user1689498 user1689498 - 6 months ago 88
Javascript Question

JavaScript isPrototypeOf vs instanceof usage

Suppose we have the following:

function Super() {
// init code
}

function Sub() {
Super.call(this);
// other init code
}

Sub.prototype = new Super();

var sub = new Sub();


Then, in some other part of our ocde, we can use either of the following to check for the relationship:

sub instanceof Super;


or

Super.prototype.isPrototypeOf( sub )


Either way, we need to have both the object (sub), and the parent constructor (Super). So, is there any reason why you'd use one vs the other? Is there some other situation where the distinction is more clear?

I've already carefully read 2464426, but didn't find a specific enough answer.

Answer

Imagine you don't use constructors in your code, but instead use Object.create to generate objects with a particular prototype. Your program might be architected to use no constructors at all:

var superProto = {
    // some super properties
}

var subProto = Object.create(superProto);
subProto.someProp = 5;

var sub = Object.create(subProto);

console.log(superProto.isPrototypeOf(sub));  // true
console.log(sub instanceof superProto);      // TypeError

Here, you don't have a constructor function to use with instanceof. You can only use subProto.isPrototypeOf(sub).