Why You Shouldn't Extend JavaScript Built-ins (And What to Do Instead)

JavaScript is a versatile language that gives you the freedom to bend it to your will—but not every freedom should be exercised. One of the most common mistakes developers make is extending built-in prototypes like Array.prototype or String.prototype. While it might seem like a good idea at first, it comes with significant risks and pitfalls. In this article, we'll explore why extending built-ins is a bad practice and discuss better alternatives to achieve the same functionality without the downsides.

Why Extending Built-ins is Dangerous

1. Global Pollution

When you add a method to a built-in prototype, you're modifying a global object. This means every instance of that type in your entire application (and any third-party libraries) will inherit your new method. This can lead to unpredictable behavior, especially if different parts of your code or external libraries rely on the original prototype behavior.

Example of Conflict:

Array.prototype.average = function () {
    return this.reduce((acc, elem) => acc + elem, 0) / this.length;
};

// Some third-party library might expect this:
const result = ["a", "b", "c"].average(); // BOOM! Unexpected behavior.

2. Future Compatibility Issues

JavaScript evolves over time, with new methods being added to built-ins regularly. By extending prototypes, you risk overwriting future native methods that browsers may implement, causing major headaches when upgrading.

For example, if a future JavaScript version introduces an Array.prototype.average method with different behavior than yours, it could break your application.

3. Debugging Complexity

Extending built-ins makes debugging much harder. If you run into unexpected behavior, you might not immediately realize that the issue stems from the modified prototype. This can lead to hours of unnecessary troubleshooting.

Better Alternatives to Extending Built-ins

1. Utility Functions

The most straightforward alternative is to create standalone utility functions. These functions can be part of a utility module and do not modify global objects.

Example:

function averageArray(list) {
    return list.reduce((acc, elem) => acc + elem, 0) / list.length;
}

const avg = averageArray([1, 2, 3]);

2. Utility Classes

For more structure and organization, you can use a utility class. This is especially useful for grouping related methods together.

Example:

class ArrayUtils {
    static average(list) {
        return list.reduce((acc, elem) => acc + elem, 0) / list.length;
    }
}

const avg = ArrayUtils.average([1, 2, 3]);

3. Composition over Extension

If you're working with custom data structures or types, consider using composition instead of extending built-ins. Create your own class that wraps the built-in functionality and adds your custom methods.

Example:

class CustomArray {
    constructor(items) {
        this.items = items;
    }

    average() {
        return this.items.reduce((acc, elem) => acc + elem, 0) / this.items.length;
    }
}

const myArray = new CustomArray([1, 2, 3]);
const avg = myArray.average();

Other Important Considerations

1. Code Readability and Maintainability

Adding methods to built-in prototypes might make your code look clean initially, but it hides functionality in unexpected places. A developer new to your project might not realize that you've modified a built-in and could spend extra time searching for the source of a mysterious method.

2. Testing Implications

Prototypes are shared globally, so modifying them can affect unrelated tests. If one test modifies a prototype, it might break other tests in the suite that rely on the unmodified prototype behavior. Keeping your utilities separate ensures isolated testing.

3. Avoid Reinventing the Wheel

Before adding custom methods to manipulate arrays, strings, or other data types, check libraries like Lodash or Ramda. These libraries are battle-tested, efficient, and widely adopted in the JavaScript ecosystem.

Example with Lodash:

import _ from 'lodash';

const avg = _.mean([1, 2, 3]);

4. Performance Considerations

Overwriting built-ins or wrapping them in unexpected ways can have performance impacts, particularly when working with large datasets or in performance-critical environments. Stick to methods that are simple, predictable, and optimized.

Finally

1. Never modify built-in prototypes. The risks outweigh the benefits in almost every scenario.
2. Use utility functions or classes to achieve the same functionality without polluting the global namespace.
3. Consider established libraries like Lodash if you're performing complex operations frequently.
4. Prioritize readability, maintainability, and compatibility in your code.

By following these guidelines, you’ll write cleaner, more predictable, and future-proof JavaScript code. Remember: just because you can doesn’t mean you should!

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