手写Promise，1.8promise实现完整代码

完整代码如下：

// 定义三个状态
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)
        }
    }
    // 定义一个表示状态的属性
    status = PENDING;
    // 定义两个实例属性，表示成功之后的值和失败后的原因
    value = undefined;
    reason = undefined;
    // 成功回调
    successCallback = [];
    // 失败回调
    failedCallback = [];

    // 两个属性,这里定义成箭头函数，是因为我们在使用的时候是直接调用，
    // 而普通函数内部this的指向window或者是undefined，定义成箭头函数使函数内部this指向指向类实例对象
    resolve = value => {
        // 如果状态不是等待，阻止程序向下进行
        if (this.status !== PENDING) return
        // 更改状态为成功
        this.status = FULFILLED
        // 保存成功的值
        this.value = value;
        // 判断成功回调是否存在，如果存在就调用
        // this.successCallback && this.successCallback(this.value);
        while (this.successCallback.length) {
            this.successCallback.shift()();
        }
    }
    reject = reason => {
        // 如果状态不是等待，阻止程序向下进行 
        if (this.status !== PENDING) return
        // 更改状态为失败
        this.status = REJECTED
        // 保存失败的原因
        this.reason = reason;
        // 判断失败回调是否存在，存在就调用
        // this.failedCallback && this.failedCallback(this.reason);
        while (this.failedCallback.length) {
            this.failedCallback.shift()();
        }
    }

    then(successCallback, failedCallback) {
        // 可选参数
        successCallback = successCallback ? successCallback : value => value;
        failedCallback = failedCallback ? failedCallback : reason => {
            throw reason
        };
        let promise2 = new Promise((resolve, reject) => {
            // 状态判断
            if (this.status === FULFILLED) {
                setTimeout(() => {
                    try {
                        // 定义成功回调返回值，传给下一个then的成功回调
                        let successRtn = 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promsie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
                        // 执行resolve方法，相当于把返回值传递给下一个then的成功回调函数
                        resolvePromise(promise2, successRtn, resolve, reject);
                    } catch (e) {
                        reject(e);
                    }
                }, 0);
            } else if (this.status === REJECTED) {
                setTimeout(() => {
                    try {
                        let failedRtn = failedCallback(this.reason);
                        resolvePromise(promise2, failedRtn, resolve, reject);
                    } catch (e) {
                        reject(e)
                    }
                }, 0);
            } else {
                // 等待，需要将成功回调和失败回调存储起来，等待需要执行的时候才执行
                this.successCallback.push(() => {
                    setTimeout(() => {
                        try {
                            let successRtn = successCallback(this.value);
                            resolvePromise(promise2, successRtn, resolve, reject);
                        } catch (e) {
                            reject(e);
                        }
                    }, 0);
                });
                this.failedCallback.push(() => {
                    setTimeout(() => {
                        try {
                            let failedRtn = failedCallback(this.reason);
                            resolvePromise(promise2, failedRtn, resolve, reject);
                        } catch (e) {
                            reject(e)
                        }
                    }, 0);
                });
            }
        });
        return promise2;
    };

    catch (failCallback) {
        this.then(undefined, failCallback)
    };

    finally(callback) {
        return this.then(value => {
            return MyPromise.resolve(callback()).then(() => value);
        }, reason => {
            return MyPromise.resolve(callback()).then(() => {
                throw reason
            })
        })
    };

    static all(array) {
        let result = [];
        // 用于判断当前执行值是否等于数组长度，相等时才执行resolve（）
        let idx = 0;

        return new MyPromise((resolve, reject) => {
            // 添加元素方法
            function addElement(index, value) {
                result[index] = value;
                idx++;
                if (idx === array.length) {
                    resolve(result)
                }
            }
            for (let i = 0; i < array.length; ++i) {
                let cur = array[i];
                // 判断cur是否是普通值，普通值直接进入result数组，
                // promise对象就执行它,调用then方法，执行成功则添加到result数组，如果失败
                if (cur instanceof MyPromise) {
                    // promise对象
                    cur.then(value => addElement(i, value), reason => reject(reason))
                } else {
                    // 普通值
                    addElement(i, array[i]);
                }
            }
        })
    }

    static race(array) {
        return new MyPromise((resolve, reject) => {
            for (let i = 0; i < array.length; ++i) {
                let cur = array[i];
                if (cur instanceof MyPromise) {
                    cur.then(resolve, reject);
                } else {
                    resolve(cur)
                }
            }
        })
    }

    static resolve(e) {
        if (e instanceof MyPromise) {
            return e;
        } else {
            return new MyPromise(resolve => resolve(e))
        }
    }
}

function resolvePromise(promise2, e, resolve, reject) {
    if (promise2 === e) {
        return reject(new TypeError("Chaining cycle detected for promise #"))
    }
    if (e instanceof MyPromise) {
        // promise对象
        // e.then((value) => {
        //     resolve(value)
        // }, (reason) => {
        //     reject(reason)
        // })
        // 简化代码
        e.then(resolve, reject);
    } else {
        //普通值
        resolve(e);
    }
}

module.exports = MyPromise;