In JavaScript, where do prototype's functions hide when we instantiate the derived object?-Collection of common programming errors

3 years ago
admin
2 minutes

I am currently trying to figure out exactly how the prototypical inheritance works in JavaScript. Here is the mystery I currently trying to solve.

Suppose we set up the following structure:

var base = { greet : "hello" }

var child = function () {}

child.prototype = base;
child.prototype.protoSomething = function () {}

var instance = new child();

Nothing fancy. Now lets see what instance has for properties (own or otherwise):

for(prop in instance) {
    console.log(prop); // prints 'greet', 'protoSomething'
}

Alright, so it has greet and protoSomething, are they instance‘s own members?

for(prop in instance) {
    if(instance.hasOwnProperty(prop))
        console.log(prop); // nothing is printed
}

Nope, own property list is empty. Are they in instance‘s prototype?

if(instance.prototype === undefined) {
    console.log("'instance' has no prototype"); // gets printed
}

Well, bummer, instance doesn’t have a prototype assigned. So, if the properties are not own and there is no prototype, where are they coming from? I feel like some sort of a diagram would be very nice at this point.

  1. Only a function has the property prototype.

    instance.constructor.prototype === base // return true

    To get a prototype of an object, you should use the Object.getPrototypeOf method.

    Object.getPrototypeOf(instance) === base // return true

  2. This page should help you out:

    http://joost.zeekat.nl/constructors-considered-mildly-confusing.html

    In short, the ‘behind the scenes’ prototype of an instance (created via new) is not accessible in any way, hence why it returns undefined. Only the constructor (on which you’ve manually set a .prototype property) will return anything when you try to retrieve it.

Originally posted 2013-11-09 23:21:29.