🔢 Array.prototype.indexOf() Polyfill

The indexOf() method returns the first index at which a given element can be found, or -1 if not present.

✅ Implementation

Array.prototype.customIndexOf = function(searchElement, fromIndex = 0) {
    // Normalize negative fromIndex to count from the end of the array
    if (fromIndex < 0) {
        fromIndex = Math.max(this.length + fromIndex, 0); // Adjust negative index
    }

    // Iterate through the array starting from fromIndex
    for (let i = fromIndex; i < this.length; i++) {
        if (this[i] === searchElement) {
            return i; // Return the index of the first match
        }
    }

    return -1; // Return -1 if the element is not found
};

// Example usage:
const array = [2, 9, 9];

console.log(array.customIndexOf(2));       // Output: 0
console.log(array.customIndexOf(7));       // Output: -1
console.log(array.customIndexOf(9, 2));    // Output: 2
console.log(array.customIndexOf(2, -1));   // Output: -1
console.log(array.customIndexOf(2, -3));   // Output: 0

How It Works:

1. Negative fromIndex : If fromIndex is negative, it is adjusted to count from the end of the array. For example, fromIndex = -1 starts the search from the last element.
2. Array Iteration : The loop starts from the fromIndex (or 0 if not provided) and compares each element with searchElement using strict equality (===).
3. Return Index or -1 :

• If searchElement is found, the method returns the index of the first match.
• If no match is found, it returns -1.

Example Walkthrough:

• customIndexOf(2): Starts at index 0 and finds 2 at index 0, so it returns 0.
• customIndexOf(7): No match is found, so it returns -1.
• customIndexOf(9, 2): Starts at index 2 and finds 9 at index 2, so it returns 2.
• customIndexOf(2, -1): Starts searching from the last element (index 2), doesn't find 2, so it returns -1.
• customIndexOf(2, -3): Starts searching from index 0, finds 2 at index 0, so it returns 0.

This implementation mimics Array.prototype.indexOf, with additional support for negative fromIndex, ensuring the search works as expected.