🔙 Array.prototype.findLast() Polyfill

The findLast() method returns the last element in an array that satisfies a given predicate function. It iterates from the end of the array.

✅ Implementation

if (!Array.prototype.findLast) {
  Array.prototype.findLast = function(callback, thisArg) {
    // Ensure that 'callback' is a function
    if (typeof callback !== 'function') {
      throw new TypeError(callback + ' is not a function');
    }
    
    // Iterate over the array starting from the last element
    for (let i = this.length - 1; i >= 0; i--) {
      // If the element satisfies the condition, return it
      if (callback.call(thisArg, this[i], i, this)) {
        return this[i];
      }
    }
    
    // If no element satisfies the condition, return undefined
    return undefined;
  };
}

Explanation:

1. Check if findLast is defined: The if (!Array.prototype.findLast) condition ensures that the polyfill only runs if findLast is not already available.

2. Callback validation: It checks whether the callback provided is a valid function using typeof callback !== 'function'.

3. Iterate backward: The loop starts from the last element of the array (i = this.length - 1) and moves towards the first element (i--).

4. Calling the callback: callback.call(thisArg, this[i], i, this) is used to invoke the callback function. This gives you the correct value for this within the callback function, the current element (this[i]), its index (i), and the array itself (this).

5. Return the matching element: If the callback returns a truthy value, that element is returned as the result of findLast.

6. Return undefined if no match is found: If the loop completes without finding a match, undefined is returned.

Example Usage:

const arr = [1, 2, 3, 4, 5];

const result = arr.findLast(num => num % 2 === 0);
console.log(result); // Output: 4

In this example, findLast finds the last even number (4) in the array.

Edge Cases:

• If no element matches the condition, it returns undefined.
• If the array is empty, it returns undefined.

This implementation closely mimics the behavior of the modern findLast method that was introduced in newer versions of JavaScript.