promise原理解析

const PENDING = 'pending'; // 等待
const FULFILLED = 'fulfilled'; // 成功
const REJECTED = 'rejected'; // 失败

class MyPromise {
    constructor (executor) {
        try {
            executor(this.resolve, this.reject)
        }catch (e) {
            this.reject(e)
        }
    }
    // promise 状态
    status = PENDING;
    // 成功之后的值
    value = undefined
    // 失败后的原因
    reason = undefined
    // 成功回调
    successCallback = []
    // 失败回调
    failCallback = []
    resolve = value => {
        // 如果状态不是等待 阻止程序向下执行
        if(this.status !== PENDING) return;
        // 将状态更改为成功
        this.status = FULFILLED;
        // 保存成功之后的值
        this.value = value
        // 判断成功回调是否存在 如果存在 调用
        // this.successCallback && this.successCallback(this.value)
        while(this.successCallback.length) this.successCallback.shift()(this.value)
    }
    reject = reason => {
        // 如果状态不是等待 阻止程序向下执行
        if(this.status !== PENDING) return;
        // 将状态更改为失败
        this.status = REJECTED;
        // 保存失败之后的原因
        this.reason = reason
        // 判断失败回调是否存在 如果存在 调用
        // this.failCallback && this.failCallback(this.reason)
        while(this.failCallback.length) this.failCallback.shift()(this.reason)
    }
    then (successCallback, failCallback) {
            successCallback = successCallback ? successCallback : value => value
            failCallback = failCallback ? failCallback : reason => reason
            let promise2 = new MyPromise((resolve, reject) => {
            // 判断状态
            if(this.status === FULFILLED) {
                setTimeout(() => {
                    try {
                        let x = successCallback(this.value)
                        // 判断 x 的值是普通值还是promise对象
                        // 如果是普通值 直接调用resolve
                        // 如果是promise对象 查看promise对象返回的结果
                        // 再根据promise对象返回的结果 决定调用resolve 还是调用reject
                        // resolve(x)
                        resolvePromise (promise2, x, resolve, reject)
                    } catch (e) {
                        reject(e)
                    }
                }, 0)
            }else if (this.status === REJECTED) {
                failCallback(this.reason)
            } else {
                // 等待
                // 将成功回调与失败回调存储起来
                this.successCallback.push(successCallback)
                this.failCallback.push(failCallback)
            }
        });
        return promise2;
    }
    finally ( callback ) {
        return this.then(value => {
            return MyPromise.resolve(callback()).then(() => value)
        }, reason => {
            return MyPromise.resolve(callback()).then(() => { throw reason })
        })
    }
    catch (failCallback) {
        return this.then(undefined, failCallback)
    }
    static all (array) {
        let result = []
        let index = 0;
        return new MyPromise((resolve, reject){
            function addData(key, value) {
                result[key] = value
                index++;
                if(index === array.length) {
                    resolve(result);
                }
            }
            for(let i = 0; i<array.length; i++) {
                let current = array[i];
                if(current instanceof MyPromise) {
                    // promise 对象
                    current.then(value => addData(i,value), reason => reject(reason))
                }else {
                    // 普通值
                    addData(i, array[i])
                }
            }
            resolve(result)
        })
    }
    static resolve (value) {
        if(value instanceof MyPromise) return value;
        return new MyPromise(resolve => resolve(value))
    }
}

function resolvePromise (x, resolve, reject) {
    if(promise2 === x) {
        return reject(new TypeError('Chaining cycle detectd for promse #<Promise>'))
    }
    if (x instanceof MyPromise) {
        // promise 对象
        // x.then(value => resolve(value), reason => reject(reason))
        x.then(resolve, reject);
    } else {
        // 普通值
        resolve(x)
    }
}

module.exports = MyPromise;