Inject javascript into a javascript function

Question

I've got a weird question in that I need to inject some javascript into another javascript function. I am using a framework which is locked so I can not change the existing function.

What I've got is something like this

function doSomething(){ ... }

...***

I can manipulate the ***(above) however I can not change the doSomething function... Instead I need to somehow inject a few lines of code into the end of the doSomething code.

The reason I need to do this is that the custom framework calls doSomething() and this results in an ID being returned from the server that I need to extract. This ID is only referenced inside the doSomething function so I can not catch it unless I inject code to that function (unless I've missed something).

Is there a way to do this?

Answer 1

I'm not sure what you mean by "locked" - JavaScript is interpreted by your browser. Even if it has been minified or encoded, you can still copy the source of that function from source of the page of script. At that point, you reassign the original function to one of your own design, which contains the copied code plus your own.

var id;
var myFunction = function() {
  // code copied from their source, which contains the variable needed

  // insert  your own code, such as for copying the needed value
  id = theirIDvalue;

  // return the value as originally designed by the function
  return theirValue;
};

theirObject.theirFunction = myFunction;