#no-extend-native

#Configuration

import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-extend-native': 'error',
    },
  },
]);

Disallows directly modifying the prototype of native built-in objects (Object, Array, Function, String, Number, Boolean, Symbol, Map, Set, Promise, Error, RegExp, Date, BigInt, WeakRef, FinalizationRegistry, etc.).

#Rule Details

Extending native prototypes is generally regarded as a bad practice because it breaks assumptions other code makes about builtins, can collide with future language additions, and is invisible to the rest of the program until it is triggered at runtime.

The rule reports two extension patterns:

1. Direct assignment, including compound and logical assignments: Builtin.prototype.foo = ..., Builtin.prototype.foo ??= ....
2. Object.defineProperty(Builtin.prototype, ...) and Object.defineProperties(Builtin.prototype, ...).

References to the builtin that are shadowed by a local declaration in scope (e.g. function foo() { var Object = function () {}; Object.prototype.p = 0 }) are not reported.

Examples of incorrect code for this rule:

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Examples of correct code for this rule:

// Modifications to user-defined objects are allowed.
x.prototype.p = 0;

// Property access on the constructor (not on its prototype) is allowed.
Object.toString.bind = 0;

#Options

{
  "no-extend-native": ["error", { "exceptions": ["Object"] }]
}

With { "exceptions": ["Object"] }, the following becomes valid:

Object.prototype.g = 0;

#Original Documentation

• ESLint rule documentation
• Source code