实现一个简易的promise

//promise里面只有三个状态，且三个状态的转换形式有两种
//由pending转换为fulfilled，由pending转换为rejected

//Promise的构造函数参数是一个函数，函数的参数分别为resolve和reject，两者也均为一个函数
//then中是实际要执行的函数，将传递进来的值传给resolve和reject对应的参数
const PENDING = 'PENDING'
const FULFILLED = 'FULFILLED'
const REJECTED = 'REJECTED'
class YPromise {
    constructor(cb) {
        this.state = PENDING;
        this.value = null;
        //承诺完成的回调列表
        this.fulfilledCbs = [];
        //承诺被拒绝的回调列表
        this.rejectedCbs = [];

        let resolve = data => {
            setTimeout(() => {
                if (this.state !== PENDING) return;
                //更改状态
                this.state = FULFILLED;
                this.value = data;
                this.fulfilledCbs.forEach(c => {
                    this.value = c(this.value);
                })

            }, 0);

        }
        let reject = reason => {
            setTimeout(() => {
                if (this.state !== PENDING) return;
                this.state = REJECTED;
                this.reason = reason;
                this.rejectedCbs.forEach(c => {
                    this.reason = c(this.reason);
                })

            }, 0);
        }
        cb(resolve, reject);
    };
    then(onFulfilled, onRejected) {
        if (typeof onFulfilled === 'function') {
            this.fulfilledCbs.push(onFulfilled);
        }
        if (typeof onRejected === 'function') {
            this.rejectedCbs.push(onRejected);
        }
        return this;//返回整个构造函数可以继续调用then方法
    }
}

let promise = new YPromise((resolve, reject) => {
    if (4 > 1) {
        resolve('hi');
    } else {
        reject(4大于1')
    }
})

fulfilledCbs = [data => data + 'world']
promise.then(data => {
    return data + ' world';
}).then(data => {
    return data + '!';
}).then(data => {
    console.log(data);
})