React useMemo

The React useMemo Hook returns a memoized value.

Note: Think of memoization as caching a value so that it does not need to be recalculated.

The useMemo Hook only runs when one of its dependencies update.

This can improve performance.

Note: The useMemo and useCallback Hooks are similar: useMemo returns a memoized value. useCallback returns a memoized function. Learn more about useCallback in the useCallback chapter.

Without useMemo

The useMemo Hook can be used to keep expensive, resource intensive functions from needlessly running.

In this example, we have an expensive function that runs on every render.

When changing the count or adding a todo, you will notice a delay in execution.

Example:

import { useState } from 'react';
import { createRoot } from 'react-dom/client';

const App = () => {
  const [count, setCount] = useState(0);
  const [todos, setTodos] = useState([]);
  const calculation = expensiveCalculation(count);

  const increment = () => {
    setCount((c) => c + 1);
  };
  const addTodo = () => {
    setTodos((t) => [...t, "New Todo"]);
  };

  return (
    <div>
      <div>
        <h2>My Todos</h2>
        {todos.map((todo, index) => {
          return <p key={index}>{todo}</p>;
        })}
        <button onClick={addTodo}>Add Todo</button>
      </div>
      <hr />
      <div>
        Count: {count}
        <button onClick={increment}>+</button>
        <h2>Expensive Calculation</h2>
        {calculation}
        <p>Note that this example executes the expensive function also when you click on the Add Todo button.</p>
      </div>
    </div>
  );
};

const expensiveCalculation = (num) => {
  console.log("Calculating...");
  for (let i = 0; i < 1000000000; i++) {
    num += 1;
  }
  return num;
};

createRoot(document.getElementById('root')).render(
  <App />
);

Use useMemo

To fix this performance issue, we can use the useMemo Hook to memoize the expensiveCalculation function. This will cause the function to only run when needed.

We can wrap the expensive function call with useMemo.

The useMemoHook accepts a second parameter to declare dependencies. The expensive function will only run when its dependencies have changed.

In the following example, the expensive function will only run when count is changed and not when todo's are added.

Example:

import { useState, useMemo } from 'react';
import { createRoot } from 'react-dom/client';

const App = () => {
  const [count, setCount] = useState(0);
  const [todos, setTodos] = useState([]);
  const calculation = useMemo(() => expensiveCalculation(count), [count]);

  const increment = () => {
    setCount((c) => c + 1);
  };
  const addTodo = () => {
    setTodos((t) => [...t, "New Todo"]);
  };

  return (
    <div>
      <div>
        <h2>My Todos</h2>
        {todos.map((todo, index) => {
          return <p key={index}>{todo}</p>;
        })}
        <button onClick={addTodo}>Add Todo</button>
      </div>
      <hr />
      <div>
        Count: {count}
        <button onClick={increment}>+</button>
        <h2>Expensive Calculation</h2>
        {calculation}
      </div>
    </div>
  );
};

const expensiveCalculation = (num) => {
  console.log("Calculating...");
  for (let i = 0; i < 1000000000; i++) {
    num += 1;
  }
  return num;
};

createRoot(document.getElementById('root')).render(
  <App />
);