What the difference between Array.prototype.isPrototypeOf and Array.isPrototypeOf?

Question

I am wondering what is the difference between Array.prototype.isPrototypeOf and Array.isPrototypeOf I thought it should work the same because I thought it will refer to the same method isPrototypeOf but It's look like I was mistaken. Can anybody explain to me why this work like that ?

 const exampleArray = [1, 2, 3]; console.log(Array.prototype.isPrototypeOf(exampleArray)); console.log(Array.isPrototypeOf(exampleArray)); // Why this statement returns false ?

Answer 1

Those are both references to Object.prototype.isPrototypeOf() , which checks to see if the object it's called on is in the prototype chain of the passed argument.

For the exampleArray , the prototype chain is this:

Object.prototype <- Array.prototype <- exampleArray instance

See snippet:

 const exampleArray = [1, 2, 3]; console.log( Object.getPrototypeOf(exampleArray) === Array.prototype, Object.getPrototypeOf(Array.prototype) === Object.prototype );

The Array constructor function - window.Array - is not in the prototype chain, so isPrototypeOf returns false .

The Array constructor function would only have isPrototypeOf return true if a class extended Array , or if it was set to be the internal prototype of a new object via Object.create , eg:

 class ExtendedArray extends Array {} console.log(Array.isPrototypeOf(ExtendedArray)); const somethingWeird = Object.create(Array); console.log(Array.isPrototypeOf(somethingWeird));

For completeness, the Array constructor function - being a function - inherits from Function.prototype , which inherits from Object.prototype :

 console.log( Object.getPrototypeOf(Array) === Function.prototype, Object.getPrototypeOf(Function.prototype) === Object.prototype );