Syntax this

Call and apply

function add(c, d) {
  return this.a + this.b + c + d;
}

var o = {a: 1, b: 3};

// The first parameter is the object to use as
// 'this', subsequent parameters are passed as 
// arguments in the function call
console.log(add.call(o, 5, 7)); // 1 + 3 + 5 + 7 = 16

// The first parameter is the object to use as
// 'this', the second is an array whose
// members are used as the arguments in the function call
console.log(add.apply(o, [10, 20])); // 1 + 3 + 10 + 20 = 34

The bind method.

function f() {
  return this.a;
}

var g = f.bind({a: 'azerty'});
console.log(g()); // azerty

var h = g.bind({a: 'yoo'}); // bind only works once!
console.log(h()); // azerty

var o = {a: 37, f: f, g: g, h: h};
console.log(o.f(), o.g(), o.h()); // 37, azerty, azerty

This on the object's prototype chain

var o = {f: function() { return this.a + this.b; }};
var p = Object.create(o);
p.a = 1;
p.b = 4;

console.log(p.f()); // 5