javascript 39 lines · 8 steps

How lazy image loading works

Defer loading images until they near the viewport using IntersectionObserver, with a graceful fallback.

Explained by highlit
Intermediate lazy-loading intersectionobserver progressive-enhancement dom performance
1function initLazyLoad(root = document) {
2 const images = root.querySelectorAll('img[data-src]');
3 
4 if (!('IntersectionObserver' in window)) {
5 images.forEach(loadImage);
6 return;
7 }
8 
9 const observer = new IntersectionObserver((entries, obs) => {
10 for (const entry of entries) {
11 if (!entry.isIntersecting) continue;
12 loadImage(entry.target);
13 obs.unobserve(entry.target);
14 }
15 }, {
16 rootMargin: '200px 0px',
17 threshold: 0.01
18 });
19 
20 images.forEach((img) => observer.observe(img));
21}
22 
23function loadImage(img) {
24 const { src, srcset } = img.dataset;
25 
26 img.addEventListener('load', () => {
27 img.classList.add('is-loaded');
28 }, { once: true });
29 
30 img.addEventListener('error', () => {
31 img.classList.add('is-error');
32 }, { once: true });
33 
34 if (srcset) img.srcset = srcset;
35 if (src) img.src = src;
36 
37 delete img.dataset.src;
38 delete img.dataset.srcset;
39}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1IntersectionObserver lets you react to viewport proximity without scroll listeners or manual math.
  2. 2Feature-detect and fall back so the page works even where the API is missing.
  3. 3Unobserving after a hit and using once listeners keeps work one-shot and avoids leaks.

Related explainers

javascript 
'use server'
 
import { revalidatePath } from 'next/cache'
import { redirect } from 'next/navigation'

How a Next.js Server Action updates a post

server-actions authorization validation
Intermediate 7 steps
javascript 
const express = require('express');
 
const v1 = express.Router();
 

Versioning an API with Express Routers

api versioning routing modularity
Intermediate 10 steps
javascript 
const RATE_LIMIT = 100;
const WINDOW_MS = 60 * 1000;
const BLOCK_MS = 5 * 60 * 1000;
 

Building a rate-limiting middleware in Express

rate-limiting middleware closures
Intermediate 9 steps
javascript 
const transitions = {
  cart: { checkout: 'shipping' },
  shipping: { submitAddress: 'payment', back: 'cart' },
  payment: { submitPayment: 'review', back: 'shipping' },

A finite state machine for checkout flow

state-machine event-driven data-driven-design
Intermediate 7 steps
javascript 
import { useState, useEffect, useCallback, useRef } from "react";
 
export function usePersistentForm(storageKey, initialValues) {
  const [values, setValues] = useState(() => {

A React hook that persists form state to localStorage

custom-hooks localstorage debounce
Intermediate 9 steps
javascript 
function groupBy(items, keySelector) {
  const resolveKey = typeof keySelector === 'function'
    ? keySelector
    : (item) => item[keySelector];

Building a flexible groupBy in JavaScript

higher-order-functions reduce data-transformation
Intermediate 6 steps

Share this explainer

Here's the card — post it anywhere.

How lazy image loading works — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code