A simple way to understand closures...

closures-js

// a simple function to display a greeting
var sayHello = function() {
  return alert('Hello there!');
};

sayHello(); // => Hi there!

// ----------------------------------------

// function returns an object, text preset
var sayHello = function() {
  var text = 'Hello there!';

  return {
    speak: function() { return alert(text); }
  };
};

var obj = new sayHello;
obj.speak() // => Hi there!

// ----------------------------------------

var sayHello = function() {
  var text = 'Hello there!';
  
  return {
    beNice: text = "I am nice!",
    beGrim: text = "Fuck off!",
    speak: function() { return alert(text); }
  };
};

// modify function to close over the variable text which can only be set thru setter methods

var obj = new sayHello;
obj.speak(); // => Hi there!
obj.beNice;
obj.speak(); // => I am nice!
obj.beGrim;
obj.speak(); // => Fuck off!

// the variable is only modifiable through the exposed api and therefore the internal state is much controlled.
// the principle behind closures...