Overriding a JavaScript function while referencing

Question

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.

Answer 1

The answer that @Matthew Crumley provides is making use of the immediately invoked function expressions, to close the older 'a' function into the execution context of the returned function. I think this was the best answer, but personally, I would prefer passing the function 'a' as an argument to IIFE. I think it is more understandable.

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

Answer 2

Thanks guys the proxy pattern really helped.....Actually I wanted to call a global function foo.. In certain pages i need do to some checks. So I did the following.

//Saving the original func
var org_foo = window.foo;

//Assigning proxy fucnc
window.foo = function(args){
    //Performing checks
    if(checkCondition(args)){
        //Calling original funcs
        org_foo(args);
    }
};

Thnx this really helped me out

Answer 3

You can override a function using a construct like:

function override(f, g) {
    return function() {
        return g(f);
    };
}

For example:

 a = override(a, function(original_a) {
      if (condition) { new_code(); original_a(); }
      else { original_a(); other_new_code(); }
 });

Edit: Fixed a typo.

Answer 4

Passing arbitrary arguments:

a = override(a, function(original_a) {
    if (condition) { new_code(); original_a.apply(this, arguments) ; }
    else { original_a.apply(this, arguments); other_new_code(); }
});

Answer 5

I had some code written by someone else and wanted to add a line to a function which i could not find in the code. So as a workaround I wanted to override it.

None of the solutions worked for me though.

Here is what worked in my case:

if (typeof originalFunction === "undefined") {
    originalFunction = targetFunction;
    targetFunction = function(x, y) {
        //Your code
        originalFunction(a, b);
        //Your Code
    };  
}