Custom Methods for Arrays

In JavaScript, arrays are objects, and all arrays inherit from Array.prototype. This means we can add new methods to all arrays.

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How to Add Custom Methods

    Array.prototype.first = function () {
      return this[0];
    };
    
    const nums = [10, 20, 30];
    console.log(nums.first()); // 10

Here we created a first() method that returns the first element of the array.

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Why This Is Dangerous

While technically possible, this is not considered good practice. Modifying Array.prototype affects all arrays and may cause unexpected bugs, especially when using third-party libraries.

Dangerous Example

    Array.prototype.random = function () {
      return this[Math.floor(Math.random() * this.length)];
    };
    
    const a = [1, 2, 3];
    for (let i in a) {
      console.log(i); // will also print "random"
    }

Safer alternative - use Object.defineProperty

    Object.defineProperty(Array.prototype, 'random', {
      value: function () {
        return this[Math.floor(Math.random() * this.length)];
      },
      enumerable: false, // hides the method from `for...in` and `Object.keys`
    });

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Better Alternative - Composition

    const getRandom = (arr) => {
      return arr[Math.floor(Math.random() * arr.length)];
    };
    
    getRandom([10, 20, 30]); 

Composition is more predictable, easier to test, and doesn't alter global state.

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Summary

✅ Good	❌ Bad
Composition/functions	Modifying Array.prototype
Make invisible with Object.defineProperty	Omitting enumerable: false
Scoped solutions	Global impact

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Useful Custom Method Examples (potential use: when creating libraries)

    Object.defineProperty(Array.prototype, 'last', {
      value: function () {
        return this[this.length - 1];
      },
      enumerable: false,
    });
    
    [1, 2, 3].last(); // 3

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Conclusion

Adding methods to Array.prototype might be interesting for experimental or educational purposes, but in production environments it's recommended to avoid this. It's better to use regular functions, composition, or libraries.