A useful interface does not need to be complicated. Start with native HTML, then add JavaScript only when you need it.
What You Need to Know
A custom element can appear in HTML before its class is registered. When the browser learns the definition, it upgrades matching elements and runs their lifecycle callbacks.
Try the Example
Take a look at the example. We’ll explain the important lines right below it.
<hello-card id="welcome-card"></hello-card>
<script>
class HelloCard extends HTMLElement {
connectedCallback() {
this.textContent = "Hello from a reusable component!";
}
}
customElements.define("hello-card", HelloCard);
</script>Let’s Read the Code
- <script>: Script contains JavaScript, data, or other browser-understood script formats depending on type.
- id: An id uniquely identifies an element on this page.
Read the named elements and attributes together. They explain how the browser connects this feature to the rest of the page.
What Happens in the Browser?
When this example includes JavaScript, it supplies behavior that ordinary HTML does not automatically provide. Always think about a fallback.
What makes it interactive? Look for the JavaScript that finds an element and reacts to an event. HTML provides the initial controls and content; the script updates the page. Try removing the script to see which part still works natively.
Your Turn
Run this on a local development server if it loads modules or resources. Change the visible text or trigger and check what still works with scripts disabled.
Common Beginner Mistakes
- Assuming new HTML syntax works in every browser without testing.
- HTML supplies structure; JavaScript is necessary for the scripted behavior in this example. Check keyboard access and focus movement too.
Key Takeaways
- A custom element can appear in HTML before its class is registered.
- <script>: Script contains JavaScript, data, or other browser-understood script formats depending on type.
- Test the behavior you expect rather than trusting how the snippet looks in your editor.