Understand the JavaScript SEO basics

JavaScript is an important part of the web platform because it provides many features that turn the web into a powerful application platform. Making your JavaScript-powered web applications discoverable via Google Search can help you find new users and re-engage existing users as they search for the content your web app provides. While Google Search runs JavaScript with anevergreen version of Chromium,there area few things that you can optimize.

This guide describes how Google Search processes JavaScript and best practices for improving JavaScript web apps for Google Search.

How Google processes JavaScript

Google processes JavaScript web apps in three main phases:

  1. Crawling
  2. Rendering
  3. Inde xing

Googlebot takes a URL from the crawl queue, crawls it, then passes it into the processing stage. The processing stage extracts links that go back on the crawl queue and queues the page for rendering. The page goes from the render queue to the renderer which passes the rendered HTML back to processing which indexes the content and extracts links to put them into the crawl queue.

Googlebot queues pages for both crawling and rendering. It is not immediately obvious when a page is waiting for crawling and when it is waiting for rendering. When Googlebot fetches a URL from the crawling queue by making an HTTP request, it first checks if you allow crawling. Googlebot reads therobots.txtfile. If it marks the URL as disallowed, then Googlebot skips making an HTTP request to this URL and skips the URL. Google Search won't render JavaScript from blocked files or on blocked pages.

Googlebot then parses the response for other URLs in thehrefattribute of HTML links and adds the URLs to the crawl queue. To prevent link discovery, use thenofollowmechanism.

Crawling a URL and parsing the HTML response works well for classical websites or server-side rendered pages where the HTML in the HTTP response contains all content. Some JavaScript sites may use theapp shell modelwhere the initial HTML does not contain the actual content and Google needs to execute JavaScript before being able to see the actual page content that JavaScript generates.

Googlebot queues all pages for rendering, unless arobotsmetatag or headertells Google not to index the page. The page may stay on this queue for a few seconds, but it can take longer than that. Once Google's resources allow, a headless Chromium renders the page and executes the JavaScript. Googlebot parses the rendered HTML for links again and queues the URLs it finds for crawling. Google also uses the rendered HTML to index the page.

Keep in mind thatserver-side or pre-renderingis still a great idea because it makes your website faster for users and crawlers, and not all bots can run JavaScript.

Describe your page with unique titles and snippets

Unique, descriptive<title>elementsandmeta descriptionshelp users quickly identify the best result for their goal. You can use JavaScript to set or change the meta description as well as the<title>element.

Write compatible code

Browsers offer many APIs and JavaScript is a quickly-evolving language. Google has some limitations regarding which APIs and JavaScript features it supports. To make sure your code is compatible with Google, follow ourguidelines for troubleshooting JavaScript problems.

We recommendusing differential serving and polyfillsif you feature-detect a missing browser API that you need. Since some browser features cannot be polyfilled, we recommend that you check the polyfill documentation for potential limitations.

Use meaningful HTTP status codes

Googlebot usesHTTP status codesto find out if something went wrong when crawling the page.

To tell Googlebot if a page can't be crawled or indexed, use a meaningful status code, like a404for a page that could not be found or a401code for pages behind a login. You can use HTTP status codes to tell Googlebot if a page has moved to a new URL, so that the index can be updated accordingly.

Here's alist of HTTP status codesand how they effect Google Search.

Avoidsoft 404errors in single-page apps

In client-side rendered single-page apps, routing is often implemented as client-side routing. In this case, using meaningful HTTP status codes can be impossible or impractical. To avoidsoft 404errorswhen using client-side rendering and routing, use one of the following strategies:

  • Use aJavaScript redirectto a URL for which the server responds with a404HTTP status code (for example/not-found).
  • Add a<meta name= "robots" content= "noindex" >to error pages using JavaScript.

Here is sample code for the redirect approach:

fetch(`/api/products/${productId}`)
.then(response => response.json())
.then(product => {
if(product.exists) {
showProductDetails(product); // shows the product information on the page
} else {
// this product does not exist, so this is an error page.
window.location.href = '/not-found'; // redirect to 404 page on the server.
}
})

Here is sample code for thenoindextag approach:

fetch(`/api/products/${productId}`)
.then(response => response.json())
.then(product => {
if(product.exists) {
showProductDetails(product); // shows the product information on the page
} else {
// this product does not exist, so this is an error page.
// Note: This example assumes there is no other robots meta tag present in the HTML.
const metaRobots = document.createElement('meta');
metaRobots.name = 'robots';
metaRobots.content = 'noindex';
document.head.appendChild(metaRobots);
}
})

Use the History API instead of fragments

Google can only discover your links if they are<a>HTML elements with anhrefattribute.

For single-page applications with client-side routing, use theHistory APIto implement routing between different views of your web app. To ensure that Googlebot can parse and extract your URLs, avoid using fragments to load different page content. The following example is a bad practice, because Googlebot can't reliably resolve the URLs: 

<nav>
<ul>
<li><a href= "#/products" >Our products</a></li>
<li><a href= "#/services" >Our services</a></li>
</ul>
</nav>

<h1>Welcome to example!</h1>
<div id= "placeholder" >
<p>Learn more about <a href= "#/products" >our products</a> and <a href= "#/services" >our services</p>
</div>
<script>
window.addEventListener('hashchange', function goToPage() {
// this function loads different content based on the current URL fragment
const pageToLoad = window.location.hash.slice(1); // URL fragment
document.getElementById('placeholder').innerHTML = load(pageToLoad);
});
</script>

Instead, you can make sure your URLs are accessible to Googlebot by implementing the History API:

<nav>
<ul>
<li><a href= "/products" >Our products</a></li>
<li><a href= "/services" >Our services</a></li>
</ul>
</nav>

<h1>Welcome to example!</h1>
<div id= "placeholder" >
<p>Learn more about <a href= "/products" >our products</a> and <a href= "/services" >our services</p>
</div>
<script>
function goToPage(event) {
event.preventDefault(); // stop the browser from navigating to the destination URL.
const hrefUrl = event.target.getAttribute('href');
const pageToLoad = hrefUrl.slice(1); // remove the leading slash
document.getElementById('placeholder').innerHTML = load(pageToLoad);
window.history.pushState({}, window.title, hrefUrl) // Update URL as well as browser history.
}

// Enable client-side routing for all links on the page
document.querySelectorAll('a').forEach(link => link.addEventListener('click', goToPage));

</script>

While we don't recommend using JavaScript for this, it is possible to inject arel= "canonical"link tagwith JavaScript. Google Search will pick up the injected canonical URL when rendering the page. Here is an example to inject arel= "canonical"link tag with JavaScript: 

fetch('/api/cats/' + id)
.then(function (response) { return response.json(); })
.then(function (cat) {
// creates a canonical link tag and dynamically builds the URL
// e.g. https://example /cats/simba
const linkTag = document.createElement('link');
linkTag.setAttribute('rel', 'canonical');
linkTag.href = 'https://example /cats/' + cat.urlFriendlyName;
document.head.appendChild(linkTag);
});

Userobotsmetatags carefully

You can prevent Google from inde xing a page or following links through therobotsmetatag. For example, adding the followingmetatag to the top of your page blocks Google from inde xing the page: 

<!-- Google won't index this page or follow links on this page -->
<meta name= "robots" content= "noindex, nofollow" >

You can use JavaScript to add arobotsmetatag to a page or change its content. The following example code shows how to change therobotsmetatag with JavaScript to prevent inde xing of the current page if an API call doesn't return content. 

fetch('/api/products/' + productId)
.then(function (response) { return response.json(); })
.then(function (apiResponse) {
if (apiResponse.isError) {
// get therobotsmetatag
var metaRobots = document.querySelector('meta[name= "robots" ]');
// if there was norobotsmetatag, add one
if (!metaRobots) {
metaRobots = document.createElement('meta');
metaRobots.setAttribute('name', 'robots');
document.head.appendChild(metaRobots);
}
// tell Google to exclude this page from the index
metaRobots.setAttribute('content', 'noindex');
// display an error message to the user
errorMsg.textContent = 'This product is no longer available';
return;
}
// display product information
//...
});

When Google encountersnoindexin therobotsmetatag before running JavaScript, it doesn't render or index the page.

Use long-lived caching

Googlebot caches aggressively in order to reduce network requests and resource usage. WRS may ignore caching headers. This may lead WRS to use outdated JavaScript or CSS resources. Content fingerprinting avoids this problem by making a fingerprint of the content part of the filename, likemain.2bb85551.js. The fingerprint depends on the content of the file, so updates generate a different filename every time. Check out theweb.dev guide on long-lived caching strategiesto learn more.

Use structured data

When usingstructured dataon your pages, you can useJavaScript to generate the required JSON-LD and inject it into the page.Make sure totest your implementationto avoid issues.

Follow best practices for web components

Google supports web components. When Google renders a page, itflattens the shadow DOM and light DOMcontent. This means Google can only see content that's visible in the rendered HTML. To make sure that Google can still see your content after it's rendered, use theRich Results Testor theURL Inspection Tooland look at the rendered HTML.

If the content isn't visible in the rendered HTML, Google won't be able to index it.

The following example creates a web component that displays its light DOM content inside its shadow DOM. One way to make sure both light DOM and shadow DOM content is displayed in the rendered HTML is to use aSlotelement. 

<script>
class MyComponent extends HTMLElement {
constructor() {
super();
this.attachShadow({ mode: 'open' });
}

connectedCallback() {
let p = document.createElement('p');
p.innerHTML = 'Hello World, this is shadow DOM content. Here comes the light DOM: <slot></slot>';
this.shadowRoot.appendChild(p);
}
}

window.customElements.define('my-component', MyComponent);
</script>

<my-component>
<p>This is light DOM content. It's projected into the shadow DOM.</p>
<p>WRS renders this content as well as the shadow DOM content.</p>
</my-component>

After rendering, Google can index this content:

<my-component>
Hello World, this is shadow DOM content. Here comes the light DOM:
<p>This is light DOM content. It's projected into the shadow DOM<p>
<p>WRS renders this content as well as the shadow DOM content.</p>
</my-component>

Fix images and lazy-loaded content

Images can be quite costly on bandwidth and performance. A good strategy is to use lazy-loading to only load images when the user is about to see them. To make sure you're implementing lazy-loading in a search-friendly way, followour lazy-loading guidelines.

Design for accessibility

Create pages for users, not just search engines. When you're designing your site, think about the needs of your users, including those who may not be using a JavaScript-capable browser (for example, people who use screen readers or less advanced mobile devices). One of the easiest ways to test your site's accessibility is to preview it in your browser with JavaScript turned off, or to view it in a text-only browser such as Lynx. Viewing a site as text-only can also help you identify other content which may be hard for Google to see, such as text embedded in images.