Promise的实现

class Promise {
    constructor(executor) {
        this.status = 'pending';
        this.value = undefined;
        this.reason = undefined;
        //存放成功的回调
        this.onFulFilledCallbacks = [];
        //存放失败的回调
        this.onRejectedCallbacks = [];

        let resolve = (value) => {
            if (this.status == 'pending') {
                this.status = 'fulfilled';
                this.value = value;
                this.onFulFilledCallbacks.forEach(fn => fn())
            }
        }

        let reject = (err) => {
            if (this.status == 'pending') {
                this.status = 'rejected';
                this.reason = err;
                this.onRejectedCallbacks.forEach(fn => fn())
            }
        }

        try {
            executor(resolve, reject)
        } catch (e) {
            reject(e)
        }

    }

    then(onFulFilled, onRejected) {
        let promise2
        if (this.status = 'fulfilled') {
            promise2 = new Promise((resolve, reject) => {
                let x = onFulFilled(this.value);
                //解析promise2和x之间的关系
                resolvePromise(promise2, x, resolve, reject)
            })
        }
        if (this.status = 'rejected') {
            promise2 = new Promise((resolve, reject) => {
                let x = onRejected(this.reason);
                resolvePromise(promise2, x, resolve, reject)
            })
        }
        //异步调用时进不到resolve，状态始终为pending，此时可以先把callback存起来待到resolve或reject再逐一执行
        if (this.status = 'pending') {
            this.onFulFilledCallbacks.push(() => {
                promise2 = new Promise((resolve, reject) => {
                    let x = onFulFilled(this.value)
                    resolvePromise(promise2, x, resolve, reject)
                })
            })
            this.onRejectedCallbacks.push(() => {
                promise2 = new Promise((resolve, reject) => {
                    let x = onRejected(this.reason)
                    resolvePromise(promise2, x, resolve, reject)
                })
            })
        }
        return promise2;
    }
}

//判断x是不是promise，规范规定代码确定不同的promise间可以交互。
resolvePromise(promise2, x, resolve, reject) {
    if (x === promise2) {
        return reject(new TypeError('Chaining cycle detected for promise!'))
    }
    if (x !== null && typeof x == 'object' || typeof x == 'function') {
        //防止取then时出现异常 Object.definedProperty
        try {
            if (typeof x == 'function') {
                x.then.call(x, y => {
                    resolvePromise(promise2, y, resolve, reject)
                })
            } else {
                resolve(x)
            }
        } catch (e) {
            reject(e)
        }
    } else {
        resolve(x)
    }
}