用JavaScript实现单例模式的最简单/最干净的方法是什么?
当前回答
我相信这是最简单/最干净和最直观的方式,尽管它需要ECMAScript 2016 (ES7):
导出默认类Singleton { 静态实例; 构造函数(){ 如果(){ 返回实例; } 这一点。State = "公爵"; 这一点。实例= this; } }
源代码来自:adam-bien.com
其他回答
最明确的答案应该是Addy Osmani所著的《学习JavaScript设计模式》一书。
var mySingleton = (function () { // Instance stores a reference to the singleton var instance; function init() { // Singleton // Private methods and variables function privateMethod(){ console.log( "I am private" ); } var privateVariable = "I'm also private"; var privateRandomNumber = Math.random(); return { // Public methods and variables publicMethod: function () { console.log( "The public can see me!" ); }, publicProperty: "I am also public", getRandomNumber: function() { return privateRandomNumber; } }; }; return { // Get the singleton instance if one exists // or create one if it doesn't getInstance: function () { if ( !instance ) { instance = init(); } return instance; } }; })();
在Node.js版本6中工作:
class Foo {
constructor(msg) {
if (Foo.singleton) {
return Foo.singleton;
}
this.msg = msg;
Foo.singleton = this;
return Foo.singleton;
}
}
我们测试:
const f = new Foo('blah');
const d = new Foo('nope');
console.log(f); // => Foo { msg: 'blah' }
console.log(d); // => Foo { msg: 'blah' }
这个知识是基于我正在学习Java,虽然Java和Javascript是不同的,但单例的概念和Java如何做到这一点是一样的。在我看来,JS的类样式本身是干净的,而不是var初始化。
class Singleton {
// use hashtag which entails that the variable can only be accessed from self scope
static #instance = null;
static getInstance() {
if (this.#instance === null) this.#instance = new Singleton();
return this.#instance;
}
// some class property
hello = 'world';
// or initialize the variable in the constructor, depend on your preference
constructor() {
// this.hello = 'world';
}
/* you can also add parameters on the constructor & getInstance
* e.g.
* static getInstance(param1, param2) {...new Singleton(param1, param2)}
* constructor(param1, param2) {...}
*/
}
// this is the same code for java and normal way for singleton for class
// just use static so you can get instance
// testing the singleton
var s1,s2;
s1 = Singleton.getInstance();
s2 = Singleton.getInstance();
// you cannot access the property, immediately
if (Singleton.hello === undefined) console.log('getInstance so you can access this');
console.log(s1.hello);
// result: "world"
console.log(s2.hello);
// result: "world"
// set the value of Singleton object
s2.hello = "hi";
console.log(s1.hello);
// result: "hi"
console.log(s2.hello);
// result: "hi"
// this is just an evidence which means that they are the same even in property level
if (s1 === s2) console.log("S1 & S2 is the same object");
// result: "S1 & S2 is the same object"
// don't use something like `var s1 = new Singleton();`
// this will defeat your purpose of just (1 object), one instance of class
使用ES6类和私有静态字段。调用Singleton类的新实例将返回相同的实例。实例变量也是私有的,不能在类外部访问。
class Singleton {
// # is a new Javascript feature that denotes private
static #instance;
constructor() {
if (!Singleton.#instance) {
Singleton.#instance = this
}
return Singleton.#instance
}
get() {
return Singleton.#instance;
}
}
const a = new Singleton();
const b = new Singleton();
console.log(a.get() === b.get()) // true
console.log(Singleton.instance === undefined) // true
function Unicode()
{
var i = 0, unicode = {}, zero_padding = "0000", max = 9999;
// Loop through code points
while (i < max) {
// Convert decimal to hex value, find the character,
// and then pad zeroes to the code point
unicode[String.fromCharCode(parseInt(i, 16))] = ("u" + zero_padding + i).substr(-4);
i = i + 1;
}
// Replace this function with the resulting lookup table
Unicode = unicode;
}
// Usage
Unicode();
// Lookup
Unicode["%"]; // Returns 0025
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