上一篇文章實(shí)現(xiàn)了Promise最基礎(chǔ)功能辽社，本文將實(shí)現(xiàn)promise核心的鏈?zhǔn)秸{(diào)用

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

class MyPromise {
   callBacks = [];// {onFulfilled, resolve}組合push到這個(gè)數(shù)組中
   state = PENDING_STATUS;// state初始狀態(tài)
   value = null;// 保存resolve傳遞過(guò)來(lái)的參數(shù)

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

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

    handle = (cb) => {
        if(this.state === PENDING_STATUS) { // 異步的話就先存著刺彩，后面再執(zhí)行
            this.callBacks.push(cb);
            return;
        }
        if (!cb.onFullfilled) {// onFullfilled如果是空渤昌，就把結(jié)果直接傳給下一個(gè)then
            cb.resolve(this.value);
            return;
        }
        var ret = cb.onFullfilled(this.value);
        cb.resolve(ret);// 鏈?zhǔn)秸{(diào)用的關(guān)鍵，把返回結(jié)果傳遞給下一個(gè)resolve
        
    }

    resolve = (value) => {
        // resolve傳過(guò)來(lái)的，除了數(shù)值，還有可能是上一個(gè)then返回的promise驹愚，下面這個(gè)if主要處理這種情況
        if (value && (typeof value === 'object' || typeof value === 'function')) {
            var then = value.then;
            if (typeof then === 'function') {
                then.call(value, this.resolve); // 如果是一個(gè)promise，則調(diào)用它的then
                return;
            }
        }
        this.state = FULFILLED_STATUS;
        this.value = value;
        this.callBacks.forEach(cb => {
            this.handle(cb);
        });
    }
}

// 鏈?zhǔn)秸{(diào)用的使用舉例
const promise = new MyPromise((resolve) => {
    setTimeout(() => {
        resolve('第一個(gè)結(jié)果');
    },3000);
})
.then((result) => {
    return new MyPromise((resolve) => {
        setTimeout(() => {
            resolve(result + ' 第二個(gè)結(jié)果');
    }, 3000);
    });
})
.then((result) => {
    return new MyPromise((resolve) => {
        setTimeout(() => {
            resolve(result + ' 第三個(gè)結(jié)果');
    }, 3000);
    });
}).then((result) => {
    console.log(result);
});```