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import { useState, useMemo } from 'react';
// Usage
function App() {
// State for our counter
const [count, setCount] = useState(0);
// State to keep track of current word in array we want to show
const [wordIndex, setWordIndex] = useState(0);
// Words we can flip through and view letter count
const words = ['hey', 'this', 'is', 'cool'];
const word = words[wordIndex];
// Returns number of letters in a word
// We make it slow by including a large and completely unnecessary loop
const computeLetterCount = word => {
let i = 0;
while (i < 1000000000) i++;
return word.length;
// Memoize computeLetterCount so it uses cached return value if input array ...
// ... values are the same as last time the function was run.
const letterCount = useMemo(() => computeLetterCount(word), [word]);
// This would result in lag when incrementing the counter because ...
// ... we'd have to wait for expensive function when re-rendering.
//const letterCount = computeLetterCount(word);
return (
<div style={{ padding: '15px' }}>
<h2>Compute number of letters (slow 🐌)</h2>
<p>"{word}" has {letterCount} letters</p>
onClick={() => {
const next = wordIndex + 1 === words.length ? 0 : wordIndex + 1;
Next word
<h2>Increment a counter (fast ⚡️)</h2>
<p>Counter: {count}</p>
<button onClick={() => setCount(count + 1)}>Increment</button>

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danielsdesk commented Jun 11, 2020

FYI looks like this example errors in codesandbox due to infinite loop

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