Access DOM nodes and cache calculations.
✍️ 3 exercises + code challenge in this lessonconst inputRef = useRef(null); attach it with ref={inputRef}, then use inputRef.current to reach the DOM element (e.g. to focus it). A ref can also store a value that does not trigger re-renders.
useMemo(() => expensiveCalc(a), [a]) remembers a calculated value and only recalculates when its dependencies change.
Mastering React useRef & useMemo helps you write clean and reliable React code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.
<div id="root"></div>
<script src="https://unpkg.com/react@18.3.1/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@18.3.1/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/@babel/standalone@7.25.6/babel.min.js"></script>
<script type="text/babel">
const { useState, useEffect, useRef, useMemo, createContext, useContext } = React;
function App() {
const inputRef = useRef(null);
const renders = useRef(0);
const [query, setQuery] = useState("");
const [, force] = useState(0);
renders.current++;
const words = ["react", "redux", "router", "python", "pandas", "sql", "svelte"];
const results = useMemo(
() => words.filter(w => w.includes(query.toLowerCase())),
[query]
);
return (
<div>
<input ref={inputRef} value={query} onChange={e => setQuery(e.target.value)} placeholder="Search..." />{" "}
<button onClick={() => inputRef.current.focus()}>Focus input</button>{" "}
<button onClick={() => force(n => n + 1)}>Re-render</button>
<p>{results.join(", ") || "No matches"}</p>
<small>Rendered {renders.current} times</small>
</div>
);
}
ReactDOM.createRoot(document.getElementById("root")).render(<App />);
</script>Check what you learned in “React useRef & useMemo”.
Create a ref.
const inputRef = (null);
Does changing ref.current cause a re-render?
Attach a ref to the input and focus it in a useEffect when the component mounts.
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