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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Overwriting existing fucntion
Date: Mon, 19 Oct 2015 19:49:59 +0200
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Andrew Poulos wrote:

> I'm using a 3rd party elearning product that produces web pages. The
> product allows me to add "custom" code to a page.
> 
> I need to "overwrite" one of the functions that the product produces
> with one of my own. Custom code seems to always get added before their
> code eg.
> 
> […]
> function ap() {
>    function action70(){
>      // my customised function
>    }
> }
> if (window.addEventListener) {
> window.addEventListener("load", ap);
> }
> 
> to get my function to "overwrite" the existing one but only their one
> ever gets triggered.

This works insofar that it is proper executable code, but the event handler 
calls the event listener referred by “ap”, which declares a local function 
and then exits, so that the Function instance just created by the 
declaration is immediately available for garbage collection – functionally a 
no-op, it may even be optimized to function ap () {} or, further, the 
addEventListener() call may be optimized out.

> How can I add my function before theirs and still have my one trigger?

You cannot.  But (towards which you should have asked, see “X–Y problem”) 
you can overwrite their function afterwards by assigning a value to the same 
identifier, provided that they have not made the corresponding property 
read-only:

  document.addEventListener("load", function () { action70 = … }, false);

If the property is read-only but still configurable, you can make it 
writable again before that:

  Object.defineProperty(global, "action70", {
    writable: true,
    enumerable: true,   // or false
    configurable: true  // or false
  });

Properties resulting from variable declarations should not be configurable 
in the first place, so in that case you do not have to worry about read-
onlyness.

Different to what Stefan suggested, you should not be using “window” to 
refer to the global object, but if you need to make it explicit, “this” in 
the global execution context.  For example

  /* global context */
  var global = this;

  /* any context */
  document.addEventListener("load",
    function () { global.action70 = … }, false);

(A reference to the global object can also be obtained in other ways.)

Also, the host object referred to by “window” might not implement an 
“addEventListener()” method; “document” is safer, “document.body” is 
probably safest, given the history of the “load” event, but might not yet be 
available at this time (before the “body” element has been parsed).  
Consider using the “defer” attribute.  On the other hand, you appear to be 
using HTML5, so this difference might not of importance to you.  However, as 
in theory the document can be loaded before loading of the window content 
has been completed, using “document” or “document.body” instead might speed 
things up a little.

Note that this approach depends on the definition in W3C UI Events 
(formerly, W3C DOM Level 3 Events) that event listeners are added and 
executed in source code order.  If this needs to work where 
addEventListener() is not available (IE < 9 or IE 9 in Compatibility Mode), 
using window.attachEvent() instead of .addEventListener() does not suffice 
as, among other peculiarities, .attachEvent() does not observe addition 
order.

-- 
PointedEars
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