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JavaScript React Components React Component Patterns The React Challenge: Solution

Simon Coates
Simon Coates
8,377 Points

Is there are reason to use useRef for the solution rather than just using a normal javascript variable?

Unless I'm missing something, couldn't you use a normal javascript variable and save yourself a rerender? Something like:

import React, { useState, useRef } from "react";

import Header from './Header';
import Player from "./Player";
import AddPlayerForm from "./AddPlayerForm";

const App = () => {
  const [players, setPlayers] = useState([
    {
      name: "Guil",
      score: 0,
      id: 1
    },
    {
      name: "Treasure",
      score: 0,
      id: 2
    },
    {
      name: "Ashley",
      score: 0,
      id: 3
    },
    {
      name: "James",
      score: 0,
      id: 4
    }
  ]);

  const nextPlayerId = useRef(5);
  const scores = players.map(p => p.score);
  const highScore = Math.max(...scores);

  const handleRemovePlayer = (id) => {
    setPlayers(prevPlayers => prevPlayers.filter(p => p.id !== id));
  }

  const handleScoreChange = (id, delta) => {
    setPlayers(prevPlayers => prevPlayers.map(player => {
      if (player.id === id) {
        return {
          name: player.name,
          score: player.score + delta,
          id: player.id
        }
      }
      return player;
    }));
  }

  const handleAddPlayer = (name) => {
    setPlayers(prevPlayers => [
      ...prevPlayers,
      {
        name,
        score: 0,
        id: nextPlayerId.current++
      }
    ]);
  }

  return (
    <div className="scoreboard">
      <Header
        players={players}
      />

      {/* Players list */}
      {players.map(player =>
        <Player
          name={player.name}
          score={player.score}
          id={player.id}
          key={player.id.toString()}
          removePlayer={handleRemovePlayer}
          changeScore={handleScoreChange}
          isHighScore={player.score === highScore}
        />
      )}
      <AddPlayerForm addPlayer={handleAddPlayer} />
    </div>
  );
}

export default App;

I'm just concerned I'm missing something terribly important. If anyone has any insight, I'd appreciate it.

1 Answer

Clayton Perszyk
MOD
Clayton Perszyk
Treehouse Moderator 48,850 Points

I believe it is because the nextPlayerId is not used in the component's view, so you don't want to cause a re-render of the view and any possible children... If you used a variable and update it, it will cause a re-render...

In short it's used to prevent re-rendering.

Additional details:

  • Only update variables when necessary. Unnecessary re-renders can slow down your application.
  • You can also use the useMemo and useCallback hooks to prevent unnecessary re-renders. (although they're use cases are different than useRef)
Simon Coates
Simon Coates
8,377 Points

Any update to App state that would alter the value of a Player highscore attribute already triggers a rerender. Then 'highScore' variable is only local and shouldn't alter anything in terms of rerenders, I wouldn't have thought. Sorry for not getting back to you at the time. I didn't really understand what you were getting at and just had that general sense that my brain was melting. But I rewatched the series today and wound up writing the same solution (more or less). It's nice to hit a particular wall and then 8 months later find myself beating my head on the same spot.