Child elements in webcomponent

Question

Suppose we have a custom element to use in this way:

<list-image>
    <img src="" /> 
     .... 
</list-image>

where list-image displays img tags in a slider way. If the user of component inserts an img tag with

document.querySelector('list-image').insertAdjacentHTML('beforeend', '<img src="..." />');

is it possible for the component to know the new element img?

Answer 1

The solution is to use a MutationObserver on the <list-image> custom element itself.

In the connectedCallback() method, observe mutations on child elements:

customElements.define('list-image', class extends HTMLElement {
  connectedCallback() {
    var observer = new MutationObserver(function(mutations) {
      mutations.forEach(function(mutation) {
        //Detect <img> insertion
        if (mutation.addedNodes.length)
          console.info('Node added: ', mutation.addedNodes[0])
      })
    })

    observer.observe(this, { childList: true })
  }
})

function add() {
  document.querySelector('list-image')
          .insertAdjacentHTML('beforeend', '<img alt="TOTO" />')
}

<list-image>
  <img alt="Image1">
  <img alt="Image2">
</list-image>
<button onclick="add()">Add image</button>