JavaScript passed by value or reference

gistfile1.js

// primitives are passed by value
var a = 4;
function fn1(val) {
  val = 5;
} 
fn1(a);
alert(a) // still 4

// objects are passed by reference
var o = {a: 4};
function fn2(obj) {
  obj.a = 5;
}
fn2(o);
alert(o.a) // changed to 5

// functions loose their context
var o = {a: 4};
o.prototype.change = function () {
  this.a = 5;
}

function f3(fn) {
  fn();
}

f3(o.change);
o.a // still 4 because change executed in different context