees that the caller won't be able to notice the difference between the original and the proxied method.