Iterate and change javascript functions on-the-fly

Question

I want to be able to iterate through all JavaScript functions in my document and, for instance, add an alert('ok'); to them.

so if I have:

function a(num) {
 return num;
}

after I run my javascript code and call this function, it will actually do:

{
 alert('ok');
 return num;
}

I know I can access a function using window.[funcName], but how can I iterate thru all functions and edit them?

thanks

PS: I know this is not a very good idea, but it's just for a debug environment

Answer 1

for (var i in window) {
 if (typeof window[i] == 'function') console.log(i); // logs the function name
}

This gets you a list of functions. You cannot modify functions in JavaScript, you can only wrap them. This means you make a copy of the function and replace the original with a wrapper. Say, we have this function:

function a(num) { return num;}

then we wrap it like this:

functionName = 'a'; // original function name
window['_original_' + functionName] = window[functionName]; // putting it in a safe place
window[functionName] = function() { // creating wrapper
 alert('ok'); //logger function
 return window['_original_' + functionName].apply(null, arguments); //invoke original
}

This gives you only global-space functions, e.g. it won't log jQuery plugins. If you want to log all functions use Firebug's profiler.

• Fernando: thanks! but just a point, it won't work the right way is: window[i] = function(x) { return function() { alert(x); return window['original' + x].apply(null, arguments); } }(i)

Answer 2

Instead of modifying the code of an existing function, I tried using an interceptor function wrapping an existing function. Just a quick implementation (we can improve this further)

function intercept(object, methodBindName, interceptor) {
 var oldMethod = object[methodBindName];
 object[methodBindName] = function() {
 interceptor.apply(null, arguments);
 return oldMethod.apply(object, arguments);
 }
}
function test1(msg) {alert(msg);}
function test2() {alert('hi');}
var methodNames = ['test1', 'test2'];
for(var i = 0; i < methodNames.length; i++) {
 intercept(window, methodNames[i], function(){alert('Hello');})
}
test1('foo');
test2();

• Alexey Lebedev: oldMethod.apply(object, arguments); should also return value
• gnarf: why not .apply(this, arguments) for both functions? sometimes those functions might not be called in the objects context.