js单例模式

function Sequence(){
window.singleSequence;//这样定义很不友好，没找到好的方法，所以先这样了
if (window.singleSequence === undefined) {
window.singleSequence = (function createGenerator(){
// 缓存单例的变量
var instance;
function next(){
if (instance === undefined) {
instance = new Construct();
}
return instance.start++;
}
function Construct(){
this.start =1;
}
return {
next: next
}})();
}
return window.singleSequence;
}