上一篇文章实现了Promise最基础功能，本文将实现promise核心的链式调用

const PENDING_STATUS = 'pending';
const FULFILLED_STATUS = 'fulfilled';

class MyPromise {
   callBacks = [];// {onFulfilled, resolve}组合push到这个数组中
   state = PENDING_STATUS;// state初始状态
   value = null;// 保存resolve传递过来的参数

    constructor(fn) {
        fn(this.resolve)
    }

    then = (onFullfilled) => {
        return new MyPromise((resolve) => {
            this.handle({
                onFullfilled,
                resolve
            });
        });
    }

    handle = (cb) => {
        if(this.state === PENDING_STATUS) { // 异步的话就先存着，后面再执行
            this.callBacks.push(cb);
            return;
        }
        if (!cb.onFullfilled) {// onFullfilled如果是空，就把结果直接传给下一个then
            cb.resolve(this.value);
            return;
        }
        var ret = cb.onFullfilled(this.value);
        cb.resolve(ret);// 链式调用的关键，把返回结果传递给下一个resolve
        
    }

    resolve = (value) => {
        // resolve传过来的，除了数值，还有可能是上一个then返回的promise，下面这个if主要处理这种情况
        if (value && (typeof value === 'object' || typeof value === 'function')) {
            var then = value.then;
            if (typeof then === 'function') {
                then.call(value, this.resolve); // 如果是一个promise，则调用它的then
                return;
            }
        }
        this.state = FULFILLED_STATUS;
        this.value = value;
        this.callBacks.forEach(cb => {
            this.handle(cb);
        });
    }
}

// 链式调用的使用举例
const promise = new MyPromise((resolve) => {
    setTimeout(() => {
        resolve('第一个结果');
    },3000);
})
.then((result) => {
    return new MyPromise((resolve) => {
        setTimeout(() => {
            resolve(result + ' 第二个结果');
    }, 3000);
    });
})
.then((result) => {
    return new MyPromise((resolve) => {
        setTimeout(() => {
            resolve(result + ' 第三个结果');
    }, 3000);
    });
}).then((result) => {
    console.log(result);
});```