React Memo
Using memo
will cause React to skip rendering a component if its props have not changed.
This can improve performance.
This section uses React Hooks. See the React Hooks section for more information on Hooks.
Problem
In this example, the Todos
component re-renders even when the todos have not changed.
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(["todo 1", "todo 2"]);
const increment = () => {
setCount((c) => c + 1);
};
return (
<>
<Todos todos={todos} />
<hr />
<div>
Count: {count}
<button onClick={increment}>+</button>
</div>
</>
);
};
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<App />);
Todos.js
:
const Todos = ({ todos }) => {
console.log("child render");
return (
<>
<h2>My Todos</h2>
{todos.map((todo, index) => {
return <p key={index}>{todo}</p>;
})}
</>
);
};
export default Todos;
When you click the increment button, the Todos
component re-renders.
If this component was complex, it could cause performance issues.
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Solution
To fix this, we can use memo
.
Use memo
to keep the Todos
component from needlessly re-rendering.
Wrap the Todos
component export in memo
:
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(["todo 1", "todo 2"]);
const increment = () => {
setCount((c) => c + 1);
};
return (
<>
<Todos todos={todos} />
<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 }) => {
console.log("child render");
return (
<>
<h2>My Todos</h2>
{todos.map((todo, index) => {
return <p key={index}>{todo}</p>;
})}
</>
);
};
export default memo(Todos);
Now the Todos
component only re-renders when the todos
that are passed to it through props are updated.