inheritance.js

"use strict";

// Base Girl object.
function Girl() {
    this.name = '';
    this.traits = {};
}

// Scout `is a` Girl
function Scout() {
    Girl.call(this);
    this.team = '';
}

Scout.prototype = new Girl();
Scout.prototype.constructor = Scout;

// Setup scout: Sarah
var sarah = new Scout();
sarah.name = "Sarah";
sarah.team = "Blue Scouts";
sarah.traits.age = 30;
sarah.traits.weight = 125;

// Setup scout: Tricia
var tricia = new Scout();
tricia.name = "Tricia";
tricia.team = "Green Scouts";
tricia.traits.age = 32;
tricia.traits.weight = 140;

// Log the girls
console.log("Sarah:", sarah, "\n");
console.log("Tricia:", tricia, "\n");

// Sarah (and Tricia) are both girl scouts
console.log(sarah instanceof Scout);
console.log(sarah instanceof Girl);

Note lines #15 and #16:

Scout.prototype = new Girl();
Scout.prototype.constructor = Scout;

If you set the prototype property of a constructor function, it "breaks" the constructor property of its instances (that you'll create in the future). As a reminder, in JavaScript all instances have a constructor property that point to the function that created it. For example:

console.log((5).constructor); // logs [Function: Number]

Now let's say you comment out line #16. Given the example above, it follows that

console.log(sarah.constructor);

will output [Function: Scout]. However, it instead outputs [Function: Girl].

That's because assigning Scout.prototype broke constructor property of its instances. To fix that, we need to remember to always set constructor property as shown in line #16 of the Gist.

In summary the "native pattern" (as in what's followed in creation of JavaScript's native objects) for inheritance in JavaScript requires the following three steps:

1. Remember to call super constructor function from the child constructor function (line #11).
2. Set prototype of child to new instance of super (line #15).
3. Set prototype.constructor of child to child (line #16).

A simple and clean example of this pattern can be seen here: http://bit.ly/1PESNli