手寫Promise，1.8promise實現完整代碼

完整代碼以下：

// 定義三個狀態
const PENDING = 'pending'; // 等待
const FULFILLED = 'fulfilled'; // 成功
const REJECTED = 'rejected'; // 失敗

class MyPromise {
    constructor(executor) {
        try {
            executor(this.resolve, this.reject);
        } catch (e) {
            this.reject(e)
        }
    }
    // 定義一個表示狀態的屬性
    status = PENDING;
    // 定義兩個實例屬性，表示成功以後的值和失敗後的緣由
    value = undefined;
    reason = undefined;
    // 成功回調
    successCallback = [];
    // 失敗回調
    failedCallback = [];

    // 兩個屬性,這裏定義成箭頭函數，是由於咱們在使用的時候是直接調用，
    // 而普通函數內部this的指向window或者是undefined，定義成箭頭函數使函數內部this指向指向類實例對象
    resolve = value => {
        // 若是狀態不是等待，阻止程序向下進行
        if (this.status !== PENDING) return
        // 更改狀態爲成功
        this.status = FULFILLED
        // 保存成功的值
        this.value = value;
        // 判斷成功回調是否存在，若是存在就調用
        // this.successCallback && this.successCallback(this.value);
        while (this.successCallback.length) {
            this.successCallback.shift()();
        }
    }
    reject = reason => {
        // 若是狀態不是等待，阻止程序向下進行 
        if (this.status !== PENDING) return
        // 更改狀態爲失敗
        this.status = REJECTED
        // 保存失敗的緣由
        this.reason = reason;
        // 判斷失敗回調是否存在，存在就調用
        // this.failedCallback && this.failedCallback(this.reason);
        while (this.failedCallback.length) {
            this.failedCallback.shift()();
        }
    }

    then(successCallback, failedCallback) {
        // 可選參數
        successCallback = successCallback ? successCallback : value => value;
        failedCallback = failedCallback ? failedCallback : reason => {
            throw reason
        };
        let promise2 = new Promise((resolve, reject) => {
            // 狀態判斷
            if (this.status === FULFILLED) {
                setTimeout(() => {
                    try {
                        // 定義成功回調返回值，傳給下一個then的成功回調
                        let successRtn = successCallback(this.value);
                        // 判斷 x 的值是普通值仍是promise對象
                        // 若是是普通值 直接調用resolve 
                        // 若是是promise對象 查看promsie對象返回的結果 
                        // 再根據promise對象返回的結果 決定調用resolve 仍是調用reject
                        // 執行resolve方法，至關於把返回值傳遞給下一個then的成功回調函數
                        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，須要將成功回調和失敗回調存儲起來，等待須要執行的時候才執行
                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 = [];
        // 用於判斷當前執行值是否等於數組長度，相等時才執行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];
                // 判斷cur是不是普通值，普通值直接進入result數組，
                // promise對象就執行它,調用then方法，執行成功則添加到result數組，若是失敗
                if (cur instanceof MyPromise) {
                    // promise對象
                    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 #<Promise>"))
    }
    if (e instanceof MyPromise) {
        // promise對象
        // e.then((value) => {
        //     resolve(value)
        // }, (reason) => {
        //     reject(reason)
        // })
        // 簡化代碼
        e.then(resolve, reject);
    } else {
        //普通值
        resolve(e);
    }
}

module.exports = MyPromise;