By reference vs by value

ref.js

var a = {},
    b = {};

// primitives are passed by value
a.num = 1;
b.num = a.num;
b.num = 5;

a.num; // still equals 1
b.num; // equals 5


// objects are passed by reference
a.foo = [ 1, 2, 3 ];
b.foo = a.foo;
b.foo[ 0 ] = 'bar';

a.foo; // [ 'bar', 2, 3 ]
b.foo; // [ 'bar', 2, 3 ]

// reassigning the property only changes the reference on THAT object
b.foo = {};
a.foo; // [ 'bar', 2, 3 ]

// deleting only removes the reference on that object
delete b.foo;
b.foo; // undefined
a.foo; // [ 'bar', 2, 3 ]