What Is JavaScript? Adding Interactivity to Your Website

JavaScript is the programming language that gives web pages behavior. Drop-down menus, form checks, shopping carts that update without reloading the page, maps and galleries all run on JavaScript. HTML builds the structure of a page and CSS its appearance. JavaScript makes the page respond when a visitor does something. Used in the right amount, it makes a site more useful; too much of it slows the site down and can make search engines' job harder.

What is JavaScript?

MDN describes JavaScript as a lightweight, interpreted (or just-in-time compiled) programming language with first-class functions. Although it is best known as the scripting language for web pages, it is also used in many environments outside the browser, such as Node.js.

Let's clear up two common confusions right away:

• JavaScript is not Java. MDN states this clearly: although the two names are similar, their syntax, semantics and uses are very different.
• JavaScript and ECMAScript are two sides of the same thing. ECMAScript is the language standard published by Ecma International (ECMA-262). Browsers implement JavaScript according to this standard. The 15th edition of the standard (ECMAScript 2024) was published in June 2024; since 2015, a new edition has come out every June.

A simple example:

“`html <button id="quote">Open the quote form</button> <form id="quote-form" hidden>…</form>

<script> document.getElementById("quote").addEventListener("click", () => { document.getElementById("quote-form").hidden = false; }); </script> “`

When the visitor clicks the button, JavaScript catches this "click event" and makes the hidden form visible. The page doesn't reload.

How does it work?

When the browser opens a page, it reads the HTML, downloads the JavaScript files referenced in it and runs them. JavaScript can access the page's tree structure (the DOM, Document Object Model) and change it: add a new element, hide one, update some text, request data from the server and place it on the page.

This comes at a cost: it takes time for the browser to download, parse and execute JavaScript. While that work is going on, the browser may be slow to respond to the visitor's click. On phones, which have less processing power than computers, the difference can be more noticeable.

Can Google read JavaScript?

Yes, but as part of a process. According to Google's JavaScript SEO guide, Google processes pages that use JavaScript in three phases: crawling, rendering and indexing. A page is crawled first, then placed in a rendering queue and run in a Chromium-based browser; what gets indexed is the rendered HTML. In the same guide, Google says that server-side rendering or pre-rendering is still a good idea, both for users and for other crawlers that can't run JavaScript.

The practical takeaway for a business owner: make sure important content such as your service descriptions, your quote request form and your contact details is directly in the page's HTML. Content that is loaded by JavaScript only after a click may not be seen by search engines.

How it affects your website and your business

Usability

Well-used JavaScript makes things easier for visitors: it warns about a mistyped phone number before the form is submitted, makes a product gallery swipeable and opens the mobile menu. These are direct parts of the user experience.

Speed and Core Web Vitals

In March 2024, Google started using Interaction to Next Paint (INP) instead of First Input Delay (FID) in its Core Web Vitals. INP measures how quickly a page responds to user interactions; according to Google, responding to interactions in under 200 milliseconds in at least 75% of experiences counts as "good." Heavy JavaScript is one of the main things that hurts INP: the visitor taps the menu, but it opens late because the browser is busy running another script.

SEO

As described above, pages whose content depends entirely on JavaScript can appear late or incompletely in search. If your links are clickable elements that work only through JavaScript instead of real <a href> tags, it is also harder for Google to discover your pages. For an overview of the topic, see our article What is SEO?

Security and maintenance

Every third-party script you add to your site (a chat box, an analytics tool, an ad tag, a plugin) means running someone else's code on your site. These scripts affect both speed and security. Removing the ones you don't use and keeping the rest up to date is part of maintenance.

Common mistakes

• Every plugin loading its own script on every page: for example, a form script that is only needed on the contact page also loading on the home page.
• Showing important content only through JavaScript: tabs, accordions and "read more" buttons should keep the content in the HTML and only change its visibility.
• Heavy animation: many elements moving as you scroll can cause stuttering, especially on phones.
• Turning on "optimization" settings without testing: caching settings that delay or combine scripts may raise your speed score but break menus and forms. After every change, you need to check that the site actually works.
• A click event instead of a link: menu items that are not real links.

Our experience

On corporate sites, we use JavaScript for a purpose: in places where it genuinely makes things easier for visitors, such as the mobile menu, form validation, galleries, and phone and WhatsApp actions. We keep important content such as service descriptions and contact details directly in the HTML.

We follow the same principle when tuning speed settings in our projects: after every setting that delays or combines scripts, we test one by one that menus, forms and click actions still work. A high speed score doesn't make up for a quote form that doesn't work.

In custom software projects, JavaScript (and TypeScript) is the application itself. Unlike corporate sites, we plan this kind of project as an application architecture.

Frequently asked questions

Conclusion

JavaScript is the layer of your site that talks to visitors. Used with purpose, it makes your site more useful through forms, menus and galleries. Used without control, it slows your site down, makes it harder for search engines to see your content and adds to the maintenance load. A good balance is to build the content solidly in HTML first and add JavaScript where it genuinely helps.

If you'd like to plan your site's interactive elements or a web application together, take a look at our web design and software and digital solutions services, or get in touch with us.