HTML Academy · Lesson

Custom Data Attributes data-*

Store custom data directly on elements with data-* attributes.

Lesson 4 of 413 steps

Custom Data Attributes data-* is a free HTML Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the HTML Academy learning path, and your progress syncs across the web and the CoddyKit app. The HTML Academy course includes 4 lessons in total.

What Are data-* Attributes?

Custom data attributes let you store extra information directly on HTML elements:

<button data-user-id="42" data-action="delete">Delete User</button>
<li data-category="frontend" data-level="beginner">HTML</li>

<!-- Format: data-[name]="value" -->
<!-- Any alphanumeric name after data- is valid -->
<!-- Values are always strings -->

Why data-* Exists

Before data-*, developers stored extra data in:

Hidden inputs (messy)
CSS class names (fragile)
Non-standard attributes like rel or title (invalid HTML)

HTML5 formalized data-* as the official way to embed custom data without affecting semantics or presentation.

Naming Rules

data-* attribute naming rules:

Must start with data-
Name can contain lowercase letters, digits, hyphens, underscores, periods
No uppercase letters in the name (the HTML spec lowercases them)
Name cannot start with xml

<div data-user-id="1">OK</div>
<div data-product-category="tools">OK</div>
<!-- <div data-XML-thing="1">BAD: starts with xml -->
<!-- <div data-MyProp="1">BAD: uppercase in name -->

Reading data-* in JavaScript

Access custom data via the dataset property:

<button id="btn"
  data-user-id="42"
  data-action="delete"
  data-confirm-message="Are you sure?"
>
  Delete
</button>

<script>
const btn = document.getElementById('btn');
console.log(btn.dataset.userId);          // '42'
console.log(btn.dataset.action);          // 'delete'
console.log(btn.dataset.confirmMessage);  // 'Are you sure?'
// Note: data-user-id → dataset.userId (camelCase conversion)
</script>

Writing data-* with dataset

Set and remove data attributes via dataset:

const card = document.querySelector('.card');

// Set:
card.dataset.status = 'active';
// Now: <div class="card" data-status="active">

// Delete:
delete card.dataset.status;
// Now: data-status is removed

// Alternatively with setAttribute:
card.setAttribute('data-status', 'inactive');
card.removeAttribute('data-status');

Hyphen to camelCase Conversion

The dataset API converts hyphen-separated names to camelCase:

<!-- HTML attribute         JavaScript dataset property -->
data-user-id           →   dataset.userId
data-product-category  →   dataset.productCategory
data-max-retries       →   dataset.maxRetries
data-is-active         →   dataset.isActive

CSS Attribute Selectors

Use data attributes in CSS selectors:

[data-status="active"] {
  color: green;
}

[data-status="inactive"] {
  color: gray;
  text-decoration: line-through;
}

/* Present but any value: */
[data-featured] {
  border: 2px solid gold;
}

Tooltip Pattern

Build pure-CSS tooltips with data attributes:

<span data-tooltip="This is a tooltip">Hover me</span>

/* CSS: */
/*
[data-tooltip] {
  position: relative;
}
[data-tooltip]::after {
  content: attr(data-tooltip);  /* attr() reads the attribute value */
  position: absolute;
  background: #333;
  color: white;
  padding: 0.25rem 0.5rem;
  border-radius: 4px;
  white-space: nowrap;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.2s;
}
[data-tooltip]:hover::after { opacity: 1; }
*/

State Tracking Pattern

Track component state with data attributes:

<button data-state="off" id="toggle">Toggle</button>

<script>
document.getElementById('toggle').addEventListener('click', function() {
  const state = this.dataset.state;
  this.dataset.state = state === 'off' ? 'on' : 'off';
  this.textContent = this.dataset.state === 'on' ? 'Active' : 'Toggle';
});
</script>

/* CSS: */
/* [data-state="on"] { background: green; color: white; } */

data-* vs JavaScript Variables

When to use data-* vs storing data in JavaScript:

Use data-* for values that correspond directly to a DOM element and may be read by CSS
Use JavaScript variables/objects for complex data structures not tied to specific elements
Do not store large or sensitive data in data-* — it is visible in DevTools

data-* Is Not for Presentation

Common mistakes to avoid:

<!-- BAD: storing presentation data in data-* -->
<div data-color="red" data-font-size="18">Text</div>
<!-- Use CSS classes instead: -->
<div class="text-danger text-lg">Text</div>

<!-- BAD: storing sensitive data -->
<div data-api-key="sk-123...">Content</div>
<!-- NEVER store secrets in HTML attributes -->

Quick Check

How does the dataset API access a data-product-id attribute?

Answer it in the interactive lesson

Recap: data-* Attributes

Custom data attribute essentials:

data-name="value" — store custom metadata on elements
Accessed via element.dataset.camelCase
Hyphens convert to camelCase in dataset API
Use in CSS with [data-name="value"] selectors
Never store sensitive data (keys, passwords) in data attributes

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Frequently Asked Questions

Is the “Custom Data Attributes data-*” lesson free?

Yes — any 3 lessons of the HTML Academy learning path, including “Custom Data Attributes data-*”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The HTML Academy course includes 4 lessons in total.

What will I learn in “Custom Data Attributes data-*”?

Store custom data directly on elements with data-* attributes. You practise HTML Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start HTML Academy?

No prior experience is required. HTML Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Custom Data Attributes data-*” lesson take?

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