nativeObjectExtensions

Reports extending the prototype of native JavaScript objects.

✅ This rule is included in the ts untyped presets.

Extending native JavaScript prototypes like Array.prototype or Object.prototype affects all instances of that type across the entire codebase. This can cause conflicts with other libraries that expect standard prototype behavior. Future ECMAScript versions may add methods with the same name, causing unexpected behavior when those methods are overwritten.

Examples

❌ Incorrect

Array.prototype.custom = function () {
  return this;
};

Object.prototype.method = 123;

String.prototype.toTitleCase = function () {
  return this.charAt(0).toUpperCase() + this.slice(1);
};

Object.defineProperty(Array.prototype, "custom", {
  value: function () {},
});

Object.defineProperties(Object.prototype, {
  method: { value: 123 },
});

✅ Correct

function customArrayMethod<T>(array: T[]): T[] {
  return array;
}

class ExtendedArray<T> extends Array<T> {
  custom() {
    return this;
  }
}

function toTitleCase(value: string) {
  return value.charAt(0).toUpperCase() + value.slice(1);
}

const arrayUtils = {
  custom<T>(array: T[]) {
    return array;
  },
};

Options

This rule is not configurable.

When Not To Use It

If you are developing a polyfill library that intentionally adds missing standard methods to native prototypes, you may need to disable this rule. Some frameworks also extend native prototypes as part of their core design, though this is increasingly rare in modern JavaScript development.

Further Reading

• MDN: Inheritance and the prototype chain
• MDN: Object.defineProperty()
• MDN: Object.prototype

Equivalents in Other Linters

• ESLint: no-extend-native
• Oxlint: eslint/no-extend-native