Examples of closure courtesy of Effective JavaScript by David Herman

Closure.js

//examples courtesy of Effective JavaScript by David Herman

//functions can refer to variables defined anywhere in that function's scope chain
function makeSandwich(){
  var magicIngredient = 'peanut butter'; 

  function make(filling){
    return magicIngredient + ' and ' + filling; 
  }
  return make('jelly'); 
}

makeSandwich();       // 'peanut butter and jelly'

/*
Here's what's happening above:
the inner make function refers to magicIngredient, a variable 
defined in the outer makeSandwich function. When makeSandwich is 
invoked on line 14, it returns the inner make function
*/

//functions can refer to variables defined in outer functions
//even after those outer functions have returned
function sandwichMaker() {
  var magicIngredient = 'peanut butter'; 
  function make(filling){
    return magicIngredient + ' and ' + filling; 
  }
  return make;
}

var f = sandwichMaker(); 
f('jelly');     // 'peanut butter and jelly'


//we can make sandwichMaker more general: 
function sandwichMaker(magicIngredient) {
  function make(filling){
    return magicIngredient + ' and ' + filling; 
  }
  return make; 
}

var turkeyAnd = sandwichMaker('turkey'); 
turkeyAnd('cheese')   // 'turkey and cheese'
turkeyAnd('tomatoes') // 'turkey and tomatoes'