Defining any property/function on Object.prototype will cause jQuery's addBack to throw error?

[dotnetprono1]

I think this must be a bug of jQuery, a simple custom property/function definition on Object.prototype is just normal (and fairly common). That kind of action should not trigger any bug in any framework. But here it does cause a bug in the jQuery when we use addBack with filter (selector).

So here what I've done before detecting the issue:

//define a simple dummy random property like this (before loading and running the jQuery code)
Object.prototype.xyzxxx = "";

$(function(){
     //just try a query with .addBack with selector
     $("div").addBack("div");
});

Here is the error printed in the console:

Uncaught TypeError: G[o].exec is not a function
at se.tokenize (jquery-3.5.1.min.js:2)
at se.compile (jquery-3.5.1.min.js:2)
at se.select (jquery-3.5.1.min.js:2)
at se (jquery-3.5.1.min.js:2)
at Function.se.matches (jquery-3.5.1.min.js:2)
at Function.S.filter (jquery-3.5.1.min.js:2)
at D (jquery-3.5.1.min.js:2)
at S.fn.init.filter (jquery-3.5.1.min.js:2)
at S.fn.init.addBack (jquery-3.5.1.min.js:2)
at HTMLDocument.<anonymous> (?editor_console=:111)

Note that the addBack without selector will not cause the error.
You can try out the fiddle here https://jsfiddle.net/v8hfmdn3/1/ for detailed error info.

[dotnetprono1]

So currently in my project I'm using addBack with selector (which is not a very common use case but I do need it) together with custom property/function defined on Object.prototype (this is just recently required) and boom, the issue popped out taking me several minutes to try to understand what was strangely going wrong before believing that it could be a bug in the jQuery library.

[dotnetprono1]

I've tried defining the custom property/function on Object.prototype by another way using Object.defineProperty and looks like it works fine without throwing the error. e.g: Object.defineProperty(Object.prototype, "xyzxxx", { value: "" });

[mgol]

Modifying Object.prototype is not supported. It’s generally a bad idea. See https://github.com/jquery/jquery/wiki/Won't-Fix#modifying-objectprototype