Detect when a function is getting called in JavaScript

Question

There are several elements on HTML page which triggers a js function HardCoded(). I cannot modify HardCoded() function.

I want to run some custom js code after the HardCoded() function is getting called. How can I do that? Is there any handlers for js functions?

I'm building a chrome extension that's why I cannot modify page source code.
I have access to JQuery.

One way is to find all elements who are calling HardCoded() and attach events to those elements but I would like to avoid this method.

Have a look: https://stackoverflow.com/questions/3406467/can-i-intercept-a-function-called-directly — PM 77-1, Feb 09 '18 at 21:44
If you really cannot, just chain the handler and wrap the hardcoded call in a promise — Sterling Archer, Feb 09 '18 at 21:44
what does HardCoded do? Is it making a DOM change that you can watch for? If you cant edit the function to add a callback, then you will need to determine what changes you can watch for when the function fires. — Korgrue, Feb 09 '18 at 21:44
@SterlingArcher this is for building chrome extension so cant modify — GorvGoyl, Feb 09 '18 at 21:45
@Korgrue yes that method is changing the width of div and unfortunately there's no jquery event for div size change :( — GorvGoyl, Feb 09 '18 at 21:46
How is the function invoked? Via HTML attributes like `onclick`? Via event listeners like `x.onclick = ` or `x.addEventListener()`? — trincot, Feb 09 '18 at 21:46
Do you even have access to HardCoded? if not... you're options are rather limited. If you do, then you can simply replace it or monkey patch it like anything else. — Kevin B, Feb 09 '18 at 21:51
@trincot yes but hardCoded method is also getting called internally by other methods — GorvGoyl, Feb 09 '18 at 21:52

Titus · Answer 1 · 2018-02-09T21:54:41.223

5

You could do something like this:

var oldFn = HardCoded;
window.HardCoded = function(){
   var res = oldFn.apply(this, arguments);
   // New Code ....
   return res;
}

What this does is to create a reference to the HardCoded function, redefine this function and then call the old implementation using the previously created reference.

edited Feb 09 '18 at 21:54

answered Feb 09 '18 at 21:44

Titus

22,031
1
23
33

StackOverflow (too many recursions) detected! Try to execute `HardCode()` – Ele Feb 09 '18 at 21:47
1

Make sure you pass the arguments along – Cory Danielson Feb 09 '18 at 21:49
1

Still going to say: `Uncaught RangeError: Maximum call stack size exceeded` – epascarello Feb 09 '18 at 21:51
3

hoisting is why this fails. – Kevin B Feb 09 '18 at 21:52
It needs to be declared as `var oldFn = HardCoded; HardCoded = function (){` – epascarello Feb 09 '18 at 21:54
@epascarello Yes, you're right, the only time I've done this was to override a function defined as a property of an object. I forgot the way functions and variables are dealt with by the engine. – Titus Feb 09 '18 at 21:56

Detect when a function is getting called in JavaScript

1 Answers1