LeetCode #2618 — MEDIUM

Check if Object Instance of Class

Move from brute-force thinking to an efficient approach using core interview patterns strategy.

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The Problem

Problem Statement

Write a function that checks if a given value is an instance of a given class or superclass. For this problem, an object is considered an instance of a given class if that object has access to that class's methods.

There are no constraints on the data types that can be passed to the function. For example, the value or the class could be undefined.

Example 1:

Input: func = () => checkIfInstanceOf(new Date(), Date)
Output: true
Explanation: The object returned by the Date constructor is, by definition, an instance of Date.

Example 2:

Input: func = () => { class Animal {}; class Dog extends Animal {}; return checkIfInstanceOf(new Dog(), Animal); }
Output: true
Explanation:
class Animal {};
class Dog extends Animal {};
checkIfInstanceOf(new Dog(), Animal); // true

Dog is a subclass of Animal. Therefore, a Dog object is an instance of both Dog and Animal.

Example 3:

Input: func = () => checkIfInstanceOf(Date, Date)
Output: false
Explanation: A date constructor cannot logically be an instance of itself.

Example 4:

Input: func = () => checkIfInstanceOf(5, Number)
Output: true
Explanation: 5 is a Number. Note that the "instanceof" keyword would return false. However, it is still considered an instance of Number because it accesses the Number methods. For example "toFixed()".

Roadmap

  1. Brute Force Baseline
  2. Core Insight
  3. Algorithm Walkthrough
  4. Edge Cases
  5. Full Annotated Code
  6. Interactive Study Demo
  7. Complexity Analysis
Step 01

Brute Force Baseline

Problem summary: Write a function that checks if a given value is an instance of a given class or superclass. For this problem, an object is considered an instance of a given class if that object has access to that class's methods. There are no constraints on the data types that can be passed to the function. For example, the value or the class could be undefined.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: General problem-solving

Example 1

() => checkIfInstanceOf(new Date(), Date)

Example 2

() => { class Animal {}; class Dog extends Animal {}; return checkIfInstanceOf(new Dog(), Animal); }

Example 3

() => checkIfInstanceOf(Date, Date)
Step 02

Core Insight

What unlocks the optimal approach

  • In Javascript, inheritance is achieved with the prototype chain.
  • You can get the prototype of an object with the Object.getPrototypeOf(obj) function. Alternatively, you can code obj['__proto__'].
  • You can compare an object's __proto__ with classFunction.prototype.
  • Traverse the entire prototype chain until you find a match.
Interview move: turn each hint into an invariant you can check after every iteration/recursion step.
Step 03

Algorithm Walkthrough

Iteration Checklist

  1. Define state (indices, window, stack, map, DP cell, or recursion frame).
  2. Apply one transition step and update the invariant.
  3. Record answer candidate when condition is met.
  4. Continue until all input is consumed.
Use the first example testcase as your mental trace to verify each transition.
Step 04

Edge Cases

Minimum Input
Single element / shortest valid input
Validate boundary behavior before entering the main loop or recursion.
Duplicates & Repeats
Repeated values / repeated states
Decide whether duplicates should be merged, skipped, or counted explicitly.
Extreme Constraints
Upper-end input sizes
Re-check complexity target against constraints to avoid time-limit issues.
Invalid / Corner Shape
Empty collections, zeros, or disconnected structures
Handle special-case structure before the core algorithm path.
Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #2618: Check if Object Instance of Class
// Auto-generated Java example from ts.
class Solution {
    public void exampleSolution() {
    }
}
// Reference (ts):
// // Accepted solution for LeetCode #2618: Check if Object Instance of Class
// function checkIfInstanceOf(obj: any, classFunction: any): boolean {
//     if (classFunction === null || classFunction === undefined) {
//         return false;
//     }
//     while (obj !== null && obj !== undefined) {
//         const proto = Object.getPrototypeOf(obj);
//         if (proto === classFunction.prototype) {
//             return true;
//         }
//         obj = proto;
//     }
//     return false;
// }
// 
// /**
//  * checkIfInstanceOf(new Date(), Date); // true
//  */
Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Press Step or Run All to begin.
Step 07

Complexity Analysis

Time
O(n)
Space
O(1)

Approach Breakdown

BRUTE FORCE
O(n²) time
O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED
O(n) time
O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.
Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.