Closure.js

var addNumber=function(x){
  //first class function : a function treated as an object
  return function(y){
      //x is a free variable. (i.e. not a local variable or an argument of this function.)
      //x is bound to the argument x in lexical environment defined by the addNumber function
      return x+y;
  };
};

jQuery Click event.js

var foo="bar";
$("#aButton").click(function(){//passing a function in as an object (i.e. first class function)
  alert(foo);//foo is bound in the global lexical environment
});

Module Pattern.js

var myModule=(function(){
  var x=1;
  return getX(){//return a function as an object (i.e. first class function)
    return x;//x is bound to the lexical environment defined by the outer function
  };
})();