proto and inheritance in JavaScript

Question

I've been studying JavaScript inheritance for a couple of days, and although I've made quite a lot of progress there are some things I don't quite understand yet.

For example, I find this behaviour quite confusing:

var Employee = function Employee() { this.company = 'xyz'; };
var Manager = function Manager() { this.wage = 'high'; };

var m = new Manager();

m; // { "wage": "high", __proto__ : Manager } -- no problems so far.

Manager.prototype = new Employee();

var n = new Manager;

m.company; // undefined
n.company; // "xyz"

m's __proto__ property points to an object which is not Manager's current prototype. This is a little counterintuitive, given that:

An object inherits properties even if they are added to its prototype after the object is created.

Taken from JavaScript: The Definitive Guide, 5th Edition, By David Flanagan

Couldn't this behaviour be applied to the aforementioned case, too?

Can anyone clarify?

user324312 · Accepted Answer · 2010-08-13 20:27:11Z

4

It's a little confusing because functions are themselves objects:

function Employee() {this.company = 'xyz';}
function Manager() {}

var m = new Manager();
Manager.prototype = new Employee();

/* You set the prototype on the Manager function (object), 
   your m instance and Manager function are separate objects.
   At this point the prototype of m is still undefined */

m = new Manager();
m.company; // 'xyz'

/* Creating a new Manager copies the new prototype */

answered Aug 13, 2010 at 20:27

user324312

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