promise

https://www.jianshu.com/p/c633a22f9e8c

function Promise(executor) {
    var _this = this;
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;

    executor(resolve, reject); //马上执行
    
    function resolve(value) {}
    function reject(reason) {}

    this.onFulfilledFunc = [];//保存成功回调
    this.onRejectedFunc = [];//保存失败回调

}

    function resolve(value) {
        if (_this.state === 'pending') {
            _this.value = value;
            //依次执行成功回调
            _this.onFulfilledFunc.forEach(fn => fn(value));
            _this.state = 'resolved';
        }

    }

    function reject(reason) {
        if (_this.state === 'pending') {
            _this.reason = reason;
            //依次执行失败回调
            _this.onRejectedFunc.forEach(fn => fn(reason));
            _this.state = 'rejected';
        }
    }

Promise.prototype.then = function (onFulfilled, onRejected) {
    //等待态，此时异步代码还没有走完
    if (this.state === 'pending') {
        if (typeof onFulfilled === 'function') {
            this.onFulfilledFunc.push(onFulfilled);//保存回调
        }
        if (typeof onRejected === 'function') {
            this.onRejectedFunc.push(onRejected);//保存回调
        }
    }
    //其它代码略...
};

Promise.prototype.then = function (onFulfilled, onRejected) {
    var promise2 = new Promise((resolve, reject) => {
    //代码略...
    }
    return promise2;
};