js单例模式

js实现单例模式

1.最简单的方式

let SingleTest=(function () {
    let _instance = null;
    return function () {
        if(_instance==null){
            _instance=this;
        }
        return _instance;
    }
})();

let i1=new SingleTest();
let i2=new SingleTest();
console.log(i1===i2);  结果返回 true

2.如果要求加上参数

let SingleTest=(function () {
    let _instance = null;
    function _init(ops) {
        for(let i in ops){
            this[i]=ops[i];
        }
    }
    return function (args) {
        if(_instance==null){
            _instance=this;
        }
        _init.call(_instance,args);  如果是一个单纯的函数，调用call方法需要将对象传递进去，这样就相当于，实例方法了 return _instance;            但是这样做有点不好看，可以使用 原型链将 _init函数变成实例方法
    }
})();

let i1=new SingleTest({name:"escapist1"});
let i2=new SingleTest({name:"escapist3"});
console.log(i1===i2);  结果是true
console.log(i1.name);  结果是escapist3

3.改进一下  不使用call方法来

let SingleTest=(function () {
    let _instance = null;
    SingleInstance.prototype._init=function (ops) {
        for(let i in ops){
            this[i]=ops[i];
        }
    };
    function SingleInstance(args) {
        if(_instance==null){
            _instance=this;
        }
        _instance._init(args);
        return _instance;
    }
    return SingleInstance;
})();

let i1=new SingleTest({name:"escapist1"});
let i2=new SingleTest({name:"escapist3"});
console.log(i1===i2);  结果是true
console.log(i1.name);  结果是escapist3

 4.如果给出的参数没有就用默认值,有就用给出的let SingleTest = (function () {    let _instance = null;

let SingleTest = (function () {
    let _instance = null;
    let _default = {age: 25};
    SingleInstance.prototype._init = function (ops) {
        for(let j in _default){
            this[j]=_default[j];
        }
        for (let i in ops) {
            this[i] = ops[i];
        }
    };
    function SingleInstance(args) {
        if (_instance == null) {
            _instance = this;
        }
        _instance._init(args);
        return _instance;
    }
    return SingleInstance;
})();

let i1 = new SingleTest({name: "escapist1"}); 
let i2 = new SingleTest({name: "escapist3"}); 
console.log(i1 === i2); 结果是 true 
console.log(i1.name);　 结果是 escapist3
console.log(i1.age);    结果是 25

5.还有最后一个问题，有的人不适用new来创建对象，而是使用 let i1=SingleTest() 这样来创建对象 

这种情况下，怎样保证只有一个实let SingleTest = (function () {

let SingleTest = (function () {
    let _instance = null;
    let _default = {age: 25};
    SingleInstance.prototype._init = function (ops) {
        for (let a in _default) {
            this[a] = _default[a];
        }
        for (let b in ops) {
            this[b] = ops[b];
        }
    };
    function SingleInstance(args) {
        if (this instanceof SingleInstance) {
            if (_instance == null) {
                _instance = this;
            }
            _instance._init(args);
            return _instance;
        }else {
            if(_instance==null){
                _instance=new SingleInstance;
            }
　　　　　　　 _instance._init(args);
            return _instance;
        }
    }
    return SingleInstance;
})();

let i1 = new SingleTest({name: "escapist1"});
let i2 = new SingleTest({name: "escapist3"});
let i3 = SingleTest({name: "escapist3"});
console.log(i1 === i2);  结果是true
console.log(i1 === i3);  结果是true
console.log(i1.name);    结果是escapist3
console.log(i1.age);       结果是2

至此js完整的单例模式改造完毕