ReactJS Program to useEventListener Hook with Explanation
ReactJS
Hard
Hooks & Effects
30 views
1 min read
88 words
This problem helps you practice core ReactJS fundamentals in a practical way. It builds intuition around hook, react, useeventlistener. Let’s break it down step by step so you can implement it confidently.
Problem Statement
Create a hook to attach an event listener safely.
Input Format
No input.
Output Format
Render a React component.
Constraints
Use a ref for the handler and bind/unbind in an effect.
Code Solution
This explanation is written for learning purposes and to help beginners understand the concept clearly.
import React, { useEffect, useRef, useState } from 'react';
function useEventListener(target, type, handler) {
const saved = useRef(handler);
useEffect(() => {
saved.current = handler;
}, [handler]);
useEffect(() => {
if (!target) return;
function listener(e) {
saved.current(e);
}
target.addEventListener(type, listener);
return () => target.removeEventListener(type, listener);
}, [target, type]);
}
export default function App() {
const [keys, setKeys] = useState([]);
useEventListener(window, 'keydown', (e) => {
setKeys((prev) => [e.key, ...prev].slice(0, 8));
});
return (
<div style={{ padding: 16 }}>
<h2 style={{ marginTop: 0 }}>meetcode key log</h2>
<div style={{ color: '#555' }}>Last keys: {keys.join(', ') || 'Press something'}</div>
</div>
);
}
Output Example
No sample I/O is provided for this question.
Common Mistakes
- Misreading input/output format.
- Not handling constraints and edge cases.
- Off-by-one errors in loops.
- Forgetting to reset variables between test cases (if any).
Solution Guide
Problem
Create a hook to attach an event listener safely.
Input / Output
Output
Render a React component.
Constraints
Use a ref for the handler and bind/unbind in an effect.
Details
Common Mistakes
- Misreading input/output format.
- Not handling constraints and edge cases.
- Off-by-one errors in loops.
- Forgetting to reset variables between test cases (if any).
Official Solution
import React, { useEffect, useRef, useState } from 'react';
function useEventListener(target, type, handler) {
const saved = useRef(handler);
useEffect(() => {
saved.current = handler;
}, [handler]);
useEffect(() => {
if (!target) return;
function listener(e) {
saved.current(e);
}
target.addEventListener(type, listener);
return () => target.removeEventListener(type, listener);
}, [target, type]);
}
export default function App() {
const [keys, setKeys] = useState([]);
useEventListener(window, 'keydown', (e) => {
setKeys((prev) => [e.key, ...prev].slice(0, 8));
});
return (
<div style={{ padding: 16 }}>
<h2 style={{ marginTop: 0 }}>meetcode key log</h2>
<div style={{ color: '#555' }}>Last keys: {keys.join(', ') || 'Press something'}</div>
</div>
);
}
Solutions (0)
No solutions submitted yet. Be the first!
