Implementing Intersection Observer API

Introduction

The Intersection Observer API is a powerful tool for optimizing image and media loading. It allows developers to efficiently track the visibility of elements in the viewport, enabling features such as lazy loading.

What is Intersection Observer?

The Intersection Observer API provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport. It helps in reducing the load on the browser by minimizing the number of DOM updates.

How It Works

The API works by creating an observer instance that listens for visibility changes of specified elements. When the target element enters or exits the viewport, a callback function is executed.

const observer = new IntersectionObserver((entries, observer) => {
    entries.forEach(entry => {
        if (entry.isIntersecting) {
            // Load the image
            const img = entry.target;
            img.src = img.dataset.src;
            observer.unobserve(img); // Stop observing after loading
        }
    });
});
        

Implementation Steps

1. Create an image element with a placeholder src and a data-src attribute containing the actual image URL.
2. Initialize a new IntersectionObserver instance and define a callback function.
3. Observe the image elements using the observer instance.
4. In the callback, check if the image is in the viewport and then load the image.

const images = document.querySelectorAll('.lazy');
images.forEach(image => {
    observer.observe(image);
});
            

Best Practices

• Use a low-resolution placeholder image to improve perceived loading time.
• Set a reasonable threshold for the observer to trigger loading before the image enters the viewport.
• Unobserve images once they have been loaded to prevent unnecessary checks.

FAQ