broken.inheritance.js

function Parent() {
    this.array = [1, 2, 3];
}

function Child() {

}

Child.prototype = new Parent;

var c1 = new Child;

c1.array; // [1, 2, 3]
c1.array.push(4);

var c2 = new Child;

c2.array; // [1, 2, 3, 4]

fixed.inheritance.js

function Parent() {
    this.array = [1, 2, 3];
}

function Child() {
    Parent.call(this); // <--------------------- The fix
}

Child.prototype = new Parent;

var c1 = new Child;

c1.array; // [1, 2, 3]
c1.array.push(4);

var c2 = new Child;

c2.array; // [1, 2, 3]