手把手写出一个promise

const PENDING = 'PENDING'
const FULLFILLED = 'FULLFILLED'
const REJECTED = 'REJECTED'

class Promise {
    constructor(executor) {
        this.status = PENDING;
        this.value = undefined;
        this.reason = undefined;
        this.onFulfilledCallbacks = [] // 保存成功回调
        this.onRejectedCallbacks = [] // 保存失败回调

        try {
            executor(this.resolve, this.reject);
        } catch (e) {
            this.reject(e)
        }
    }

    resolve(value) {
        if (this.status === PENDING) return;
        this.status = FULLFILLED;
        this.value = value;
        while (this.onFulfilledCallbacks.length) {
            this.onFulfilledCallbacks.shift()(this.PromiseResult)
        }
    }

    reject(reason) {
        if (this.status !== PENDING) return;
        this.status = REJECTED;
        this.reason = reason;
        while (this.onRejectedCallbacks.length) {
            this.onRejectedCallbacks.shift()(this.PromiseResult)
        }
    }

    then(onFulfilled, onRejected) {
        // 接收两个回调 onFulfilled, onRejected

        // 参数校验，确保一定是函数
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : val => val
        onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason }

        if (this.status === 'fulfilled') {
            // 如果当前为成功状态，执行第一个回调
            onFulfilled(this.PromiseResult);
        } else if (this.status === 'rejected') {
            // 如果当前为失败状态，执行第二哥回调
            onRejected(this.PromiseResult);
        } else if (this.status === 'pending') {
            // 如果状态为待定状态，暂时保存两个回调
            this.onFulfilledCallbacks.push(onFulfilled.bind(this))
            this.onRejectedCallbacks.push(onRejected.bind(this))
        }

    }

}