How to inject javascript code to the beginning of every prototype method?

Question

I want to inject code in javascript, for debugging purposes, within every one of my method prototypes in javascript. This example shows one class only, but assume I have hundreds of classes and each class has dozens of methods. This mechanism should perform at prototype level without the need to specify each class/method name.

function MyClass1() {
    this.attrib = "ABC";
}

MyClass1.prototype.myMethod = function() {
   alert("first row");  // <---- THE INJECTION SHOULD OCCUR BEFORE THIS LINE OF CODE
}

The idea is to dynamically inject some code before the first row of myMethod(), during the first loading/execution of the javascript code. Such as:

MyClass1.prototype.myMethod = function() {
   alert("I was injected dynamically");
   alert("first row");
}

So for every other Class and Method, the same should happen. Is this achievable using the Function.prototype approach ?

possible duplicate of [Need to hook into a javascript function call, any way to do this?](http://stackoverflow.com/questions/10273309/need-to-hook-into-a-javascript-function-call-any-way-to-do-this) — simonzack, Jul 06 '13 at 12:25
I hope you're not planning to display debug info using `alert`. Use `console.log` or `console.debug` instead (if supported). — Bart, Jul 06 '13 at 12:30
Even ``console.log`` can't be relied on. It'll crash Internet Explorer unless the developer tools have been opened. — mzedeler, Jul 06 '13 at 12:31
I used "alert" simply for the purpose of this inquiry. The debug that I plan to perform is based on a specific controlling global object. — gextra, Jul 06 '13 at 12:43
regarding the duplication, I am trying to perform this dynamically for all classes and all methods. I am ensuring my question specifies this more clearly. thanks — gextra, Jul 06 '13 at 12:46

score 13 · Accepted Answer · edited Mar 24 '16 at 22:08

Just wrap your method. Here is the standard method:

MyClass1.prototype.myMethod = function() {
    alert("first row");
}

Then wrap it:

var orig = MyClass1.prototype.myMethod;
MyClass1.prototype.myMethod = function() {
    alert('Injected');
    return orig.apply(this, arguments);
}

You are asking two questions, and I've only answered one of them (i.e. how to wrap a function). The other part - how to do this on many functions - is best done using a specialized library. In fact, this can be done using Aspect Oriented Programming (AOP). I found a couple of JavaScript libraries that offers this, one of them is Aop.js (try googling for more yourself).

Instead of `orig = MyClass1.prototype;` maybe you meant `var orig = MyClass1.prototype.myMethod;` ? — , Jul 06 '13 at 12:58

How to inject javascript code to the beginning of every prototype method?

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