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Pre-render routes with react-snap | Articles | web.dev
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<div class="devsite-sidebar-content"> <devsite-toc class="devsite-nav" role="navigation" aria-label="On this page" depth="2" scrollbars ></devsite-toc> <devsite-recommendations-sidebar class="nocontent devsite-nav"> </devsite-recommendations-sidebar> </div> </div> <devsite-content> <article class="devsite-article"> <div class="devsite-article-meta nocontent" role="navigation"> <ul class="devsite-breadcrumb-list" aria-label="Breadcrumb"> <li class="devsite-breadcrumb-item "> <a href="https://web.dev/" class="devsite-breadcrumb-link gc-analytics-event" data-category="Site-Wide Custom Events" data-label="Breadcrumbs" data-value="1" track-type="globalNav" track-name="breadcrumb" track-metadata-position="1" track-metadata-eventdetail="" > Home </a> </li> <li class="devsite-breadcrumb-item "> <div class="devsite-breadcrumb-guillemet material-icons" aria-hidden="true"></div> <a href="https://web.dev/articles" class="devsite-breadcrumb-link gc-analytics-event" data-category="Site-Wide Custom 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Try pre-rendering!</p> <p> <div class="wd-authors" translate="no"> <div class="wd-author"> <img class="devsite-landing-row-item-icon" alt="Houssein Djirdeh" src="https://web.dev/images/authors/houssein.jpg" decoding="async" height="64" loading="lazy" width="64"> <div> <span> Houssein Djirdeh </span> <div class="wd-author__links"> <a href="https://twitter.com/hdjirdeh" aria-label="Houssein Djirdeh on X" rel="me"> <svg xmlns="http://www.w3.org/2000/svg" width="22" height="22" viewBox="0 0 300 271"> <title>X</title> <path fill="currentColor" d="m236 0h46l-101 115 118 156h-92.6l-72.5-94.8-83 94.8h-46l107-123-113-148h94.9l65.5 86.6zm-16.1 244h25.5l-165-218h-27.4z"></path> </svg></a> <a href="https://github.com/housseindjirdeh" aria-label="Houssein Djirdeh on GitHub" rel="me"> <svg xmlns="http://www.w3.org/2000/svg" width="22" height="22" viewBox="0 0 32.6 31.8"> <title>GitHub</title> <path d="M16.3 0C7.3 0 0 7.3 0 16.3c0 7.2 4.7 13.3 11.1 15.5.8.1 1.1-.4 1.1-.8v-2.8c-4.5 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0-10.359-2.786l-1.135-.063h-.063l-.458-.026c-.583-.036-1.172-.068-1.755-.094l.036.078c.234.615.396 1.255.479 1.906.042.453-.036 1.078-.224 1.88l-.203.828a24.99 24.99 0 0 1-.385 1.333l-.026.068v.036c-.068.203-.266.297-.438.229a.42.42 0 0 1-.203-.51v-.026l.042-.078.078-.292c.182-.589.344-1.177.479-1.776.161-.708.229-1.281.203-1.599-.016-.12-.031-.24-.052-.359a7.996 7.996 0 0 0-.521-1.708l-.052-.12-.031-.068-.026-.063c-.12-.271.057-.578.323-.563h.188l.531.042 2.12.104 1.135.083a28.14 28.14 0 0 1 10.573 2.823l.891.401c.172.078.266.307.188.505z"/> </svg></a> <a href="https://houssein.me/" aria-label="Houssein Djirdeh's homepage" rel="me"> <svg xmlns="http://www.w3.org/2000/svg" width="22" height="22" viewBox="0 0 30 30"> <title>Homepage</title> <circle cx="14.5" cy="14.5" r="13.5" stroke-width="2" stroke-miterlimit="10" fill="none" stroke="currentColor" /> <ellipse cx="14.5" cy="14.5" rx="6.1" ry="13.5" stroke-width="2" stroke-miterlimit="10" fill="none" stroke="currentColor" /> <path d="M1.6 9.6h25.8M1.6 19.4h25.8" stroke-width="2" stroke-miterlimit="10" fill="none" stroke="currentColor" /> </svg></a> </div> </div> </div> </div></p> <p><a href="https://github.com/stereobooster/react-snap"><code translate="no" dir="ltr">react-snap</code></a> is a third-party library that pre-renders pages on your site into static HTML files. This can improve <a href="/articles/user-centric-performance-metrics#important_metrics_to_measure">First Paint</a> times in your application.</p> <p>Here's a comparison of the same application with and without pre-rendering loaded on a simulated 3G connection and mobile device:</p> <figure> <img src="/static/articles/prerender-with-react-snap/image/a-side-side-loading-comp-38a189328548.gif" alt="A side by side loading comparison. The version using pre-rendering loads 4.2 seconds faster." width="600" height="435"> </figure> <aside class="note"><b>Note: </b> <code translate="no" dir="ltr">react-snap</code> is not the only library that can pre-render static HTML content for your React application. <a href="https://github.com/geelen/react-snapshot"><code translate="no" dir="ltr">react-snapshot</code></a> is another alternative. </aside> <h2 id="why_is_this_useful" data-text="Why is this useful?" tabindex="-1">Why is this useful?</h2> <p>The main performance problem with large single-page applications is that the user needs to wait for the JavaScript bundle(s) that make up the site to finish downloading before they can see any real content. The larger the bundles, the longer the user will have to wait.</p> <p>To solve this, many developers take the approach of rendering the application on the server instead of only booting it up on the browser. With each page/route transition, the complete HTML is generated on the server and sent to the browser, which reduces First Paint times but comes at the cost of a slower Time to First Byte.</p> <p><strong>Pre-rendering</strong> is a separate technique that is less complex than server rendering, but also provides a way to improve First Paint times in your application. A headless browser, or a browser without a user interface, is used to generate static HTML files of every route during <em>build time</em>. These files can then be shipped along with the JavaScript bundles that are needed for the application.</p> <h2 id="react-snap" data-text="react-snap" tabindex="-1">react-snap</h2> <p><code translate="no" dir="ltr">react-snap</code> uses <a href="https://github.com/GoogleChrome/puppeteer">Puppeteer</a> to create pre-rendered HTML files of different routes in your application. To begin, install it as a development dependency:</p> <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Bash"><code translate="no" dir="ltr">npm<span class="devsite-syntax-w"> </span>install<span class="devsite-syntax-w"> </span>--save-dev<span class="devsite-syntax-w"> </span>react-snap </code></pre></devsite-code> <p>Then add a <code translate="no" dir="ltr">postbuild</code> script in your <code translate="no" dir="ltr">package.json</code>:</p> <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="JSON"><code translate="no" dir="ltr"><span class="devsite-syntax-nt">"scripts"</span><span class="devsite-syntax-p">:</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-p">{</span> <span class="devsite-syntax-w"> </span><span class="devsite-syntax-c1">//...</span> <span class="devsite-syntax-w"> </span><span class="devsite-syntax-nt">"postbuild"</span><span class="devsite-syntax-p">:</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-s2">"react-snap"</span> <span class="devsite-syntax-p">}</span> </code></pre></devsite-code> <p>This would automatically run the <code translate="no" dir="ltr">react-snap</code> command every time a new build of the applications made (<code translate="no" dir="ltr">npm build</code>).</p> <aside class="note"><b>Note: </b> <code translate="no" dir="ltr">npm</code> supports <em>pre</em> and <em>post</em> commands for main and arbitrary scripts which will always run directly before or after the original script respectively. You can learn more in the <a href="https://docs.npmjs.com/misc/scripts">npm documentation</a>. </aside> <p>The last thing you will need to do is change how the application is booted. Change the <code translate="no" dir="ltr">src/index.js</code> file to the following:</p> <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Python"><code translate="no" dir="ltr"><span class="devsite-syntax-kn">import</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-nn">React</span> <span class="devsite-syntax-kn">from</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-s1">'react'</span><span class="devsite-syntax-p">;</span> <span class="devsite-syntax-kn">import</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-nn">ReactDOM</span> <span class="devsite-syntax-kn">from</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-s1">'react-dom'</span><span class="devsite-syntax-p">;</span> <span class="devsite-syntax-kn">import</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-s1">'./index.css'</span><span class="devsite-syntax-p">;</span> <span class="devsite-syntax-kn">import</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-nn">App</span> <span class="devsite-syntax-kn">from</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-s1">'./App'</span><span class="devsite-syntax-p">;</span> <span class="devsite-syntax-n">ReactDOM</span><span class="devsite-syntax-o">.</span><span class="devsite-syntax-n">render</span><span class="devsite-syntax-p">(</span><<span class="devsite-syntax-n">App</span> <span class="devsite-syntax-o">/</span>><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">document</span><span class="devsite-syntax-o">.</span><span class="devsite-syntax-n">getElementById</span><span class="devsite-syntax-p">(</span><span class="devsite-syntax-s1">'root'</span><span class="devsite-syntax-p">));</span> <span class="devsite-syntax-n">const</span> <span class="devsite-syntax-n">rootElement</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-n">document</span><span class="devsite-syntax-o">.</span><span class="devsite-syntax-n">getElementById</span><span class="devsite-syntax-p">(</span><span class="devsite-syntax-s2">"root"</span><span class="devsite-syntax-p">);</span> <span class="devsite-syntax-k">if</span> <span class="devsite-syntax-p">(</span><span class="devsite-syntax-n">rootElement</span><span class="devsite-syntax-o">.</span><span class="devsite-syntax-n">hasChildNodes</span><span class="devsite-syntax-p">())</span> <span class="devsite-syntax-p">{</span> <span class="devsite-syntax-n">ReactDOM</span><span class="devsite-syntax-o">.</span><span class="devsite-syntax-n">hydrate</span><span class="devsite-syntax-p">(</span><<span class="devsite-syntax-n">App</span> <span class="devsite-syntax-o">/</span>><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">rootElement</span><span class="devsite-syntax-p">);</span> <span class="devsite-syntax-p">}</span> <span class="devsite-syntax-k">else</span> <span class="devsite-syntax-p">{</span> <span class="devsite-syntax-n">ReactDOM</span><span class="devsite-syntax-o">.</span><span class="devsite-syntax-n">render</span><span class="devsite-syntax-p">(</span><<span class="devsite-syntax-n">App</span> <span class="devsite-syntax-o">/</span>><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">rootElement</span><span class="devsite-syntax-p">);</span> <span class="devsite-syntax-p">}</span> </code></pre></devsite-code> <p>Instead of only using <code translate="no" dir="ltr">ReactDOM.render</code> to render the root React element directly into the DOM, this checks to see if any child nodes are already present to determine whether HTML contents were pre-rendered (or rendered on the server). If that's the case, <code translate="no" dir="ltr">ReactDOM.hydrate</code> is used instead to attach event listeners to the already created HTML instead of creating it anew.</p> <p>Building the application will now generate static HTML files as payloads for each route that is crawled. You can take a look at what the HTML payload looks like by clicking the URL of the HTML request and then clicking the <strong>Previews</strong> tab within Chrome DevTools.</p> <p><img src="/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9.png" alt="A before and after comparison. The after shot shows content has rendered." width="800" height="450" srcset="https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_36.png 36w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_48.png 48w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_72.png 72w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_96.png 96w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_480.png 480w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_720.png 720w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_856.png 856w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_960.png 960w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_1440.png 1440w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_1920.png 1920w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-fd751bb2379c9_2880.png 2880w" sizes="(max-width: 840px) 100vw, 856px"></p> <aside class="note"><b>Note: </b> <code translate="no" dir="ltr">react-snap</code> can be used for other frameworks than React! This includes Vue and Preact. More instructions about this can be found in the <a href="https://github.com/stereobooster/react-snap"><code translate="no" dir="ltr">react-snap</code> README</a>. </aside> <h2 id="flash_of_unstyled_content" data-text="Flash of unstyled content" tabindex="-1">Flash of unstyled content</h2> <p>Although static HTML is now rendered almost immediately, it still remains unstyled by default which may cause the issue of showing a "flash of unstyled content" (FOUC). This can be especially noticeable if you are using a CSS-in-JS library to generate selectors since the JavaScript bundle will have to finish executing before any styles can be applied.</p> <p>To help prevent this, the <strong>critical</strong> CSS, or the minimum amount of CSS that is needed for the initial page to render, can be inlined directly to the <code translate="no" dir="ltr"><head></code> of the HTML document. <code translate="no" dir="ltr">react-snap</code> uses another third-party library under the hood, <a href="https://github.com/peterbe/minimalcss"><code translate="no" dir="ltr">minimalcss</code></a>, to extract any critical CSS for different routes. You can enable this by specifying the following in your <code translate="no" dir="ltr">package.json</code> file:</p> <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="JSON"><code translate="no" dir="ltr"><span class="devsite-syntax-nt">"reactSnap"</span><span class="devsite-syntax-p">:</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-p">{</span> <span class="devsite-syntax-w"> </span><span class="devsite-syntax-nt">"inlineCss"</span><span class="devsite-syntax-p">:</span><span class="devsite-syntax-w"> </span><span class="devsite-syntax-kc">true</span> <span class="devsite-syntax-p">}</span> </code></pre></devsite-code> <p>Taking a look at the response preview in Chrome DevTools will now show the styled page with critical CSS inlined.</p> <p><img src="/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609.png" alt="A before and after comparison. The after shot shows content has rendered and is styled because of inlined critical CSS." width="800" height="450" srcset="https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_36.png 36w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_48.png 48w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_72.png 72w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_96.png 96w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_480.png 480w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_720.png 720w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_856.png 856w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_960.png 960w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_1440.png 1440w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_1920.png 1920w,https://web.dev/static/articles/prerender-with-react-snap/image/a-before-after-compariso-88a1a76fa5609_2880.png 2880w" sizes="(max-width: 840px) 100vw, 856px"></p> <aside class="caution"><b>Caution: </b> The <code translate="no" dir="ltr">inlineCSS</code> option is still experimental. It is worth double-checking to make sure styles are being applied correctly for your routes. </aside> <h3 id="conclusion" data-text="Conclusion" tabindex="-1">Conclusion</h3> <p>If you are not server-side rendering routes in your application, use <code translate="no" dir="ltr">react-snap</code> to pre-render static HTML to your users.</p> <ol> <li>Install it as a development dependency and begin with just the default settings.</li> <li>Use the experimental <code translate="no" dir="ltr">inlineCss</code> option to inline critical CSS if it works for your site.</li> <li>If you are using code splitting on a component level within any routes, be careful not to pre-render a loading state to your users. 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