Disallow extending native types

In JavaScript, you can extend any object, including builtin or “native” objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won’t need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

Open in Playground
/*eslint no-extend-native: "error"*/

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


This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.


Examples of correct code for the sample { "exceptions": ["Object"] } option:

Open in Playground
/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.


This rule was introduced in ESLint v0.1.4.


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