What's the difference between isPrototypeOf and instanceof in Javascript?

Question

In some of my own older code, I use the following:

Object.prototype.instanceOf = function( iface )
{
 return iface.prototype.isPrototypeOf( this );
};
         


        

Answer 1

Is a instanceof b ALWAYS the same as b.prototype.isPrototypeOf(a) ?

No, a instanceof b will not always behave the same as b.prototype.isPrototypeOf(a).

CMS' answer pointed out that they differ in what they are (one is an operator and the other is a built-in method available on the Object.prototype object). This is correct, however there are also some special cases for which a instanceof b will result in a TypeError while b.prototype.isPrototypeOf(a) will work just fine and vice versa.

Difference #1

The right-hand side of instanceof is expected to be a constructor function.

If b is not a function:

• a instanceof b will result in a TypeError.

• b.prototype.isPrototypeOf(a) will work just fine.

const b = {
    prototype: {}
};
const a = Object.create( b.prototype );

console.log( b.prototype.isPrototypeOf(a) );    // true
console.log( a instanceof b );                  // TypeError: Right-hand side of 'instanceof' is not callable

Difference #2

When using b.prototype.isPrototypeOf(a), b.prototype should be inheriting from Object.prototype:

If b.prototype has not access to the Object.prototype.isPrototypeOf() method:

• b.prototype.isPrototypeOf(a) will result in a TypeError.
• a instanceof b will work just fine.

function B() {};
B.prototype = Object.create( null );

const a = new B();

console.log( a instanceof B );              // true
console.log( B.prototype.isPrototypeOf(a) ) // TypeError: B.prototype.isPrototypeOf is not a function

Difference #3

If the right-hand side of instanceof is a bound function, it is treated equivalently to its target function.

If b is a bound function:

• a instanceof b will work just fine.
• b.prototype.isPrototypeOf(a) will result in a TypeError (bound functions don't have a prototype property).

function B() {};
const BoundB = B.bind( null );
const a = new B();

console.log( a instanceof BoundB );              // true
console.log( BoundB.prototype.isPrototypeOf(a) ) // TypeError: Cannot read property 'isPrototypeOf' of undefined

Conclusion

• If you are dealing with prototypal inheritance established through Object.create(), without the use of constructors, you should probably be using the Object.prototype.isPrototypeOf() method (indeed the use cases of instanceof are more restricted in that instanceof expects its right-hand side parameter to be a constructor function).
• If you are dealing with constructors you will be slightly safer by using the instanceof operator (you will be able to cover bound functions as well as the cases where Object.prototype does not lie in the prototype chain of Constructor.prototype).