Summary - Advanced DOM

JavaScript course · Advanced · Lesson 95/100

Build a lazy-loaded image component using IntersectionObserver and custom elements.

Summary - Advanced DOM

In this group you learned four modern DOM APIs:

MutationObserver: watches DOM node and attribute changes
IntersectionObserver: reports how an element intersects the viewport
ResizeObserver: notifies when an element's size changes
Custom elements and Shadow DOM: build your own tags and isolated components

All four are callback based; they start with observe and end with disconnect. Shadow DOM starts with attachShadow.

Exercise Lazy loader: stop when visible

Create an IntersectionObserver. The callback takes entries; for each entry, if isIntersecting is true, log to console and stop watching that element with observer.unobserve. Set threshold to 0.

Show example solution

1. Create with new IntersectionObserver((entries) => {...}).
2. For each entry, if isIntersecting, log and call observer.unobserve(entry.target).
3. Set { threshold: 0 }.

Example solution:
const observer = new IntersectionObserver((entries) => {
  for (const entry of entries) {
    if (entry.isIntersecting) {
      console.log('loaded');
      observer.unobserve(entry.target);
    }
  }
}, { threshold: 0 });

Good habits

A few things to keep in mind when using these APIs:

Always call disconnect when done; it prevents memory leaks.
Use one observer for many elements; do not create a new one per element.
Do not do heavy work inside the callback; just update the DOM.
In Shadow DOM, bind styles to the component with the :host selector.

These habits matter in large apps.

Exercise Custom element with shadow DOM

Define a custom element named my-card. The class should extend HTMLElement. In connectedCallback, attach a shadow root in open mode with this.attachShadow. Finally register it with customElements.define.

Show example solution

1. Write a MyCard class that extends HTMLElement.
2. In connectedCallback, attach a shadow root with this.attachShadow({ mode: 'open' }).
3. Register it with customElements.define as 'my-card'.

Example solution:
class MyCard extends HTMLElement {
  connectedCallback() {
    const root = this.attachShadow({ mode: 'open' });
    root.innerHTML = '<p>Hello</p>';
  }
}
customElements.define('my-card', MyCard);

Test yourself

Match APIs to use cases

MutationObserver

Node added
IntersectionObserver

Entering viewport
ResizeObserver

Size change
Shadow DOM

Style isolation
Which method stops an observer?
- disconnect
- observe
- stop
- remove
Show answer

disconnect — Correct. disconnect stops watching and frees memory.

Summary - Advanced DOM