自定义 Promise

let p1 = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve('OK')
    })
})

p1.then(
    value => {
        console.log(value)
    },
    reason => {
        console.warn(reason)
    }
)

class Promise {
    //构造方法
    constructor(executor) {
        //添加属性
        this.PromiseState = 'pending'
        this.PromiseResult = null
        //声明属性
        this.callbacks = []
        //保存实例对象的 this 的值
        const self = this
        //resolve 函数
        function resolve(data) {
            //判断状态
            if (self.PromiseState !== 'pending') return
            //1. 修改对象的状态 (promiseState)
            self.PromiseState = 'fulfilled' // resolved
            //2. 设置对象结果值 (promiseResult)
            self.PromiseResult = data
            //调用成功的回调函数
            setTimeout(() => {
                self.callbacks.forEach(item => {
                    item.onResolved(data)
                })
            })
        }
        //reject 函数
        function reject(data) {
            //判断状态
            if (self.PromiseState !== 'pending') return
            //1. 修改对象的状态 (promiseState)
            self.PromiseState = 'rejected' //
            //2. 设置对象结果值 (promiseResult)
            self.PromiseResult = data
            //执行失败的回调
            setTimeout(() => {
                self.callbacks.forEach(item => {
                    item.onRejected(data)
                })
            })
        }
        try {
            //同步调用『执行器函数』
            executor(resolve, reject)
        } catch (e) {
            //修改 promise 对象状态为『失败』
            reject(e)
        }
    }

    //then 方法封装
    then(onResolved, onRejected) {
        const self = this
        //判断回调函数参数
        if (typeof onRejected !== 'function') {
            onRejected = reason => {
                throw reason
            }
        }
        if (typeof onResolved !== 'function') {
            onResolved = value => value
        }
        return new Promise((resolve, reject) => {
            //封装函数
            function callback(type) {
                try {
                    //获取回调函数的执行结果
                    let result = type(self.PromiseResult)
                    //判断
                    if (result instanceof Promise) {
                        //如果是 Promise 类型的对象
                        result.then(
                            v => {
                                resolve(v)
                            },
                            r => {
                                reject(r)
                            }
                        )
                    } else {
                        //结果的对象状态为『成功』
                        resolve(result)
                    }
                } catch (e) {
                    reject(e)
                }
            }
            //调用回调函数  PromiseState
            if (this.PromiseState === 'fulfilled') {
                setTimeout(() => {
                    callback(onResolved)
                })
            }
            if (this.PromiseState === 'rejected') {
                setTimeout(() => {
                    callback(onRejected)
                })
            }
            //判断 pending 状态
            if (this.PromiseState === 'pending') {
                //保存回调函数
                this.callbacks.push({
                    onResolved: function () {
                        callback(onResolved)
                    },
                    onRejected: function () {
                        callback(onRejected)
                    }
                })
            }
        })
    }

    //catch 方法
    catch(onRejected) {
        return this.then(undefined, onRejected)
    }

    //添加 resolve 方法
    static resolve(value) {
        //返回promise对象
        return new Promise((resolve, reject) => {
            if (value instanceof Promise) {
                value.then(
                    v => {
                        resolve(v)
                    },
                    r => {
                        reject(r)
                    }
                )
            } else {
                //状态设置为成功
                resolve(value)
            }
        })
    }

    //添加 reject 方法
    static reject(reason) {
        return new Promise((resolve, reject) => {
            reject(reason)
        })
    }

    //添加 all 方法
    static all(promises) {
        //返回结果为promise对象
        return new Promise((resolve, reject) => {
            //声明变量
            let count = 0
            let arr = []
            //遍历
            for (let i = 0; i < promises.length; i++) {
                //
                promises[i].then(
                    v => {
                        //得知对象的状态是成功
                        //每个promise对象 都成功
                        count++
                        //将当前promise对象成功的结果 存入到数组中
                        arr[i] = v
                        //判断
                        if (count === promises.length) {
                            //修改状态
                            resolve(arr)
                        }
                    },
                    r => {
                        reject(r)
                    }
                )
            }
        })
    }

    //添加 race 方法
    static race(promises) {
        return new Promise((resolve, reject) => {
            for (let i = 0; i < promises.length; i++) {
                promises[i].then(
                    v => {
                        //修改返回对象的状态为 『成功』
                        resolve(v)
                    },
                    r => {
                        //修改返回对象的状态为 『失败』
                        reject(r)
                    }
                )
            }
        })
    }
}