React Hooks: useMemo()

What are React hooks?
They are functions in React that allow you to add react functionality to function based components.

What is the useMemo hook?
This hook will only run if its dependencies change. This is especially useful in preventing expensive operations from needlessly running.

• For those that are familiar with Vue.js, the useMemo hook function in React serves the same purpose as computed properties in Vue.

The syntax follows a common pattern:

const functionToRun = (value) => return value + 1;
const computedValue = useMemo(() => functionToRun(value),[value]);

A few things to note:

• functionToRun() needs to return a value.

• [value] is the dependency. There can be multiple dependencies ex: [value1, value2, value3]. Any time the dependencies change, the useMemo function will run.

• If the dependency array is left empty [], useMemo will only execute the function on the initial component render.

Here is a practicle example using the useMemo hook:

import { useState, useEffect, useMemo } from 'react';

const App = () => {
  const [users, setUsers] = useState([]);

  useEffect(() => {
    const fetchData = async () => {
      //Fetches a list of ten users
      const response = await fetch('https://jsonplaceholder.typicode.com/users');

      // If the HTTP status code is 200-299
      if (response.ok) {
        const json = await response.json();
        setUsers(json);
      }
    };

    fetchData();
  }, []);

  //Only runs when users changes
  const profiles = useMemo(() => {
    users.map((user) => {
      return <Profile key={user.id} user={user} />;
    });
  }, [users]);

  return <div>{profiles}</div>;
};

const Profile = ({ user }) => {
  return (
    <div>
      <h1>{user.name}</h1>
    </div>
  );
};

export default App;

More information about useMemo can be found in the React docs: https://reactjs.org/docs/hooks-reference.html#usememo