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<!DOCTYPE html><html lang="zh-CN" data-default-color-scheme="&#34;auto&#34;"><head><meta charset="UTF-8"><link rel="apple-touch-icon" sizes="76x76" href="/images/img/apple-touch-icon.webp"><link rel="icon" type="image/png" href="/images/img/favicon.webp"><meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1,user-scalable=no,shrink-to-fit=no"><meta http-equiv="x-ua-compatible" content="ie=edge"><meta name="theme-color" content="#9DC8C8"><meta name="description" content=""><meta name="author" content="Defectink"><meta name="keywords" content=""><title>Node.js之旅 - 🍭Defectink</title><link rel="stylesheet" href="https://cdn.defectink.com/static/twitter-bootstrap/4.5.3/css/bootstrap.min.css"><link rel="stylesheet" href="https://cdn.defectink.com/static/github-markdown-css/4.0.0/github-markdown.min.css"><link rel="stylesheet" href="/lib/hint/hint.min.css"><link rel="stylesheet" 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PGP</a></li><li class="nav-item" id="search-btn"><a class="nav-link" data-toggle="modal" data-target="#modalSearch">&nbsp;<i class="iconfont icon-search"></i>&nbsp;</a></li><li class="nav-item" id="color-toggle-btn"><a class="nav-link" href="javascript:">&nbsp;<i class="iconfont icon-dark" id="color-toggle-icon"></i>&nbsp;</a></li></ul></div></div></nav><div class="banner intro-2" id="background" parallax="true" style="background:url(/images/img/post.webp) no-repeat center center;background-size:cover"><div class="full-bg-img"><div class="mask flex-center" style="background-color:rgba(0,0,0,.3)"><div class="container page-header text-center fade-in-up"><span class="h2" id="subtitle"></span><div class="mt-3"><span class="post-meta mr-2"><i class="iconfont icon-author" aria-hidden="true"></i> Defectink</span><span class="post-meta"><i class="iconfont icon-date-fill" aria-hidden="true"></i> <time datetime="2020-09-03 16:09" pubdate>2020年9月3日 下午</time></span></div><div class="mt-1"><span class="post-meta mr-2"><i class="iconfont icon-chart"></i> 1.6k 字</span><span class="post-meta mr-2"><i class="iconfont icon-clock-fill"></i> 18 分钟</span></div></div></div></div></div></header><main><div class="container-fluid"><div class="row"><div class="d-none d-lg-block col-lg-2"></div><div class="col-lg-8 nopadding-md"><div class="container nopadding-md" id="board-ctn"><div class="py-5" id="board"><article class="post-content mx-auto" id="post"><h1 style="display:none">Node.js之旅</h1><p class="note note-info">本文最后水于：2020年11月2日 凌晨</p><div class="markdown-body" id="post-body"><blockquote><p>Node.js® is a JavaScript runtime built on Chrome’s V8 JavaScript engine.</p></blockquote><p>Node.js不仅仅是服务器上的JavaScript。</p><p><img src="../images/Node.js%E4%B9%8B%E6%97%85/2020-09-03-14-54-53.webp" srcset="/images/img/loading.gif"></p><h2 id="并不熟悉的JavaScript"><a href="#并不熟悉的JavaScript" class="headerlink" title="并不熟悉的JavaScript"></a>并不熟悉的JavaScript</h2><p>虽然说Node直接般来了个V8来运行JavaScript，但它毕竟不运行在浏览器上，并且是由事件驱动的异步程序，它的本来的目的就是用来搭建高性能的Web服务器。在极少情况下，编写的 JavaScript 代码没有按期望的方式运行。</p><p>在客户端的运行环境下，只要是在同一个浏览器窗口运行的JS环境就属于同一个全局环境。无论是在哪里引入的JS文件，都属于同一个运行环境，全局变量能够正常工作，例如有两个js被引入：</p><pre><code class="hljs html"><span class="hljs-tag">&lt;<span class="hljs-name">script</span> <span class="hljs-attr">src</span>=<span class="hljs-string">&quot;a.js&quot;</span>&gt;</span><span class="hljs-tag">&lt;/<span class="hljs-name">script</span>&gt;</span>
<span class="hljs-tag">&lt;<span class="hljs-name">script</span> <span class="hljs-attr">src</span>=<span class="hljs-string">&quot;b.js&quot;</span>&gt;</span><span class="hljs-tag">&lt;/<span class="hljs-name">script</span>&gt;</span></code></pre><p>他们分别有一段代码：</p><pre><code class="hljs js"><span class="hljs-comment">// a.js</span>
<span class="hljs-keyword">let</span> a = <span class="hljs-string">&#x27;xxxxxfy!&#x27;</span>;
<span class="hljs-comment">// b.js</span>
alert(a);</code></pre><p>正常情况下全局变量<code>a</code>是可以被正常访问的，而在node中，a.js和b.js分别是两个文件，不做其他操作的情况下，他们分别是两个全局变量。</p><h2 id="基础架构"><a href="#基础架构" class="headerlink" title="基础架构"></a>基础架构</h2><p>Node 运行时的基础架构由两大组件构成：</p><ul><li>JavaScript 引擎</li><li>非阻塞 I/O 库</li></ul><h3 id="JavaScript引擎"><a href="#JavaScript引擎" class="headerlink" title="JavaScript引擎"></a>JavaScript引擎</h3><p>众所周知，node使用的是Chrome大排自吸V8，它可以运行任何JS代码。启动时，它会启动一个V8的引擎实例，且node可以充分的利用这个引擎实例。</p><p>V8可嵌入（/绑定）到任何C++程序中，。这意味着，除了纯 JavaScript 库外，还可以扩展 V8 来创建全新的函数（或函数模板），方法是将其与 V8 绑定在一起。并且node还支持使用编译好的二进制的C++程序。</p><h3 id="事件循环"><a href="#事件循环" class="headerlink" title="事件循环"></a>事件循环</h3><p>JavaScript是一门单线程的语言，无论换到什么地方运行，它也是单线程语言。单线程语言面临的最大的问题就是阻塞，在前面的代码没有被执行完，后面的代码都将处于等待状态。</p><p>node使用<a target="_blank" rel="noopener" href="http://libuv.org/">libuv</a>来实现事件循环。要使用事件循环可以使用异步API，可将回调函数作为参数传递到该 API 函数，在事件循环期间会执行该回调函数。node异步编程的直接体现就是回调函数。异步编程基于回调函数来实现，但不能说是使用了回调后程序就异步化了。Node 所有 API 都支持回调函数。</p><pre><code class="hljs smali"> ┌───────────────────────────┐
┌─&gt;│ timers │
│ └─────────────┬─────────────┘
│ ┌─────────────┴─────────────┐
│ │ pending callbacks │
│ └─────────────┬─────────────┘
│ ┌─────────────┴─────────────┐
│ │ idle, prepare │
│ └─────────────┬─────────────┘ ┌───────────────┐
│ ┌─────────────┴─────────────┐ │ incoming: │
│ │ poll │&lt;─────┤ connections, │
│ └─────────────┬─────────────┘ │ data, etc. │
│ ┌─────────────┴─────────────┐ └───────────────┘
│ │ <span class="hljs-built_in"> check </span> │
│ └─────────────┬─────────────┘
│ ┌─────────────┴─────────────┐
└──┤ close callbacks │
└───────────────────────────┘</code></pre><p>例如，我们可以一边读取文件，一边执行其他命令，在文件读取完成后，我们将文件内容作为回调函数的参数返回。这样在执行代码时就没有阻塞或等待文件 I/O 操作。这就大大提高了 Node.js 的性能，可以处理大量的并发请求。</p><p>事件循环包含多个调用回调函数的阶段：</p><ul><li>计时器阶段：将运行 setInterval() 和 setTimeout() 过期计时器回调函数</li><li>轮询阶段：将轮询操作系统以查看是否完成了所有 I/O 操作，如果已完成，将运行回调函数</li><li>检查阶段：将运行 setImmediate() 回调函数</li></ul><p>一个常见的误区是，认为 V8 和事件循环回调函数在不同线程上运行。但事实并非如此。V8 在同一个线程上运行所有 JavaScript 代码。</p><h2 id="非阻塞I-O"><a href="#非阻塞I-O" class="headerlink" title="非阻塞I/O"></a>非阻塞I/O</h2><p>已经简单的了解过node基于v8的单线程来使用回调函数实现异步编程，从而达到高性能与高并发。</p><h3 id="异步I-O"><a href="#异步I-O" class="headerlink" title="异步I/O"></a>异步I/O</h3><p>异步I/O能够大大的提升程序的工作效率，且不会影响剩下的代码执行。</p><h4 id="同步读取文件"><a href="#同步读取文件" class="headerlink" title="同步读取文件"></a>同步读取文件</h4><p>首先来看下同步读取文件</p><pre><code class="hljs js"><span class="hljs-keyword">let</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">&#x27;fs&#x27;</span>);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;starting process...&#x27;</span>);
<span class="hljs-keyword">let</span> data = fs.readFileSync(<span class="hljs-string">&#x27;test.txt&#x27;</span>);
<span class="hljs-built_in">console</span>.log(data);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;the end&#x27;</span>);</code></pre><p>同步读取文件会按照从上到下的顺序来执行代码，到遇到读取I/O时，剩下的代码将处于等待状态。并且在读取I/O时遇到错误（例如文件不存在），将会直接返回错误，剩下的代码将不执行完。</p><h4 id="异步读取文件"><a href="#异步读取文件" class="headerlink" title="异步读取文件"></a>异步读取文件</h4><pre><code class="hljs js"><span class="hljs-keyword">let</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">&#x27;fs&#x27;</span>);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;starting process...&#x27;</span>);
fs.readFile(<span class="hljs-string">&#x27;test.txt&#x27;</span>, <span class="hljs-function">(<span class="hljs-params">err, data</span>) =&gt;</span> &#123;
<span class="hljs-keyword">if</span> (err) &#123;
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;something was wrong: &#x27;</span> + err);
&#125; <span class="hljs-keyword">else</span> &#123;
<span class="hljs-built_in">console</span>.log(data);
&#125;
&#125;)
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;the end&#x27;</span>);</code></pre><p>异步读取文件调用了回调函数，它设置了一个事件循环，在等待读取I/O的同时，将继续执行剩下的代码。当文件读取完成之后，将会返回回调函数，并输出结果。并且当读取文件遇到错误时，在等待读取期间执行的代码将正常执行。</p><p>异步的结果看起来像是这样的：</p><pre><code class="hljs angelscript">starting process...
the end
&lt;Buffer <span class="hljs-number">64</span> <span class="hljs-number">66</span> <span class="hljs-number">6</span>a <span class="hljs-number">61</span> <span class="hljs-number">73</span>&gt;</code></pre><p>在等待其读取<code>test.txt</code>的期间，JS将剩下的<code>console.log</code>语句先执行了。</p><h3 id="同步I-O"><a href="#同步I-O" class="headerlink" title="同步I/O"></a>同步I/O</h3><p>node的设计理念就是使用异步编程，使用阻塞编程可无法写出一个高性能的web服务器。</p><p>但这不代表着同步I/O就一无是处，在某些情况下，同步 I/O 通常比异步 I/O 更快，原因在于设置和使用回调函数、轮询操作系统来获取 I/O 状态等操作都涉及到一定的开销。</p><p>如果需要写一个一次性的使用程序，仅用于处理一个文件，从命令行中启动 Node，并向其传递 JavaScript 实用程序的文件名。此时，该实用程序是唯一运行的程序，因此，即使它阻塞了 V8 线程，也没有任何影响。在此情况下，适合使用同步 I/O。</p><p>只要是谨慎使用同步 Node API 调用，就不会出现什么问题。</p><h2 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h2><ul><li>V8是有八个气缸的V型发动机.</li><li>JavaScript是单线程语言，它通过V8 引擎提供的异步执行回调接口实现了异步编程。</li></ul><p>全文均是笔记</p><ul><li><a target="_blank" rel="noopener" href="https://developer.ibm.com/zh/tutorials/learn-nodejs-tour-node/">Node.js 之旅</a></li><li><a target="_blank" rel="noopener" href="https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/">The Node.js Event Loop</a></li><li><a target="_blank" rel="noopener" href="https://www.runoob.com/nodejs/nodejs-callback.html">Node.js 回调函数</a></li><li><a target="_blank" rel="noopener" href="https://zh.wikipedia.org/wiki/V8%E5%8F%91%E5%8A%A8%E6%9C%BA">V8发动机</a></li></ul></div><hr><div><div class="post-metas mb-3"><div class="post-meta mr-3"><i class="iconfont icon-category"></i> <a class="hover-with-bg" href="/categories/%E7%AC%94%E8%AE%B0/">笔记</a></div><div class="post-meta"><i class="iconfont icon-tags"></i> <a class="hover-with-bg" href="/tags/JavaScript/">JavaScript</a> <a class="hover-with-bg" href="/tags/Node/">Node</a></div></div><p class="note note-warning"><a target="_blank" href="https://zh.wikipedia.org/wiki/Wikipedia:CC_BY-SA_3.0%E5%8D%8F%E8%AE%AE%E6%96%87%E6%9C%AC" rel="nofollow noopener noopener">CC BY-SA 3.0❤</a></p><div class="post-prevnext row"><article class="post-prev col-6"><a 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