undefined in ECMAScript 262

The static value "undefined"

Any variable which has been declared but not assigned a value, or a named property of an object which has not been assigned a value, has the value undefined.

While undefined values do compare as equal to the value null, the two are not the same. To specifically test if a value is undefined rather than null, use the typeof operator:

var foo;
var bar = {};
var someNullValue = null;

foo==null;                              // ** true
foo==someNullValue;                     // ** true
typeof(foo);                            // ** 'undefined'
typeof(someNullValue);                  // ** 'object'
typeof(bar.jimmyJammy)=='undefined';    // ** true

if (typeof(foo)=='undefined'){
	// do something here
}