🔙 Array.prototype.findLastIndex() Polyfill

The findLastIndex() method returns the index of the last element that satisfies the provided testing function, iterating from end to start. Returns -1 if none found.

✅ Implementation

if (!Array.prototype.findLastIndex) {
  Array.prototype.findLastIndex = function(callback, thisArg) {
    // Ensure callback is a function
    if (typeof callback !== 'function') {
      throw new TypeError(callback + ' is not a function');
    }

    // Iterate backward through the array
    for (let i = this.length - 1; i >= 0; i--) {
      // If the element satisfies the condition, return the index
      if (callback.call(thisArg, this[i], i, this)) {
        return i;
      }
    }

    // Return -1 if no matching element is found
    return -1;
  };
}

Explanation:

1. Check if findLastIndex exists: The if (!Array.prototype.findLastIndex) ensures that the polyfill is only applied if findLastIndex doesn't already exist on Array.prototype.

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

3. Backward iteration: We iterate from the last element (i = this.length - 1) to the first (i--).

4. Callback invocation: callback.call(thisArg, this[i], i, this) calls the provided callback for each element, passing the current element (this[i]), its index (i), and the array itself (this).

5. Return index: If the callback returns a truthy value, we return the current index (i).

6. Return -1: If no element matches the condition, we return -1, which indicates no match was found.

Example Usage:

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

const index = arr.findLastIndex(num => num % 2 === 0);
console.log(index); // Output: 3 (because the last even number is 4 at index 3)

Edge Cases:

• No elements match: If no elements satisfy the condition, -1 is returned.
• Empty array: If the array is empty, it returns -1 right away.
• Callback that always returns false: If the callback always returns false, the result will be -1.

This polyfill ensures the behavior of findLastIndex is available even in environments where it's not supported natively.