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Why is extending built-in JavaScript objects not a good idea?

Extending built-in JavaScript objects (e.g., Array, Object, String) by adding custom methods or properties may seem convenient, but it comes with several significant risks and downsides.

Reasons to Avoid Extending Built-in Objects

1. Risk of Naming Collisions

Adding properties or methods to built-in objects can clash with future updates to the JavaScript language or third-party libraries. If the same name is used for a built-in method in a future specification, your code could break.

Array.prototype.myCustomMethod = function () {
  return 'Hello!';
};

// If future versions of JavaScript introduce `myCustomMethod`, this will cause conflicts.

2. Affects Global Scope

Changes to built-in objects affect all instances globally, which can lead to unintended side effects in unrelated code or libraries.

String.prototype.reverse = function () {
  return this.split('').reverse().join('');
};

console.log('abc'.reverse()); // Works here

// Another library may rely on `String.prototype.reverse` and now behave unexpectedly.

3. Performance Overhead

Extending built-in objects can slow down JavaScript engines, as they need to handle additional properties on every instance of the object.

4. Violates the Principle of Least Astonishment

Developers expect built-in objects to behave in standard ways. Adding custom methods or properties can lead to confusion and bugs.

Array.prototype.sum = function () {
  return this.reduce((acc, curr) => acc + curr, 0);
};

const arr = [1, 2, 3];
console.log(arr.sum()); // Works

// Another developer may not expect this and assume `sum` is a built-in method.

5. Harder to Maintain Code

Custom extensions make it harder for others (or even yourself) to understand and maintain the codebase. Developers unfamiliar with the extensions may struggle to debug issues.

6. Compatibility Issues

Third-party libraries may inadvertently rely on the original behavior of the built-in object, and your extensions can interfere with their functionality.

Recommended Alternatives

1. Utility Functions

Instead of extending built-in objects, create standalone utility functions.

function reverseString(str) {
  return str.split('').reverse().join('');
}

console.log(reverseString('abc')); // "cba"

2. Subclasses

If you need to add functionality, create a custom class that extends the built-in object.

class CustomArray extends Array {
  sum() {
    return this.reduce((acc, curr) => acc + curr, 0);
  }
}

const arr = new CustomArray(1, 2, 3);
console.log(arr.sum()); // 6

3. Polyfills (with Caution)

If you must add a feature that is widely missing in older environments, use a polyfill, but only if the feature is standardized.

if (!Array.prototype.includes) {
  Array.prototype.includes = function (element) {
    return this.indexOf(element) !== -1;
  };
}

When Extending Built-in Objects Might Be Acceptable

  1. Private Use in a Controlled Environment:

    • If you control the entire codebase and no external libraries are used.

  2. Standardized Polyfills:

    • Adding features that conform to an official specification to support older environments.

Conclusion

While it is technically possible to extend built-in JavaScript objects, it is generally considered bad practice due to potential naming collisions, global scope impacts, and maintainability challenges. Instead, use utility functions, subclasses, or polyfills as safer alternatives.

References

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