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.

If you really cannot, just chain the handler and wrap the hardcoded call in a promise — Sterling Archer
– Sterling Archer, Commented Feb 9, 2018 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
– Korgrue, Commented Feb 9, 2018 at 21:44
@SterlingArcher this is for building chrome extension so cant modify — GorvGoyl
– GorvGoyl, Commented Feb 9, 2018 at 21:45

Titus · Accepted Answer · 2018-02-09 21:54:41Z

6

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 9, 2018 at 21:54

answered Feb 9, 2018 at 21:44

Titus

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6 Comments

Ele Over a year ago

StackOverflow (too many recursions) detected! Try to execute HardCode()

Cory Danielson Over a year ago

Make sure you pass the arguments along

epascarello Over a year ago

Still going to say: Uncaught RangeError: Maximum call stack size exceeded

user400654 Over a year ago

hoisting is why this fails.

epascarello Over a year ago

It needs to be declared as var oldFn = HardCoded; HardCoded = function (){

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