Javascript Closures

closures.js

/**
 * Be careful of closure behaviour, it might not give you values you expect!
 * For example when you have timeouts or event listeners. 
 */
(function(){
  var i = 0,
      arr = ['a','b','c','d'],
      doSomething = function(localKey){
        console.log('i:', i);
        console.log('localKey:', localKey);
      };
      
  arr.forEach(function(value, key){
    i = key;
    setTimeout(function(){
      console.log('key:', key);
      doSomething(key);
    }, 1000);
  });
})();


/* Output
 
key: 0 
i: 3 
localKey: 0

key: 1 
i: 3 
localKey: 1 

key: 2 
i: 3 
localKey: 2 

key: 3 
i: 3 
localKey: 3 
*/