step1.js

class myPromise {
    static PENDING = 'pending';
    static FULFILLED = 'fulfilled';
    static REJECTED = 'rejected';
    constructor(func) {
        this.PromiseState = myPromise.PENDING;
        this.PromiseResult = null;
        this.onFulfilledCallbacks = []; // 保存成功回调
        this.onRejectedCallbacks = []; // 保存失败回调
        try {
            func(this.resolve.bind(this), this.reject.bind(this));
        } catch (error) {
            this.reject(error)
        }
    }
    resolve(result) {
        if (this.PromiseState === myPromise.PENDING) {
            setTimeout(() => {
                this.PromiseState = myPromise.FULFILLED;
                this.PromiseResult = result;
                this.onFulfilledCallbacks.forEach(callback => {
                    callback(result)
                })
            });
        }
    }
    reject(reason) {
        if (this.PromiseState === myPromise.PENDING) {
            setTimeout(() => {
                this.PromiseState = myPromise.REJECTED;
                this.PromiseResult = reason;
                this.onRejectedCallbacks.forEach(callback => {
                    callback(reason)
                })
            });
        }
    }
    then(onFulfilled, onRejected) {
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
        onRejected = typeof onRejected === 'function' ? onRejected : reason => {
            throw reason;
        };
        if (this.PromiseState === myPromise.PENDING) {
            this.onFulfilledCallbacks.push(onFulfilled);
            this.onRejectedCallbacks.push(onRejected);
        }
        if (this.PromiseState === myPromise.FULFILLED) {
            setTimeout(() => {
                onFulfilled(this.PromiseResult);
            });
        }
        if (this.PromiseState === myPromise.REJECTED) {
            setTimeout(() => {
                onRejected(this.PromiseResult);
            });
        }
    }
}


// 测试代码
let p1 = new myPromise((resolve, reject) => {
    resolve(10)
})
p1.then(res => {
    console.log('fulfilled', res);
    return 2 * res
}).then(res => {
    console.log('fulfilled', res)
}) 

// [referer](https://juejin.cn/post/7043758954496655397)