Promise/a+规范

promise/a+规范概念

Promise/A+ 是 Promise 最小的一个规范，它包括：

1 promise状态
2 then方法
3 promise解析过程

任何能跑通测试的 Promise 实现都被 Promise/A+ 承认。

promise/a+出现的缘由

1 咱们不知道异步请求何时返回数据，因此咱们就须要些回调函数。可是在某些状况下咱们须要知道数据是在何时返回的，而后进行一些处理。
2 当咱们在异步回调里面处理的操做仍是异步操做的时候，这样就造成了异步回调的嵌套

那么咱们就按照这个文档来写一个可经过测试的promise吧(https://www.ituring.com.cn/ar... 写以前最好读几遍规范内容。话很少少，上代码，为了更好的展现我给代码打了不少注释，虽然很乱可是挺清楚的

//规范三种状态，默认是pending;定义宏变量是为了书写时方便一点
const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';

function resolvePromise(promise2, x, resolve, reject) {
`若是promise和x指向同一对象，以TypeError为据因拒绝执行promise`
    if (promise2 === x) {
        reject(new TypeError('Chaining cycle detected for promise #<Promise>--'))
    }
    let called;
    `判断若是是对象或者函数`
    if ((typeof x === 'object' && x != null) || typeof  x === 'function') {  
        try {
        `文档规定`
            let then = x.then;
            if (typeof then === 'function') {
            `若是then是函数，将x做为函数的做用域this调用之，传递两个回调函数做为参数`
                then.call(x, y => {
                `防止重复调用`
                    if (called) return;
                    called = true;
                    `若是是嵌套的promise那么须要递归遍历到普通值为止`
                    resolvePromise(promise2, y, resolve, reject)
                }, r => {
                 `防止重复调用`
                    if (called) return;
                    called = true;
                    reject(r)
                })
            } else {
          `若是then不是函数，以x为参数执行promise`
                resolve(x)
            }
        } catch (e) {
         `防止重复调用`
            if (called) return;
            called = true;
            `若是取x.then的值时抛出错误e，则以e为据因拒绝promise`
            reject(e)
        }
    } else {
    `若是是普通值`
        resolve(x)
    }
}
`建立一个promise类`
class Promise {
    constructor(executor) {
        this.status = PENDING;
        this.value = undefined;
        this.reason = undefined;
        this.onFulfilledCallbacks = [];
        this.onRejectedCallbacks = [];
        `定义executor(resolve, reject)中的resolve，参数value`
        let resolve = value => {
        `只有状态为pending的状况下才能转换状态`
            if (this.status === PENDING) {
            `调用reslove函数 状态变为成功态`
                this.status = FULFILLED;
                `定义变量接受传进来的参数`
                this.value = value;
                `遍历成功事件池中的方法并依次执行（解决异步）`
                this.onFulfilledCallbacks.forEach(fn => fn())
            }
        };
      `定义executor(resolve, reject)中的reject,参数reason`
        let reject = reason => {
       `只有状态为pending的状况下才能转换状态`
            if (this.status === PENDING) {
              `调用reject函数 状态变为失败态`
                this.status = REJECTED;
                `定义变量接受传进来的参数`
                this.reason = reason;
                `遍历失败事件池中的方法并依次执行（解决异步）`
                this.onRejectedCallbacks.forEach(fn => fn())
            }
        };
        try {
         `默认传进一个函数executor，而且执行，promise中的提供两个函数`
            executor(resolve, reject)
        } catch (e) {
        `若是抛错调用reject（文档写的）`
            reject(e)
        }

    }

    then(onFulfilled, onRejected) {
    `（成功态）连续调用时若是函数没有写给他附一个默认值继续向下传递`
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : d => d;
         `（失败态）若是函数没有写，那就把错误抛出去`
        onRejected = typeof onRejected === 'function' ? onRejected : e => {
            throw e
        };
        `规范上写调用.then时必须返回promise`
        let promise2 = new Promise((resolve, reject) => {
        `若是状态为成功态`
            if (this.status === FULFILLED) {
                setTimeout(() => {
                    try {
                    `接受.then后面第一个函数的返回值，为了判断有没有抛错使用了try包裹`
                        let x = onFulfilled(this.value);
                        `判断x是promise仍是普通值，从而进行不一样的处理，当调用这个函数时获取不到promise因此这里使用了异步`
                        resolvePromise(promise2, x, resolve, reject)
                    } catch (e) {
                    `若是抛错以后调用reject方法以e做为缘由`
                        reject(e)
                    }
                }, 0);
            }
            if (this.status === REJECTED) {
                setTimeout(() => {
                    try {
                        let x = onRejected(this.reason);
                        resolvePromise(promise2, x, resolve, reject)
                    } catch (e) {
                        reject(e)
                    }
                }, 0);
            }
            if (this.status === PENDING) {
          `若是是异步的话，使用发布订阅模式，将事件放到对应的事件池中`
                this.onFulfilledCallbacks.push(() => {
                    setTimeout(() => {
                        try {
                            let x = onFulfilled(this.value);
                            resolvePromise(promise2, x, resolve, reject)
                        } catch (e) {
                            reject(e)
                        }
                    }, 0)
                });
                this.onRejectedCallbacks.push(() => {
                    setTimeout(() => {
                        try {
                            let x = onRejected(this.reason);
                            resolvePromise(promise2, x, resolve, reject)
                        } catch (e) {
                            reject(e)
                        }
                    }, 0)
                })
            }
        });
        return promise2;//返回一个promise
    }
}

//导出咱们本身写的promise
module.exports = Promise;

测试

1.安装测试脚本

npm i -g promises-aplus-tests

2.在咱们写的promise中写入代码

//测试代码
Promise.defer = Promise.deferred = function () {
    let dfd = {};
    dfd.promise = new Promise((resolve, reject) => {
        dfd.resolve = resolve;
        dfd.reject = reject;
    })
    return dfd
}

3.在当前文件夹下执行

promises-aplus-tests xxx.js(本身写promise的文件)