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
