Usbrth

I am following a Udemy course on how to register events with hooks, the instructor gave the below code:

 const [userText, setUserText] = useState('');

 const handleUserKeyPress = event => (keyCode >= 65 && keyCode <= 90)) 
 setUserText(`$userText$key`);
 
 ;

 useEffect(() => 
 window.addEventListener('keydown', handleUserKeyPress);

 return () => 
 window.removeEventListener('keydown', handleUserKeyPress);
 ;
 );

 return (
 <div>
 <h1>Feel free to type!</h1>
 <blockquote>userText</blockquote>
 </div>
 );

Now it works great but I'm not convinced that this is the right way. The reason is, if I understand correctly, on each and every re-render, events will keep registering and deregistering every time and I simply don't think it is the right way to go about it.

So I made a slight modification to the useEffect hooks to below

useEffect(() => 
 window.addEventListener('keydown', handleUserKeyPress);

 return () => 
 window.removeEventListener('keydown', handleUserKeyPress);
 ;
, []);

By having an empty array as the second argument, letting the component to only run the effect once, imitating componentDidMount. And when I try out the result, it's weird that on every key I type, instead of appending, it's overwritten instead.

I was expecting setUserText($userText$key); to have new typed key append to current state and set as a new state but instead, it's forgetting the old state and rewriting with the new state.

Was it really the correct way that we should register and deregister event on every re-render?

The best way to go about such scenarios is to see what you are doing in the event handler. If you are simply setting state using previous state, its best to use the callback pattern and register the event listeners only on initial mount. If you do not use the callback pattern (https://reactjs.org/docs/hooks-reference.html#usecallback) the listeners reference along with its lexical scope is being used by the event listener but a new function is created with updated closure on new render and hence in the handler you will not be able to the updated state

const [userText, setUserText] = useState('');

 const handleUserKeyPress = useCallback(event => 
 const key, keyCode = event;

 if (keyCode === 32 , []);

 useEffect(() => 
 window.addEventListener('keydown', handleUserKeyPress);

 return () => 
 window.removeEventListener('keydown', handleUserKeyPress);
 ;
 , [handleUserKeyPress]);

 return (
 <div>
 <h1>Feel free to type!</h1>
 <blockquote>userText</blockquote>
 </div>
 );

For your use case, useEffect needs a dependency array to track changes and based on the dependency it can determine whether to re-render or not. It is always advised to pass a dependency array to useEffect. Kindly see the code below:

I have introduced useCallback hook.

const useCallback, useState, useEffect = React;

 const [userText, setUserText] = useState("");

 const handleUserKeyPress = useCallback(event => 
 const key, keyCode = event;

 if (keyCode === 32 , []);

 useEffect(() => 
 window.addEventListener("keydown", handleUserKeyPress);

 return () => 
 window.removeEventListener("keydown", handleUserKeyPress);
 ;
 , [handleUserKeyPress]);

 return (
 <div>
 <blockquote>userText</blockquote>
 </div>
 );

new answer:

useEffect(() => 
 function handlekeydownEvent(event) (keyCode >= 65 && keyCode <= 90)) 
 setUserText(prevUserText => `$prevUserText$key`);
 
 

 document.addEventListener('keyup', handlekeydownEvent)
 return () => 
 document.removeEventListener('keyup', handlekeydownEvent)
 
, [])

when using setUserText, pass the function as the argument instead of the object, the prevUserText will be always the newest state.

old answer:

try this, it works same as your original code:

useEffect(() => 
 function handlekeydownEvent(event) 
 const key, keyCode = event;
 if (keyCode === 32 

 document.addEventListener('keyup', handlekeydownEvent)
 return () => 
 document.removeEventListener('keyup', handlekeydownEvent)
 
, [userText])

because in your useEffect() method, it depends on the userText variable but you don't put it inside the second argument, else the userText will always be bound to the initial value '' with argument [].

you don't need to do like this, just want to let you know why your second solution doesn't work.

You'll need a way to keep track of the previous state. useState helps you keep track of the current state only. From the docs, there is a way to access the old state, by using another hook.

const prevRef = useRef();
useEffect(() => 
 prevRef.current = userText;
);

I've updated your example to use this. And it works out.

const useState, useEffect, useRef = React;

const App = () => 
 const [userText, setUserText] = useState("");
 const prevRef = useRef();
 useEffect(() => 
 prevRef.current = userText;
 );

 const handleUserKeyPress = event => (keyCode >= 65 && keyCode <= 90)) 
 setUserText(`$prevRef.current$key`);
 
 ;

 useEffect(() => 
 window.addEventListener("keydown", handleUserKeyPress);

 return () => 
 window.removeEventListener("keydown", handleUserKeyPress);
 ;
 , []);

 return (
 <div>
 <h1>Feel free to type!</h1>
 <blockquote>userText</blockquote>
 </div>
 );
;

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.3/umd/react-dom.production.min.js"></script>
<div id="root"></div>