Implement custom instanceof method

Let us see how we can implement the custom instanceof method in JavaScript.

According to MDN –

The instanceof operator tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object. The return value is a boolean value.

From the definition, we can derive that to create a custom instanceof method we will have to check if the prototype property of a constructor appears anywhere in the prototype chain of an object.

Thus we will have first to check if the provided input is an object or not and later keep checking if the prototype property appears anywhere in the prototype chain.

To get the prototype of a constructor we can use __proto__ or getPrototypeOf(object) method.

Iterative implementation with __proto__

const instanceOf = (obj, target) => {
  
  // if provided input is not object type, return false
  if(obj === null || typeof obj !== 'object') return false;
  
  // keep checking in the prototype chain
  while(obj){
    if(obj.__proto__ === target.prototype) return true;
    obj = obj.__proto__;
  }
  
  return false;
}

Input:
class P {}
class Q extends P {}

const q = new Q()
console.log(instanceOf(q, Q)) // true
console.log(instanceOf(q, P)) // true
console.log(instanceOf(q, Object)) // true

function R() {}
console.log(instanceOf(q, R)) // false
R.prototype = Q.prototype
console.log(instanceOf(q, R)) // true
R.prototype = {}
console.log(instanceOf(q, R)) // false

Recursive implementation with getPrototypeOf(obj)

const instanceOf = (obj, target) => {
  // if provided input is not object type, return false
  if (obj == null || typeof obj !== 'object') return false;
  
  // get the prototype 
  const proto = Object.getPrototypeOf(obj);
  
  // recursively test if prototype matches to the target's prototype
  return proto === target.prototype ? true : instanceOf(proto, target);
}

Input:
class P {}
class Q extends P {}

const q = new Q()
console.log(instanceOf(q, Q)) // true
console.log(instanceOf(q, P)) // true
console.log(instanceOf(q, Object)) // true

function R() {}
console.log(instanceOf(q, R)) // false
R.prototype = Q.prototype
console.log(instanceOf(q, R)) // true
R.prototype = {}
console.log(instanceOf(q, R)) // false