[React.js] - Part 15. React Hooks - useMemo Hook

Ace Lennox

2025年01月19日読了時間：3分

The useMemo Hook in React helps optimize performance by memoizing values. This means it caches a computed value and only recalculates it when its dependencies change. This guide will walk you through how to use the useMemo Hook with clear examples and practical use cases.

What is `useMemo`?

The useMemo Hook is used to memoize the result of a computationally expensive function. By caching the result, React avoids unnecessary recalculations during renders.

Key Features of `useMemo`:

Performance Optimization: Prevents unnecessary recalculations for expensive operations.
Dependencies: The function only re-runs when dependencies change.
Comparison to useCallback: While useCallback memoizes functions, useMemo memoizes values.

Problem: Expensive Computations

Imagine you have an app with a computationally intensive function that runs on every render. Here’s an example:

Example: Without `useMemo`

import { useState } from "react";
import ReactDOM from "react-dom/client";

const App = () => {
  const [count, setCount] = useState(0);
  const [tasks, setTasks] = useState([]);

  const heavyComputation = (num) => {
    console.log("Running heavy computation...");
    let result = 0;
    for (let i = 0; i < 1000000000; i++) {
      result += num;
    }
    return result;
  };

  const result = heavyComputation(count);

  const increment = () => setCount((c) => c + 1);
  const addTask = () => setTasks((prev) => [...prev, `Task ${prev.length + 1}`]);

  return (
    <div>
      <h2>Tasks</h2>
      {tasks.map((task, index) => (
        <p key={index}>{task}</p>
      ))}
      <button onClick={addTask}>Add Task</button>

      <hr />

      <h2>Count: {count}</h2>
      <button onClick={increment}>Increment</button>
      <h3>Computation Result: {result}</h3>
    </div>
  );
};

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

Problem: Poor Performance

Every time you add a task or increment the counter, the heavyComputation function runs, even when it’s unnecessary. This causes a noticeable delay.

Solution: Using `useMemo`

By wrapping the computation in useMemo, we ensure it only recalculates when the relevant dependency (count) changes.

Example: Optimized with `useMemo`

import { useState, useMemo } from "react";
import ReactDOM from "react-dom/client";

const App = () => {
  const [count, setCount] = useState(0);
  const [tasks, setTasks] = useState([]);

  const heavyComputation = (num) => {
    console.log("Running heavy computation...");
    let result = 0;
    for (let i = 0; i < 1000000000; i++) {
      result += num;
    }
    return result;
  };

  const result = useMemo(() => heavyComputation(count), [count]);

  const increment = () => setCount((c) => c + 1);
  const addTask = () => setTasks((prev) => [...prev, `Task ${prev.length + 1}`]);

  return (
    <div>
      <h2>Tasks</h2>
      {tasks.map((task, index) => (
        <p key={index}>{task}</p>
      ))}
      <button onClick={addTask}>Add Task</button>

      <hr />

      <h2>Count: {count}</h2>
      <button onClick={increment}>Increment</button>
      <h3>Computation Result: {result}</h3>
    </div>
  );
};

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

Why This Works

The useMemo Hook memoizes the result of heavyComputation.
The function only re-runs when count changes, saving resources when tasks are added.

When to Use `useMemo`

Expensive Computations: Use useMemo for functions that require significant processing power.
Frequent Renders: Optimize components that render often but don’t always need recalculations.
Complex Calculations: Cache derived values from state or props to avoid unnecessary processing.

Best Practices

Avoid Overusing useMemo: Only use it for performance-critical computations. Adding unnecessary memoization can complicate your code.
Understand Dependencies: Ensure all values used inside the memoized function are included in the dependency array.
Debugging: Log statements like console.log can help you confirm when a function re-runs.

Conclusion

The useMemo Hook is a valuable tool for optimizing React applications, especially for expensive computations. By memoizing results, you can improve performance and provide a smoother user experience.

Experiment with the examples above and see how useMemo can make your React applications faster and more efficient!

Source Code

The complete source code for this part is available on GitHub

← [React.js] - Part 14. React Hooks - useCallback Hook

[React.js] - Part 16. React Hooks - Custom Hook →

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[React.js] - Part 15. React Hooks - useMemo Hook

Ace Lennox

What is useMemo?

Key Features of useMemo: