javascript 44 lines · 7 steps

Optimistic like buttons in Next.js

A client component that flips the UI instantly with useOptimistic, then reconciles with a server action.

Explained by highlit
Next.js Intermediate optimistic-ui react-hooks server-actions transitions client-components
1'use client';
2 
3import { useOptimistic, useTransition } from 'react';
4import { toggleLike } from '@/app/actions/likes';
5 
6export function LikeButton({ postId, initialLiked, initialCount }) {
7 const [isPending, startTransition] = useTransition();
8 const [optimistic, setOptimistic] = useOptimistic(
9 { liked: initialLiked, count: initialCount },
10 (state, liked) => ({
11 liked,
12 count: state.count + (liked ? 1 : -1),
13 })
14 );
15 
16 function handleClick() {
17 const nextLiked = !optimistic.liked;
18 startTransition(async () => {
19 setOptimistic(nextLiked);
20 await toggleLike(postId, nextLiked);
21 });
22 }
23 
24 return (
25 <button
26 type="button"
27 onClick={handleClick}
28 disabled={isPending}
29 aria-pressed={optimistic.liked}
30 className="flex items-center gap-1.5 text-sm font-medium disabled:opacity-60"
31 >
32 <svg
33 viewBox="0 0 24 24"
34 className={`h-5 w-5 transition-colors ${
35 optimistic.liked ? 'fill-red-500 stroke-red-500' : 'fill-none stroke-current'
36 }`}
37 strokeWidth={2}
38 >
39 <path d="M12 21s-7.5-4.6-10-9.3C.7 8.9 2.2 5.5 5.5 5.5c2 0 3.4 1.2 4.5 2.8 1.1-1.6 2.5-2.8 4.5-2.8 3.3 0 4.8 3.4 3.5 6.2C19.5 16.4 12 21 12 21z" />
40 </svg>
41 <span>{optimistic.count}</span>
42 </button>
43 );
44}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1useOptimistic lets you render a provisional state that automatically reverts if the underlying data doesn't confirm it.
  2. 2Wrapping a server action in startTransition keeps the UI responsive and exposes a pending flag for disabling controls.
  3. 3Deriving both the icon fill and count from one optimistic value keeps the whole button visually consistent during the round trip.

Related explainers

javascript 
function evaluate(expression) {
  const tokens = tokenize(expression);
  let pos = 0;
 

Building a recursive descent calculator

parsing recursion operator-precedence
Intermediate 8 steps
javascript 
import { useState, useEffect, useCallback, useRef } from 'react';
 
const cache = new Map();
const inflight = new Map();

Building a stale-while-revalidate hook in React

caching request-deduplication custom-hooks
Advanced 10 steps
javascript 
import { useEffect, useRef, useState } from 'react';
 
export function useDelayedFlag(active, delay = 300) {
  const [visible, setVisible] = useState(false);

Delaying a loading spinner with a React hook

custom-hooks debouncing cleanup
Intermediate 8 steps
javascript 
const SWIPE_THRESHOLD = 80;
const MAX_TRANSLATE = 120;
 
export function attachSwipeToDismiss(element, onDismiss) {

Building a swipe-to-dismiss gesture in JS

touch-events gesture-detection dom-manipulation
Intermediate 10 steps
javascript 
const { pool } = require('./db');
 
function withTransaction() {
  return async (req, res, next) => {

A per-request transaction middleware in Express

middleware database-transactions connection-pooling
Advanced 7 steps
javascript 
function collapseConsecutiveLogs(lines, { keyFn = (l) => l.message } = {}) {
  const groups = [];
 
  for (const line of lines) {

Collapsing consecutive log lines in JavaScript

grouping run-length-encoding data-transformation
Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

Optimistic like buttons in Next.js — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code