1, in js, when a string is''(empty string), its value is false 
//var str = null;
var str ='';

alert(isNull(str));

/**
 * if a str is space or null return true
 * esle return false;
 */
function isNull(str) {
	if(!str){//when str = '', it's false
		return true;
	}
	if (str.length > 0) {
		return false;
	} else {
		return true;
	}
}