Check If a Value Is an Instance of a Class in JavaScript

Introduction

Instance checking in JavaScript appears simple at first—just use instanceof.
But real-world scenarios (and tasks like LeetCode 2618) require correctly handling:

primitive values (42, 'hi', true, 5n)
wrapper constructors (Number, String, etc.)
invalid constructor inputs
functions as instances of Function
prototype chain traversal
custom instance logic via Symbol.hasInstance

This snippet provides three robust solutions, rewritten and improved for clarity, correctness, and edge-case safety.

Solution 1 — Recommended

Using `instanceof` + Primitive Mapping

Works for objects and functions, with explicit special rules for built-in primitive wrappers.

function isInstance(candidate, ctor) {
  if (ctor == null || typeof ctor !== "function") return false;
  if (candidate == null) return false;

  if (candidate instanceof ctor) return true;

  const type = typeof candidate;
  if (ctor === Number) return type === "number";
  if (ctor === String) return type === "string";
  if (ctor === Boolean) return type === "boolean";
  if (ctor === BigInt) return type === "bigint";
  if (ctor === Symbol) return type === "symbol";

  return false;
}

Solution 2

Manual Prototype Chain Traversal

function isInstanceByPrototype(value, ctor) {
  if (ctor == null || typeof ctor !== "function") return false;
  if (value == null) return false;

  const type = typeof value;

  if (ctor === Number) return type === "number";
  if (ctor === String) return type === "string";
  if (ctor === Boolean) return type === "boolean";
  if (ctor === BigInt) return type === "bigint";
  if (ctor === Symbol) return type === "symbol";

  if (type !== "object" && type !== "function") return false;

  let proto = Object.getPrototypeOf(value);
  const target = ctor.prototype;

  while (proto) {
    if (proto === target) return true;
    proto = Object.getPrototypeOf(proto);
  }

  return false;
}

Solution 3

Using `Symbol.hasInstance`

function isInstanceBySymbol(value, ctor) {
  if (ctor == null || typeof ctor !== "function") return false;
  if (value == null) return false;

  const hook = ctor[Symbol.hasInstance];
  if (typeof hook === "function" && hook.call(ctor, value)) {
    return true;
  }

  const type = typeof value;
  if (ctor === Number) return type === "number";
  if (ctor === String) return type === "string";
  if (ctor === Boolean) return type === "boolean";
  if (ctor === BigInt) return type === "bigint";
  if (ctor === Symbol) return type === "symbol";

  return false;
}

Examples

class Creature {}
class Feline extends Creature {}
function Demo() {}

console.log(isInstance(new Date(), Date));        
console.log(isInstance(new Feline(), Creature));  
console.log(isInstance(123, Number));             
console.log(isInstance("ok", String));            
console.log(isInstance(true, Boolean));           
console.log(isInstance(7n, BigInt));              
console.log(isInstance(Symbol("x"), Symbol));     

console.log(isInstance(123, Object));             
console.log(isInstance(null, Object));            
console.log(isInstance(undefined, Number));       
console.log(isInstance(Creature, Creature));      
console.log(isInstance(Demo, Function));