แอตทริบิวต์ข้อมูลกำหนดเอง data-*

<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 -->

<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 -->

<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>

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');

<!-- 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

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

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

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

<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; }
*/

<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; } */

<!-- 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 -->