es5實現簡單 Promise ，手動實現 簡單 Promise

function MyPromise(callback) {
    this.subs = [];
    // 這裏的 resolve 和 reject 需要手動綁定 this ,
    //要不然，resolve函數只是一個函數，
    //綁定不到當前 MyPromise 的實例
    callback(this.resolve.bind(this), this.reject.bind(this))
}
MyPromise.prototype.resolve = function (data) {
    if (this.subs && this.subs.length) {
        this.subs.shift()(data)
    }else{
        if(this.finalyFn){
            this.finalyFn('resolved')
        }
    }
}

MyPromise.prototype.reject = function (data) {
    this.subs = [];
    if(this.finalyFn){
        this.finalyFn('rejected')
    }
}
MyPromise.prototype.then = function (fn) {
    this.subs.push(fn);
    // 這裏，需要 return this ，保證當前調用者仍是 當前的 MyPromise 實例
    return this;
}
MyPromise.prototype.finaly = function (fn) {
    this.finalyFn = fn
}
var a = new MyPromise(function (resolve, reject) {
    setTimeout(function () {
        console.log('resolve')
        resolve(1)
    }, 1250)
})
a.then(function (data) {
    setTimeout(function () {
        console.log(data)
        a.resolve('then---1')
    }, 2000)
}).then(function (data) {
    setTimeout(function () {
        console.log(data)
        a.reject('reject')
    }, 1000)
}).then(function (data) {
    setTimeout(function () {
        console.log(data)
    }, 1000)
})