promise 实现

ES6 内置类 promise 的实现

参考尚硅谷 promise 解析视频

class Promise {
    // promise 状态
    PromiseState = 'pending'
    // promise 结果
    PromiseResult = null
    // promise 回调函数列表
    callback = []

    constructor(executor) {
        // 成功的回调
        // 箭头函数避免 this 指向问题
        const resolve = (data) => {
            if (this.PromiseState !== 'pending') return
            // 1. 修改 Promise 状态（promiseState）
            // 2. 设置对象结果值（promiseResult）
            this.PromiseState = 'fulfilled'
            this.PromiseResult = data
            // 异步调用 then 方法
            setTimeout(() => {
                // 调用成功的回调函数
                this.callback.forEach(item => {
                    item.onResolved(data)
                })
            })
        }

        // 失败的回调
        const reject = (data) => {
            if (this.PromiseState !== 'pending') return
            this.PromiseState = 'rejected'
            this.PromiseResult = data
            setTimeout(() => {
                this.callback.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(onResolved, onRejected) {
        // onRejected 是否传递
        if (typeof onRejected !== 'function') {
            onRejected = (reason) => {
                throw reason
            }
        }
        // onResolved 是否传递
        if (typeof onResolved !== 'function') {
            onResolved = value => value
        }

        return new Promise((resolve, reject) => {
            const callback = (type) => {
                try {
                    let result = type(this.PromiseResult)
                    if (result instanceof Promise) {
                        // 是 Promise 类型
                        result.then(r => {
                            resolve(r)
                        }, e => {
                            reject(e)
                        })
                    } else {
                        resolve(result)
                    }
                } catch (e) {
                    reject(e)
                }
            }


            // 调用回调函数 PromiseState
            if (this.PromiseState === 'fulfilled') {
                // 回调函数异步执行
                setTimeout(() => {
                    callback(onResolved)
                })
            }
            if (this.PromiseState === 'rejected') {
                setTimeout(() => {
                    callback(onRejected)
                })
            }

            // pending 状态
            if (this.PromiseState === 'pending') {
                // 保存回调函数
                this.callback.push({
                    onResolved: function () {
                        // 执行成功的回调函数
                        callback(onResolved)
                    },
                    onRejected: function () {
                        callback(onRejected)
                    }
                })
            }
        })
    }
    catch(onRejected) {
        return this.then(undefined, onRejected)
    }

    // 静态方法
    static resolve(value) {
        return new Promise((resolve, reject) => {
            if (value instanceof Promise) {
                value.then(v => {
                    resolve(v)
                }, r => {
                    reject(r)
                })
            } else {
                resolve(value)
            }
        })
    }

    static reject(value) {
        return new Promise((resolve, reject) => {
            reject(value)
        })
    }

    static all(promises) {
        return new Promise((resolve, reject) => {
            let count = 0
            const res = []
            for (let i = 0; i < promises.length; i++) {
                promises[i].then(v => {
                    count++
                    // 可能成功的时间不一致
                    // 使用push可能导致结果与传入数组不一致
                    res[i] = v
                    // 每个 promise 都成功
                    if (count === promises.length) {
                        resolve(res)
                    }
                }, r => {
                    // 只要有一个失败
                    reject(r)
                })
            }
        })
    }

    static race(promises) {
        return new Promise((resolve, reject) => {
            for (let i = 0; i < promises.length; i++) {
                promises[i].then(v => {
                    resolve(v)
                }, r => {
                    reject(r)
                })
            }
        })
    }

}