[JavaScript] undefined, null, empty value

Empty value

var car = "";                // The value is "", the typeof is string

Null vs Undefined

var person = null;           // Value is null, but type is still an object
var person = undefined;     // Value is undefined, type is undefined
 
typeof undefined             // undefined
typeof null                  // object
null === undefined           // false
null == undefined            // true

출처 : http://www.w3schools.com/js/js_datatypes.asp