Check if Object Instance of Class

Problem Statement

In this task, your objective is to implement a function checkIfInstanceOf that determines whether a given value is an instance of a specified class or any of its superclasses. This concept is rooted in object-oriented programming, where objects inherit behavior from their constructors and their prototype chains.

However, unlike JavaScript's built-in instanceof operator, your implementation must be more flexible:

• It should work with all data types — including primitives like numbers and strings.
• It must return false for invalid inputs, such as when the constructor is undefined or not a function.
• It should correctly identify inherited instances across prototype chains.

This generalized approach ensures that your function can determine instance relationships reliably, even for unconventional cases that would typically fail with instanceof.

Examples

Example 1

Input:

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

Output:

true

Explanation:
The object returned by the Date constructor is an instance of Date.

Example 2

Input:

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

Output:

true

Explanation:
Dog is a subclass of Animal. So, an instance of Dog is also an instance of Animal.

Example 3

Input:

func = () => checkIfInstanceOf(Date, Date)

Output:

false

Explanation:
A constructor function like Date is not an instance of itself.

Example 4

Input:

func = () => checkIfInstanceOf(5, Number)

Output:

true

Explanation:
Although 5 is a primitive, it can access methods from the Number prototype, like toFixed(). Thus, it's considered an instance of Number in this context.

Constraints

• Both obj and constructor can be of any type — including undefined, null, or primitives.
• The function must not throw exceptions for invalid types.
• The return type is boolean.
• You cannot use the built-in instanceof operator.

Approach and Intuition

To implement this logic, we need to replicate how JavaScript's prototype chain works, but with more inclusive input handling.

1. Validate Inputs

Before any logic runs, check if:

• The constructor is a function — if not, return false.
• The obj is non-null and of type "object" or "function" — otherwise, box it using Object(obj) to access its prototype.

This step ensures safety and allows handling of primitive values like numbers or strings.

2. Traverse the Prototype Chain

Use Object.getPrototypeOf() to walk up the prototype chain of the given object. At each level:

• Compare the current prototype to constructor.prototype.
• If a match is found, return true.
• If the chain ends (null), return false.

This mimics the internal behavior of instanceof.

3. Handle Primitives Gracefully

Primitives like 5, 'hello', or true are not objects, but in JavaScript they can still access methods from their respective wrapper types (Number, String, Boolean). For example:

5..toFixed(2); // valid, due to temporary boxing

To account for this, box primitive values using Object(obj) before beginning the prototype walk.

4. Edge Case Considerations

• checkIfInstanceOf(Date, Date) should return false — constructors are functions, not instances.
• If either argument is undefined or null, return false safely.

Final Thoughts

This problem highlights deep understanding of JavaScript's type system and prototype chain. By abstracting away the strictness of instanceof and handling edge cases explicitly, your function will be more robust and aligned with real-world behavior.

Solutions

function* generatePrototypes(instance) {
  let tempPrototype = Object.getPrototypeOf(instance);
  while (tempPrototype !== null) {
    yield tempPrototype;
    tempPrototype = Object.getPrototypeOf(tempPrototype);
  }
}

var isInstanceOf = function (object, constructorFunc) {
  if (object === null || object === undefined || typeof constructorFunc !== "function")
    return false;

  for (const proto of generatePrototypes(object)) {
    if (proto === constructorFunc.prototype) return true;
  }

  return false;
};