2619. Array Prototype Last - Detailed Explanation

Free Coding Questions Catalog

Boost your coding skills with our essential coding questions catalog. Take a step towards a better tech career now!

Problem Statement

You need to add a method last() to JavaScript’s Array prototype so that calling someArray.last() returns the last element of the array, or -1 if the array is empty. You may assume any array you get is the result of JSON.parse, so it behaves like a normal array.

Examples

Example 1

Input:  nums = [null, {}, 3]
Output: 3
Explanation: nums.last() returns the last entry, 3.

Example 2

Input:  nums = []
Output: -1
Explanation: nums.last() returns -1 because the array is empty.

Constraints

You must attach last to Array.prototype.
Calling last() must run in O(1) time.
If this.length === 0, return -1; otherwise return the last element.

Hints

How do you extend all arrays at once in JavaScript?
What does this refer to inside a function on Array.prototype?
How can you grab the last element in constant time?

Approach 1 – Bracket Indexing on Prototype

Attach a regular function to Array.prototype and use this[this.length – 1].

Array.prototype.last = function() {
  // 'this' is the array on which .last() was called
  return this.length === 0 ? -1 : this[this.length - 1];
};

Code (Python equivalent demonstration)

Python3

. . . .

Code (Java demonstration utility)

Java

. . . .

Approach 2 – Using ES2022 `at()`

Modern JavaScript offers Array.prototype.at(index), where a negative index counts from the end. We can write:

Array.prototype.last = function() {
  return this.length === 0 ? -1 : this.at(-1);
};

This is equally O(1) and perhaps more expressive.

Complexity Analysis

Time: O(1) per call, just a couple of property accesses.
Space: O(1) extra.

Step‑by‑Step Walkthrough

Call [1,2,3].last() → inside the function, this points to [1,2,3].
Check this.length===0? No (3).
Return this[2] (or this.at(-1)), which is 3.

Calling [].last() → this.length===0 is true, so return -1.

Common Mistakes

Arrow functions: Array.prototype.last = () => … will bind this incorrectly.
Forgetting -1 check: omitting the empty‑array guard leads to undefined.
Overwriting built‑ins: avoid reassigning Array.prototype wholesale.

Edge Cases

Arrays with holes (e.g. [1,,3]) – bracket indexing returns undefined at a hole, but the problem guarantees normal arrays from JSON.parse.
Non‑primitive entries (null, {}) still work since we return whatever sits at that index.
Very large arrays – still O(1).

Alternative Variations

first() – return the first element or -1.
last(n) – return an array of the last n elements.
safe chaining: Array.prototype.lastSafe = function() { … } to avoid collisions with future native methods.