React useCallback Hook example in react js
const cachedFn = useCallback(fn, dependencies)
The React useCallback Hook returns a memoized callback function.
Think of memoization as caching a value so that it does not need to be recalculated.
This allows us to isolate resource intensive functions so that they will not automatically run on every render.
The useCallback Hook only runs when one of its dependencies update.
This can improve performance.
The useCallback and useMemo Hooks are similar. The main difference is that useMemo returns a memoized value and useCallback returns a memoized function.
Problem
One reason to use useCallback is to prevent a component from re-rendering unless its props have changed.
In this example, you might think that the Todos component will not re-render unless the todos change:
Example:
index.js
import { useState } from "react";
import ReactDOM from "react-dom/client";
import Todos from "./Todos";
const App = () => {
const [count, setCount] = useState(0);
const [todos, setTodos] = useState([]);
const increment = () => {
setCount((c) => c + 1);
};
const addTodo = () => {
setTodos((t) => [...t, "New Todo"]);
};
return (
<>
<Todos todos={todos} addTodo={addTodo} />
<hr />
<div>
Count: {count}
<button onClick={increment}>+</button>
</div>
</>
);
};
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<App />);
Todos.js
import { memo } from "react";
const Todos = ({ todos, addTodo }) => {
console.log("child render");
return (
<>
<h2>My Todos</h2>
{todos.map((todo, index) => {
return <p key={index}>{todo}</p>;
})}
<button onClick={addTodo}>Add Todo</button>
</>
);
};
export default memo(Todos);
