The at() method takes an integer and returns the item at that index, allowing for positive and negative integers.
✅ Implementation
Array.prototype.customAt = function(index) { // Adjust negative index to count from the end of the array if (index < 0) { index += this.length; // Convert negative index to a positive one } // Return the element at the adjusted index, or undefined if out of bounds return index >= 0 && index < this.length ? this[index] : undefined; }; // Example usage: const array = [5, 12, 8, 130, 44]; console.log(array.customAt(2)); // Output: 8 console.log(array.customAt(-1)); // Output: 44 console.log(array.customAt(100)); // Output: undefined
Explanation:
-
Negative index handling :
If the index is negative, it’s adjusted by adding the array’s length. This makes
-1refer to the last element,-2to the second-to-last, and so on. -
Bounds check :
After adjusting the index, the function checks if it’s within the valid range (i.e.,
0 <= index < this.length). If it’s valid, the element at that index is returned; otherwise,undefinedis returned. -
Edge case :
If the index is out of bounds (e.g.,
100for an array of length 5), it returnsundefined.
Examples:
customAt(2)returns the element at index 2 (which is 8).customAt(-1)returns the last element (44).customAt(100)returnsundefinedsince the index is out of bounds.
This mimics Array.prototype.at while supporting both positive and negative indices.
Quick Quiz
Test your understanding with 3 quick questions
Q1What does `[5, 12, 8].customAt(-1)` return?
Q2How does the `at()` polyfill handle negative indices?
Q3What does `[1, 2, 3].customAt(10)` return?