> For the complete documentation index, see [llms.txt](https://sugar-at.gitbook.io/blog-article/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sugar-at.gitbook.io/blog-article/design/singleton.md).
# 单例模式
## 箭头函数写法
```javascript
const singleton = function () { };
singleton.getInstance = (() => {
let instance = null;
return () => {
if (!instance) {
instance = new singleton();
}
return instance;
}
})()
const s1 = singleton.getInstance();
const s2 = singleton.getInstance();
console.log(s1 === s2);//true
```
## function写法
```javascript
singleton.getInstance=(function(){
let instance = null;
return function(){
if(!instance){
instance = new singleton();
}
return instance;
}
})();
const s1 = singleton.getInstance();
const s2 = singleton.getInstance();
console.log(s1===s2);
```
## class写法
```javascript
class Singleton{
static getInstance(){
if(!Singleton.instance){
Singleton.instance = new Singleton();
}
return Singleton.instance;
}
}
let s1 = Singleton.getInstance();
let s2 = Singleton.getInstance();
console.log(s1 === s2);//true
```
## 实例1
实现一个 Storage
**①Storage--class实现**
```javascript
class Storage {
static getInstance() {
if (!Storage.instance) {
Storage.instance = new Storage();
}
return Storage.instance;
}
setItem(key, value) {
return localStorage.setItem(key, value);
}
getItem(key) {
return localStorage.getItem(key);
}
}
//调用
let s1 = Storage.getInstance();
s1.setItem("a", 666);
let s2 = Storage.getInstance();
console.log(s2.getItem("a"));
```
**②闭包实现**
```javascript
// 基础的StorageBase类，把getItem和setItem方法放在它的原型链上
function storageBase() {
storageBase.prototype.getItem = function(key) {
return localStorage.getItem(key);
}
storageBase.prototype.setItem = function(key, value) {
return localStorage.setItem(key, value);
}
}
const Storage = (function() {
let instance = null;
return function() {
if (!instance) {
instance = new storageBase();
}
return instance;
}
})()
let s3 = new Storage();
console.log(s3.getItem('a'));
```
## 实例2
实现一个全局唯一的Modal弹框
index.html
```markup
Document
```
modal.js
```javascript
class Modal {
static getInstance() {
if (!Modal.instance) {
let modal = document.createElement('div');
modal.innerHTML = '全局弹窗';
Modal.instance = modal;
modal.id = 'my-modal';
modal.style.display = 'none';
document.body.appendChild(modal);
}
return Modal.instance;
}
}
```
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