手写Promise—基于es6的Promise实现

const PENDING = 'pending';// 等待
const FULFILLED = 'fulfilled';// 等待
const REJECT = 'reject';// 等待

class MyPromise {
    constructor(executor) {
        try {
            executor(this.resolve, this.reject)
        } catch (e) {
            console.log(e, e)
            this.reject(e);
        }
    }
    status = PENDING;// promise状态
    value = undefined;// 成功之后的值
    reason = undefined;// 失败后的原因
    successCallback = [];// 成功回调
    failCallback = [];// 失败回调
    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 阻止程序向下执行
        if (this.status !== PENDING) return
        // 将状态更改为失败
        this.status = REJECT;
        // 保存失败之后的原因
        this.reason = reason;
        // 判断失败回调是否存在，如果存在就调用
        // this.failCallback && this.failCallback(this.reason);
        while (this.failCallback.length) this.failCallback.shift()()
    }
    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的值事普通值还是promise对象，如果是普通之，直接调用resolve
                        // 如果是promise对象，查看promiase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
                        resolvePromise(promise2, x, resolve, reject)
                    } catch (e) {
                        reject(e)
                    }
                }, 0);
            } else if (this.status === REJECT) {
                setTimeout(() => {
                    try {
                        let x = failCallback(this.value);
                        // 判断x的值事普通值还是promise对象，如果是普通之，直接调用resolve
                        // 如果是promise对象，查看promiase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
                        resolvePromise(promise2, x, resolve, reject)
                    } catch (e) {
                        reject(e)
                    }

                }, 0);
            } else {
                // 等待
                //将成功回调和失败回调存储起来
                this.successCallback.push(() => {
                    setTimeout(() => {
                        try {
                            let x = successCallback(this.value);
                            // 判断x的值事普通值还是promise对象，如果是普通之，直接调用resolve
                            // 如果是promise对象，查看promiase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
                            resolvePromise(promise2, x, resolve, reject)
                        } catch (e) {
                            reject(e)
                        }
                    }, 0);
                });
                this.failCallback.push(() => {
                    setTimeout(() => {
                        try {
                            let x = failCallback(this.value);
                            // 判断x的值事普通值还是promise对象，如果是普通之，直接调用resolve
                            // 如果是promise对象，查看promiase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
                            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(() => value);
            throw reason;
        })
    }
    catch(failCallback) {
        return this.then(undefined, failCallback)
    }
    static all(array) {
        const result = [];
        let index = 0;
        return new MyPromise((resolve, reject) => {
            const 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));
    }
}
const resolvePromise = (promise2, x, resolve, reject) => {
    if (promise2 === x) {
        return reject('TypeError: Chaining cycle detected for promise #<Promise>')
    }
    if (x instanceof MyPromise) {
        // promise对象
        x.then(resolve, reject)
    } else {
        // 普通值
        resolve(x)
    }
}
module.exports = MyPromise