When Object.prototype is polluted, polyfilling can fail

[kellym]

When working on a library that is run on sites with other scripts, we ran into an issue where, while trying to avoid global pollution, we found ourselves polluted by another script.

In essence, another script had modified Object.prototype and had defined a property includes that was not configurable on all new objects. This threw an error for us while trying to polyfill [].includes.

A quick and dirty solution was to replace some references of {} in core-js/packages/core-js-pure/override/internals/export.js to Object.create(null).

I'm trying to submit a proper PR to fix this, but was wondering, is there a proper way to use a polyfill inside this export file?

[zloirock]

Could you create a reproducible example for understanding with which subset of the problem we are dealing? core-js have some such assumptions on initialization to minimize minimalistic bundles size and coupling.

[kellym]

Sure, for a quick example of something we've actually run into:

// third party script does a polyfill like so
Object.defineProperty(Object.prototype, 'includes', {
  configurable: false,
  writable: false,
  value: function(i) { return this.indexOf(i) != -1; }
});

// core-js now does this: 
'use strict';

const path = {};
path['String'] = {}
path['String'].includes = () => 'new function';
// ^ Throws: Uncaught TypeError: Cannot assign to read only property 'includes' of object '<Object>'

Here's an example of a commit that fixes the error: activeprospect@ef571d6. Note, though, it checks for Object.create, which is only present in IE9+. Fortunately, Object.defineProperty is only fully implemented in IE9+ as well, so it's fairly likely that commit will effectively resolve the issue.

[zloirock]

I'm not sure should such workaround be added on the core-js side or not since it's the case of a broken environment and it could cause some other problems. I'll think about it.

[zloirock]

I'll add a workaround, but in a little different way.

[kellym]

Awesome, thanks! I appreciate the help!