React useMemo Hook

The React useMemo Hook returns a memoized value.
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.
The useMemo and useCallback Hooks are similar.
The main difference is that useMemo returns a memoized value and
useCallback returns a memoized function.
You can learn more about useCallback in the useCallback chapter.
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.
A poor performing function. The expensiveCalculation function runs on every render:
import { useState } from "react";
import ReactDOM 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}
      </div>
    </div>
  );
};

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

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


Run 
Example »

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.
Performance example using the useMemo Hook:
import { useState, useMemo } from "react";
import ReactDOM 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;
};

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


Run 
Example »

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