Promise

一朝刊、Promise 是一個(gè)異步操作返回的對象，用來傳遞異步操作的消息蜈缤，
根據(jù)自己對 Promise 的理解拾氓，實(shí)現(xiàn)一個(gè)Promise :
Promise 有三種狀態(tài)：Pending 初始態(tài)； Fulfilled 成功態(tài)底哥； Rejected失敗態(tài)

function Promise(executor) {
    let self = this;
    self.statuts = 'pending'; //等狀態(tài)
    self.value = undefined; //成功的返回值
    self.reason = undefined; //失敗的返回值

    function resolve(value) {
        if (self.statuts === 'pending') {
            self.statuts = 'resolved';
            self.value = value;
        }
    }
    function reject(reason) {
        if (self.statuts === 'pending') {
            self.statuts = 'rejected';
            self.reason = reason;
        }
    }
    try {
        executor(resolve, reject)
    } catch (e) {
        reject(e); //捕獲時(shí)如果發(fā)生異常咙鞍，就直接失敗
    }
}

//onFufiled 成功的回調(diào)
//onRejected 失敗的回調(diào)
Promise.prototype.then = function (onFufiled, onRejected) {
    let self = this;
    if (self.status === 'resolved') {
        onFufiled(self.value);
    }
    if (self.status === 'rejected') {
        onRejected(self.reason);
    }
}
module.exports = Promise;

來測試一下子：

let Promise = require('./Promise');
    let Promise = new Promise(function (resolve, reject) {
        resolve(100);
    });

    Promise.then(function (data) {
        console.log('data:', data);
    }, function (err) {
        console.log('err', err);
    });

    //輸出：data: 100
}

Promise 實(shí)例可以多次then，當(dāng)成功后會(huì)將 then 中的成功方法按順序執(zhí)行趾徽，我們可以先將 then 中成功的回調(diào)和失敗的回調(diào)存到數(shù)組內(nèi)续滋。當(dāng)成功的時(shí)候調(diào)用成功的數(shù)組即可

self.onResolvedCallbacks = [];  /* 存放then成功的回調(diào)*/
self.onRejectedCallbacks = [];  /* 存放then失敗的回調(diào)*/
function resolve(value){
    if(self.status === 'pending'){
        self.status = 'resolved';
        self.value = value;
        self.onResolvedCallbacks.forEach(function (fn) {
            fn();
        })
    }
}
function reject(reason) {
    if(self.status === 'pending') {
        self.status = 'rejected';
        self.reason = reason;
        self.onRejectedCallbacks.forEach(function (fn) {
            fn();
        })
    }
}

復(fù)制代碼if(self.status === 'pending'){
    self.onResolvedCallbacks.push(function () {
        onFufiled(self.value);
    })
    self.onRejectedCallbacks.push(function () {
        onRejected(self.reason);
    })
}

手寫 AJAX

//背代碼，完整版
    var request = new XMLHttpRequest()
    request.open('GET', 'a/b/c?name=ff', true);
    request.onreadystatechange = function () {
        if (request.readyState === 4 && request.status === 200) {
            console.log(request.responseText);
        }
    }
    request.send()

    // 背代碼孵奶，簡化版
    var request = new XMLHttpRequest()
    request.open('GET', 'a/b/c?name=ff', true)
    request.onload = () => console.log(request.responseText)
    request.send()