Promise 类核心逻辑实现

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) //执行器  立即执行
        } catch (e) {
            this.reject(e);
        }
    }
    // promsie 状态 
    status = PENDING;
    // 成功后的值
    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()();
        }
    }
    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()()
        }
    }
    then = (successCallback, failCallback) => {
        // 参数可选
        successCallback = successCallback ? successCallback : value => value;
        // 参数可选
        failCallback = failCallback ? failCallback : reason => {
            throw reason
        };
        let promise2 = new MyPromise((resolve, reject) => {

            // 判断状态
            if (this.status == FULFILLED) {
                /* 要判断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reject 
                 */
                setTimeout(() => {
                    try {
                        let x = successCallback(this.value); //传递成功后的值
                        resolvePromise(promise2, x, resolve, reject)
                    } catch (e) {
                        reject(e);
                    }
                }, 0)
            } else if (this.status == REJECTED) {
                setTimeout(() => {
                    try {
                        let x = failCallback(this.reason); //传递失败后的值
                        resolvePromise(promise2, x, resolve, reject)
                    } catch (e) {
                        reject(e);
                    }
                }, 0)
            } else {
                // 状态为等待时(相当于异步)
                // 将成功回调和失败回调储存起来
                this.successCallback.push(() => {
                    setTimeout(() => {
                        try {
                            let x = successCallback(this.value); //传递成功后的值
                            resolvePromise(promise2, x, resolve, reject)
                        } catch (e) {
                            reject(e);
                        }
                    }, 0)
                })
                this.failCallback.push(() => {
                    setTimeout(() => {
                        try {
                            let x = failCallback(this.reason); //传递失败后的值
                            resolvePromise(promise2, x, resolve, reject)
                        } catch (e) {
                            reject(e);
                        }
                    }, 0)
                })
            }
        });
        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])
                }
            }
        })
    }
    static resolve(value) {
        if (value instanceof MyPromise) return value;
        return new MyPromise(resolve => resolve(value))
    }
}

function resolvePromise(promise2, x, resolve, reject) {
    if (promise2 === x) {
        return reject(new Error('promise 重复调用了！'))
    }
    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)
    }
}

// 导出Mypromise
module.exports = MyPromise