this

Simple example of 'this' changing context.

var obj = {
  text: 'hello',
  show: function show(){
    console.log(this.text);
  }
}

obj.show();//hello

var text = 'me';

//potential problem as the behaviour is unexpected most the time
setTimeout(obj.show, 100);//'me' this binding is to the current scope as it is that called as a object method

var funcObj = obj.show
funcObj();//'me' this binding is to the current scope same problem as above

//to fix this you can bind 'this' to the object you want this to be
var funcObjBound = obj.show.bind(obj)
funcObjBound();//hello

Prototypical inheritance example. Just showing that context of 'this' changes.

//simple constructor
function Testa(a,b,c){
  this.a = a;
  this.b = b;
  this.c = c;
}

//add a prototype function to Testa
Testa.prototype.blurb = function(){
  return console.log(this.a,this.b,this.c);
}

//another constructor that 'inherits' from Testa
function Testb(a,b,c,d,e){
  this.d = d;
  this.e = e;

  //call the TestA as a function but giving 'this' as the context
  //so you are effectively assigning the a,b,c to 'this' TestB object
  Testa.call(this, a,b,c);
}

//setup your prototype chain
Testb.prototype = new Testa(); //could do Testb.prototype = Object.create(Testa.prototype); creates a new object with the 'Testa.prototype' as the prototype
Testb.prototype.constructor = Testb; //see reference below


var objectB = new Testb(1,2,3,4,5);
console.log(objectB); //
console.log(objectB instanceof Testb,' instance of TestB'); // true instance of TestB
console.log(objectB instanceof Testa, ' instance of TestA'); // true instance of TestA
console.log(typeof objectB, 'typeOf'); //object typeOf

objectB.blurb();//1,2,3 which is the a,b,c from the testB object

For more reading see:

this

Prototype Constructor

Instance Of