手写promise

// 本来的编写方法
const promise = new promise((reslove,reject)=>{})
// 手写一个promise
class Commitment {
    // 状态 PENDING FULFILLED REJECTED,三种状态分别为待定成功拒绝，并且三种方法不能逆转
    static PENDING = '待定'; static FULFILLED = '成功'; static REJECTED = '拒绝';
    constructor(func) {
        this.status = Commitment.PENDING //开始时进入的状态是待定状态
        this.result = null
        this.resloveCallbacks = []
        this.rejectCallbacks = []
        // 然后传入的参数函数方法，这里使用bind方法是因为类原型方法中使用了this.status，但是在函数外部并没有定义这个变量，因此要改变当前this指向
        try {
            func(this.reslove.bind(this), this.reject.bind(this))
        } catch (error) {
            this.reject(error)
        }
    }
    // 成功时执行的方法
    reslove(result) {
        setTimeout(() => {
            if (this.status === Commitment.PENDING) {
                this.status = Commitment.FULFILLED
                this.result = result
                this.resloveCallbacks.forEach(callback => {
                    callback(result)
                })
            }
        })
    }
    // 拒绝时执行的方法
    reject(result) {
        setTimeout(() => {
            if (this.status === Commitment.PENDING) {
                this.status = Commitment.REJECTED
                this.result = result
                this.rejectCallbacks.forEach(callback => {
                    callback(result)
                })
            }
        })
    }
    then(ONFILFILLED, ONREJECTED) {
        return new Commitment((reslove, reject) => {
            ONFILFILLED = typeof (ONFILFILLED) === 'function' ? ONFILFILLED : () => { }
            ONREJECTED = typeof (ONREJECTED) === 'function' ? ONREJECTED : () => { }
            if (this.status === Commitment.PENDING) {
                this.resloveCallbacks.push(ONFILFILLED)
                this.rejectCallbacks.push(ONREJECTED)
            }
            if (this.status === Commitment.FULFILLED) {
                setTimeout(() => {
                    ONFILFILLED(this.result)
                })
            }
            if (this.status === Commitment.REJECTED) {
                setTimeout(() => {
                    ONREJECTED(this.result)
                })
            }
        })
    }
}
console.log(1);
const a = new Commitment(function (reslove, reject) {
    // throw new Error('抛出错误')
    console.log(2);
    setTimeout(() => {
        reslove(4)
        console.log(5);
    })
})
a.then(
    result => console.log(result),
    result => { console.log(result.message); }
)
console.log(3);