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JavaScript SEO – Best Practice Guide

Using JavaScript has an impact on how Google crawls and indexes your site. Make sure your use of JavaScript isn’t getting in the way of users finding your website via search engines.

JavaScript and SEO didn't always go well together. Search engine bots, including the Googlebot, couldn't crawl Javascript, so were unable to see any content embedded with JavaScript. Now, Google and other search engines can crawl and render JavaScript. In this article, we show you how you can implement JavaScript to ensure that your SEO isn't affected.

Can Search Engines Crawl JavaScript?

Both Google and Bing can crawl and render JavaScript. In the United States, Google and Bing combined have a market share of over 90%. However, although they can crawl and render JavaScript, there's no guarantee that they will.

Tests show that the amount of JavaScript crawled and rendered on different websites differs greatly. Using JavaScript is therefore always associated with a certain risk; that the crawlers do not crawl and index the content, meaning users will not find it in search engines. This shouldn't scare you away from JavaScript, but from am SEO perspective, there are many things you should watch out for.

BING restricts the rendering capabilities of its bots with respect to JavaScript, and does not necessarily support all the same JavaScript frameworks supported in the latest version of your browser. Therefore BING recommends using dynamic rendering. Google also advises webmasters with quickly changing JavaScript content to use dynamic rendering.

Will JavaScript increase a website's loading time?

JavaScript websites must first be rendered by a client or a bot so that they can be displayed. This rendering takes time. Therefore, JS sites have a higher loading time than pure HTML sites, but with certain tools, loading times can be optimized even with JavaScript.

In this article, Andy Favell from searchenginewatch shows in this article how JavaScript extends the loading time of mobile websites.

If JavaScript is used in the form of a tracking code third party script, you should allow the code to load at the end asynchronously so that the page speed is not affected.

How does Google see JavaScript?

Google is able to render websites, including JavaScript and CSS. No important site components including those in JavaScript should be excluded from crawling.

Google has been able to render JavaScript and CSS since 2015, and recommended at the time that no important site components, including those in JavaScript or CSS are excluded from crawling via the robots.txt. Google also indicated that it prefers the principle of progressive enhancement, an approach involving the successive improvement of HTML websites. However, JavaScript would still be crawled and rendered.

In October 2018, in a response on Reddit, John Mueller pointed out that JavaScript would become increasingly important in the coming years, and gives SEOs the tip to focus more on JavaScript SEO: "If you're keen on technical SEO, then past HTML you're going to need to understand JS more and more".

Why is it difficult for search engines to crawl JavaScript?

It's difficult for search engines to crawl JavaScript due to computing capacity. HTML, which functions with PHP, CSS, and other technologies, can be directly read by a crawler - in this case, the source code is rendered immediately when the URL is called up.

With JavaScript, this direct access is not possible. Firstly, the crawler must analyze the DOM (Document Object Model). The DOM's code is loaded and listed, and only when this is done can the website be rendered. Every browser does this automatically when surfing the web. If you want to test JavaScript, you can use so-called "headless browsers." This is a browser without a graphic user interface that is operated by a command line.

5 Rules for JavaScript and SEO

  • The content must be indexable within the load event so that JavaScript content makes sense for search engines.

  • Content dependent on user events is not indexable.

  • JS sites require an indexable URL with server-side support.

  • The same SEO best practices used for HTML sites apply to rendering JS sites.

  • There should be no contradiction between HTML and JavaScript versions of the website.

Basics for the development of JavaScript websites.

JavaScript is more complex than HTML. To realize what is most important for SEO, you should firstly try to understand how JavaScript works:

1. Initial Request: The browser and the search engine bot start a GET request for the HTML code of the website and its affiliated assets.

2. DOM rendering: The JS script site delivers the DOM (Document Object Model) to the browser or the bot. The document shows how the content will be formed on the website and what the connections are between the individual elements on the site. The browser renders this information and makes it visible and usable for the user.

3. IDOM load: While the target site is being processed, the browser triggers events that belong to the DOMContentLoaded. The initial HTML document is then loaded and stored. The browser or the bot is now ready to implement JavaScript elements.

4. JavaScript implementation: JavaScript elements can now change the contents or functions of the website without the HTML source code having to be changed, removed, or expanded.

5. Load Event: As soon as the resources and the JS resources dependent on these are loaded, the browser starts the load, and the site is finished.

6. Post Load Events: After the JS site has been loaded, further content or functional elements can be changed or adapted by the user.

JavvaScript-rendering-process SEO JavaScript

Figure 1: JavaScript process

Search engines such as Google use so-called headless browsers in order to simulate access to a traditional browser. In contrast to the "normal" browser, the headless browser calls up the code via the DOM to render a website from it. In this way, the Googlebot can, for example, check which elements JavaScript inserts in order to modify the HTML site. After the rendering, the Googlebot can analyze and index the rendered elements like an HTML source.

With JavaScript, there are two versions for crawlers, the pre-DOM HTML code and the rendered post-DOM HTML code.

Important events from JavaScript that have an influence on SEO

Load events and user events can clearly influence your SEO. This is why:

  • Load Event: The load event is "fired" by the browser when a site is completely loaded. Search engine robots imitate common browsers when rendering JavaScript. The load event is therefore important for them - it makes it possible for them to take a snapshot of the rendered content. Events loaded after the load event has been fired will not be considered by JavaScript crawling, and therefore won't be indexed, because JavaScript can quickly change site content. This is especially the case for news sites or social networks feeds, such as Twitter and Facebook.

The time frame of the DOMContentLoaded can be measured with the Google development tool:

  • User Events: After the load event, further events can be triggered via JavaScript. Among these, for example, are the so-called "onClick-Events." These are user-triggered events, such as the restriction of site content or interactive navigation. However, this content, which is created or changed by user events, is usually not indexed by search engines, because it takes place after the load event.

Errors that you should avoid with JavaScript

If you use JavaScript on your website, Google can now render the elements after the load event quite well, and can finally read and index the snapshot like a traditional HTML site.

Most problems with JavaScript and SEO result from improper implementation. Many common SEO best practices can therefore also be used for JavaScript SEO. These are a few of the most common errors that can occur:

1. Indexable URLs: Every website requires unique and distinctive URLs so that the sites can be indexed at all. A pushState, as is created with JavaScript, however, does not generate a URL. Therefore, your JavaScript site also requires its own web document that can give a status code 200 OK as a server answer to a client or bot inquiry. Every product presented with JS (or each category of your website realized with JS) must therefore be equipped with a server-side URL so that your site can be indexed.

2. pushState errors: With the pushState method, JavaScript URLs can be changed. Therefore, you must absolutely ensure that the original URL is relayed with server-side support. Otherwise, you risk duplicate content.

3. Missing Metadata: With the use of JavaScript, many webmasters or SEO forget the basics and do not transfer meta data to the bot. However, the same SEO standards hold for JavaScript content as for HTML sites. So, think about title and meta description of ALT tags for images.

4. a href and img src: The Googlebot requires links that it can follow so that it can find further sites. Therefore, you should also provide links with href- or src-attributes in your JS documents.

5. Create unified versions: Through the rendering of JavaScript, preDOM and postDOM versions arise. Ensure that, if possible, no contradictions slip in and, for example, that canonical tags or paginations can be correctly interpreted. In this way, you will avoid cloaking.

6. Create access for all bots: Not all bots can deal with JavaScript like the Googlebot. It is therefore recommended to place title, meta information, and social tags in the HTML code.

7. Don't disable JS over robots.txt: Ensure that your JavaScript can also be crawled by the Googlebot. For this, the directories should not be excluded in the robots.txt.

8. Use a current sitemap: In order to show Google any possible changes in the JS contents, you should always keep the attribute "lastmod" current in your XML sitemap.

Check JavaScript websites: What To Do in an Audit

A JS website audit is primarily a manual inspection of individual elements.

To check your JavaScript, you can use the Google Developer Tools from Google Chrome as well as the Web Developer Expansion for Chrome.

1. Visual inspection

To get a feel for how a visitor will see a website, you should divide the content on the website into:

  • Visible contents on the site

  • Content that requires an interaction

  • Hidden content

  • Content that come from third parties, such as ratings

  • Content that contain product recommendations

This way, you can narrow down the selection to elements that can be realized with JavaScript. You should be checking the JavaScript elements with the goal of making these elements crawl-able.

2. Check HTML code

With web developer tools, you can turn off CSS, JavaScript, and cookies. You can see what is missing on the site in the remaining code. These elements are controlled by JavaScript.

Then, you can control meta elements such as the title and website description. So that bots can index these elements, they must be accessible via the load event. In general, however, only Google can currently read these elements. It is therefore recommended to write title and meta tags in the HTMl code even with JS sites.

3. Check rendered HTML

Load the site with deactivated JavaScript and CSS anew. Finally, right-click on the site and choose "Examine element" in the Chrome menu. On the right side, another window will appear. Click on the HTML tag. With a right click, an options menu will appear. Choose "Copy External HTML" here. Finally, insert the code into an editor. This code can then be indexed by search engines such as Google.

You can also test JavaScript with the Google Search Console using the URL inspection tool or the mobile friendly test tool.

Further things to consider

  • Indexable URLs: In order for a website to be indexed and ranked at all, it requires an indexable URL. If your website uses versions of JavaScript that cannot be called up via their own URL, it is not possible for search engines to index this site. If a URL exists but cannot be linked in JS code, the site similarly cannot rank.

  • pushState: If an improper URL is produced by a pushState, duplicate content can arise. Therefore, be especially sure to check internal links that are created with JavaScript.

  • Data attributes: On JS sites, further resources such as images can be stored in data attributes. However, in general, these resources cannot be loaded, rendered, and indexed by bots. It is therefore recommended that you incorporate resources, such as images, traditionally via HTML.

What tools can you use for checking JavaScript??

There are various tools that you can use in editing, creating or checking your JavaScript content, such as prerender.iom brombone, angular JS, SEO.JS, , Google Search Console, Isomorphes, and Ryte.

1. prerender.io

prerender.io is an open-source software that optimizes the rendering of a JS site. With this, the site is cached after rendering and can be pulled up more quickly when accessed by a bot.

2. Brombone

This program downloads your website and renders it in a web browser, meaning you can easily check whether AJAX retrievals and JavaScript are functioning correctly. DOM changes can be tested in the same way. If the rendering is working, these sites are stored as HTML. If a crawler is accessing your site, you can allow the rendered JS sites to be issued by a proxy from Brombone. In order to correctly implement the tool, you also require an XML sitemap.


With Angular JS, HTML snapshots can be prerendered so that the Googlebot can more quickly grasp and index JS sites.


With this program, JS code is likewise rendered as HTML and made crawl-able by Google. The program code is hereby transferred to your server. Having your own dashboard will help you to manage those of your JS elements and sites that have to be rendered. Moreover, the tool creates an XML sitemap with your JS sites.

5. Google Search Console

With the old version of the Search Console, Google helps you check JS elements by rendering individual sites. The tool also shows potential crawling problems.

6. Isomorphes JavaScript

With isomorphic JavaScript, applications can be carried out on the part of the server or the client. Back-end and front-end thus share the same code. By implementing the missing rendering, JavaScript is less error-prone with regards to SEO.

7. Ryte

Many SEO tools are now able to crawl JavaScript to give users a more complete analysis of their sites. As of February 2019, the Ryte crawler is also able to render and crawl JavaScript and CSS using an optimized Chrome browser. It executes the JavaScript on each page for up to 30 seconds, so all elements that are triggered when the page is first loaded are rendered and crawled. You can find out more in this article.


JavaScript can enormously expand the functionality of your website. However, there are many things you need to take into account to ensure that JavaScript fits into your SEO strategy. Sometimes it make more sense to use progressive enhancement rather than building a site solely with JavaScript, especially when considering AMP or progressive web apps. You should also make the use of the tools available to help create, edit or check the JavaScript elements on your site.

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Published on Jul 22, 2020 by Olivia Willson