Mutation Observer

Mutation observers watch child elements and dispatch an event when they mutate.

The component uses a MutationObserver to monitor when its direct children are added, removed, or modified. A quiet-mutation event is dispatched for each observed change, providing detailed information about what was mutated.

The component is styled with display: contents, allowing you to easily apply flex and grid layouts to the containing element without the component interfering.

<div id="mutation__overview">
  <quiet-mutation-observer child-list>
    <div class="box">1 <button aria-label="Remove">✕</button></div>
    <div class="box">2 <button aria-label="Remove">✕</button></div>
    <div class="box">3 <button aria-label="Remove">✕</button></div>
  </quiet-mutation-observer>
</div>

<quiet-button>Add box</quiet-button>

<small>Add and remove some items, then open the console to inspect the output.</small>

<script>
  const container = document.getElementById('mutation__overview');
  const mutationObserver = container.querySelector('quiet-mutation-observer');
  const appendButton = container.nextElementSibling;
  let count = mutationObserver.querySelectorAll('.box').length;

  // Listen for mutations
  mutationObserver.addEventListener('quiet-mutation', event => {
    console.log(event.detail.record);
  });

  // Remove a card
  mutationObserver.addEventListener('click', event => {
    const button = event.target.closest('button');
    const box = button?.closest('.box');
    box?.remove();
  });

  // Append a box
  appendButton.addEventListener('click', () => {
    const html = `<div class="box">${++count} <button aria-label="Remove">✕</button></div>`;
    mutationObserver.insertAdjacentHTML('beforeEnd', html);
  });
</script>

<style>
  #mutation__overview {
    display: flex;
    flex-wrap: wrap;
    gap: 1.5rem;
    min-height: 5rem;
    margin-block-end: 1rem;

    .box {
      display: flex;
      align-items: center;
      justify-content: center;
      position: relative;
      min-width: 5rem;
      height: 5rem;
      border-radius: var(--quiet-border-radius);
      background-color: var(--quiet-neutral-fill-softer);
      box-shadow: var(--quiet-shadow-softer);
      text-align: center;
      padding: 1rem;

      button {
        display: flex;
        align-items: center;
        justify-content: center;
        background-color: var(--quiet-neutral-fill-loud);
        color: var(--quiet-neutral-text-on-loud);
        font-size: .75em;
        padding: 0;
        width: 1.25rem;
        height: 1.25rem;
        min-height: 0;
        position: absolute;
        top: -0.625rem;
        right: -0.625rem;
        border-radius: 50%;
      }
    }

    ~ small {
      display: block;
      margin-block-start: 1rem;
    }
  }
</style>

Examples Jump to heading #

Providing content Jump to heading #

Slot one or more elements into the mutation observer and listen for the quiet-mutation event. The event includes event.detail.record, which is a MutationRecord object that corresponds to the mutated element.

In its simplest form, a mutation observer can be used like this.

<quiet-mutation-observer child-list>
  <div>Item 1</div>
  <div>Item 2</div>
  <div>Item 3</div>
</quiet-mutation-observer>

<script>
  const mutationObserver = document.querySelector('quiet-mutation-observer');

  // Listen for mutations
  mutationObserver.addEventListener('quiet-mutation', event => {
    console.log(event.detail.record); // MutationRecord
  });
</script>

Observing attribute changes Jump to heading #

Use the attr attribute to monitor changes to element attributes. You can optionally specify attr-old-value to record the previous attribute value.

<quiet-mutation-observer attr attr-old-value>
  <div id="target" class="example">Watch my attributes</div>
</quiet-mutation-observer>

<script>
  const observer = document.querySelector('quiet-mutation-observer');
  const target = observer.querySelector('#target');

  observer.addEventListener('quiet-mutation', event => {
    const record = event.detail.record;
    console.log(`Attribute "${record.attributeName}" changed`);
    console.log(`Old value: ${record.oldValue}`);
    console.log(`New value: ${record.target.getAttribute(record.attributeName)}`);
  });

  // Change an attribute to trigger the observer
  target.className = 'modified';
</script>

Filtering specific attributes Jump to heading #

Limit observations to specific attributes using the attr-filter attribute. Separate multiple attribute names with spaces.

<quiet-mutation-observer attr attr-filter="class data-state">
  <div class="example" data-state="active" id="example">
    Only class and data-state changes are observed
  </div>
</quiet-mutation-observer>

<script>
  const observer = document.querySelector('quiet-mutation-observer');
  const target = observer.querySelector('#example');

  observer.addEventListener('quiet-mutation', event => {
    console.log(`Observed attribute: ${event.detail.record.attributeName}`);
  });

  // These will trigger events
  target.className = 'modified';
  target.setAttribute('data-state', 'inactive');

  // This will NOT trigger an event
  target.id = 'new-id';
</script>

Observing text content changes Jump to heading #

Use the character-data attribute to monitor changes to text nodes. The character-data-old-value attribute records the previous text content.

<quiet-mutation-observer character-data character-data-old-value subtree>
  <p id="text-content">Original text content</p>
</quiet-mutation-observer>

<script>
  const observer = document.querySelector('quiet-mutation-observer');
  const paragraph = observer.querySelector('#text-content');

  observer.addEventListener('quiet-mutation', event => {
    const record = event.detail.record;
    console.log(`Text changed from "${record.oldValue}" to "${record.target.textContent}"`);
  });

  // Change text content to trigger the observer
  paragraph.textContent = 'Updated text content';
</script>

Observing nested elements Jump to heading #

By default, only direct children are observed. Add the subtree attribute to observe changes in nested elements as well.

<quiet-mutation-observer child-list subtree>
  <div class="container">
    <div class="nested">
      <span>Nested content</span>
    </div>
  </div>
</quiet-mutation-observer>

<script>
  const observer = document.querySelector('quiet-mutation-observer');
  const nested = observer.querySelector('.nested');

  observer.addEventListener('quiet-mutation', event => {
    console.log('Mutation detected in subtree:', event.detail.record);
  });

  // This will trigger the observer even though span is deeply nested
  nested.innerHTML = '<span>New nested content</span>';
</script>

Combining multiple observation types Jump to heading #

You can observe multiple types of mutations simultaneously by combining attributes.

<quiet-mutation-observer 
  child-list 
  attr 
  character-data 
  subtree 
  attr-old-value 
  character-data-old-value
>
  <div class="comprehensive-example">
    <p>This observer watches everything</p>
  </div>
</quiet-mutation-observer>

<script>
  const observer = document.querySelector('quiet-mutation-observer');

  observer.addEventListener('quiet-mutation', event => {
    const record = event.detail.record;
    
    switch (record.type) {
      case 'childList':
        console.log('Child nodes changed');
        break;
      case 'attributes':
        console.log(`Attribute "${record.attributeName}" changed`);
        break;
      case 'characterData':
        console.log('Text content changed');
        break;
    }
  });
</script>

Disabling the observer Jump to heading #

Use the disabled attribute to temporarily stop observing mutations without removing the component.

<quiet-mutation-observer child-list disabled>
  <div>Changes to this content won't be observed</div>
</quiet-mutation-observer>

<script>
  const observer = document.querySelector('quiet-mutation-observer');

  // Re-enable observation
  observer.disabled = false;

  // Disable observation again
  observer.disabled = true;
</script>

API Jump to heading #

Importing Jump to heading #

The autoloader is the recommended way to import components but, if you prefer to do it manually, the following code snippets will be helpful.

import 'https://cdn.jsdelivr.net/npm/@quietui/quiet-browser@1.0.0/dist/components/mutation-observer/mutation-observer.js';

import '@quietui/quiet/dist/components/mutation-observer/mutation-observer.js';

Slots Jump to heading #

Mutation Observer supports the following slots. Learn more about using slots

Properties Jump to heading #

Mutation Observer has the following properties that can be set with corresponding attributes. In many cases, the attribute's name is the same as the property's name. If an attribute is different, it will be displayed after the property. Learn more about attributes and properties

Events Jump to heading #

Mutation Observer dispatches the following custom events. You can listen to them the same way was native events. Learn more about custom events