Lap Memory Stopwatch using React
Last Updated :
23 Jul, 2025
Stopwatch is an application which helps to track time in hours, minutes, seconds, and milliseconds. This application implements all the basic operations of a stopwatch such as start, pause and reset button. It has an additional feature using which we can keep a record of laps which is useful when we have to note time for certain checkpoints.
Final Preview
Prerequisites
Approach
This program creates a functional stopwatch timer with start, pause, reset, and lap features. It maintains state variables for hours, minutes, seconds, and milliseconds, ensuring real-time updates using the "useEffect" hook. Proper interval clearing prevents memory leaks, delivering a straightforward and reliable stopwatch for time tracking.
Functionalities:
- When users click "Start," the timer begins, disabling the button with a "not-allowed" cursor.
- "Pause" stops the timer, and "Reset" sets all time units to zero.
- The program also offers a "Lap" feature for recording lap times, allowing users to track multiple intervals
Steps to create the application
Step 1: Set up React project using the command
npm create vite@latest <<name-of-project>> --template react
Step 2: Navigate to the project folder using
cd <<name-of-project>>
Step 3: Install the required dependencies:
npm install
Step 4: Create a folder “components” and add three new files in it namely StopWatch.js, Lap.js and StopWatch.css.
Project Structure:
Project StructureThe updated dependencies in package.json will look like:
"dependencies": {
"react": "^18.2.0",
"react-dom": "^18.2.0",
"react-scripts": "5.0.1",
"vite": "^4.0.0"
}Example: Write the following code in respective files
- App.js: This file imports the StopWatch components and exports it.
- StopWatch.js: This file contains the logic to track time in hours, minutes, seconds, and milliseconds, offering user-friendly controls for starting, pausing, and resetting the timer.
- Lap.js : This file contains the logic for displaying the laps in a list. From StopWatch.js file we send a prop laps to this file.
- StopWatch.css: This file contains the design of the StopWatch.js elements.
CSS
/* StopWatch.css */
body {
margin: 0;
padding: 0;
display: flex;
justify-content: center;
align-items: center;
min-height: 100vh;
background-color: #f0f0f0;
font-family: Arial, sans-serif;
}
.container {
padding: 30px;
border: 1px solid #ced4da;
border-radius: 10px;
background: #fff;
box-shadow: 0 0 6px rgba(0, 0, 0, 0.25);
margin: 5px;
width: 100%;
max-width: 400px;
box-sizing: border-box;
}
.timeDisplay {
padding: 20px;
font-size: 30px;
box-shadow: 0 0 6px rgba(0, 0, 0, 0.25);
border-radius: 0.25em;
text-align: center;
}
.buttons {
display: flex;
justify-content: center;
margin-top: 20px;
}
button.btn {
padding: 10px 20px;
outline: none;
border: 1px solid #ced4da;
margin: 5px;
border-radius: 4px;
cursor: pointer;
background: #1d9bf0;
color: #fff;
font-size: 16px;
transition: all 0.5s;
}
button.btn:hover {
background-color: #1879ba;
}
.laps {
margin-top: 20px;
}
.laps h3 {
text-align: left;
display: flex;
justify-content: space-between;
margin: 5px;
}
.laps ul {
list-style-type: none;
padding: 0;
}
.laps li {
display: flex;
justify-content: space-between;
align-items: center;
margin: 0px 5px 10px 5px;
}
@media screen and (max-width: 420px) {
.buttons {
display: grid;
grid-template-columns: 1fr 1fr;
}
}
// App.js
import './App.css';
import StopWatch from './components/StopWatch';
function App() {
return (
<div className="App">
<StopWatch />
</div>
);
}
export default App;
// StopWatch.js
import React, { useState, useEffect } from "react";
import "./StopWatch.css";
import Lap from "./Lap";
export default function StopWatch() {
const [seconds, setSeconds] = useState(0);
const [minutes, setMinutes] = useState(0);
const [hours, setHours] = useState(0);
const [miliSeconds, setMiliSeconds] = useState(0);
const [laps, setLaps] = useState([]);
const [isRunning, setIsRunning] = useState(false);
const formatWithLeadingZero = (number) => {
if (number < 10) return "0" + number;
else return number.toString();
};
useEffect(() => {
let interval;
if (isRunning) {
interval = setInterval(() => {
setMiliSeconds((miliSeconds) => {
if (miliSeconds >= 99) {
setSeconds((seconds) => {
if (seconds >= 59) {
setMinutes((minutes) => {
if (minutes >= 59) {
setHours((prevHours) => prevHours + 1);
return 0;
} else {
return minutes + 1;
}
});
return 0;
} else {
return seconds + 1;
}
});
return 0;
} else {
return miliSeconds + 1;
}
});
}, 10);
}
return () => clearInterval(interval);
}, [isRunning]);
const handleStart = () => {
setIsRunning(true);
};
const handlePause = () => {
setIsRunning(false);
};
const handleLap = () => {
const lapTime =
formatWithLeadingZero(hours) +
":" +
formatWithLeadingZero(minutes) +
":" +
formatWithLeadingZero(seconds) +
"." +
formatWithLeadingZero(miliSeconds);
setLaps((prevLaps) => [...prevLaps, lapTime]);
};
const handleReset = () => {
setIsRunning(false);
setMiliSeconds(0);
setSeconds(0);
setMinutes(0);
setHours(0);
setLaps([]);
};
return (
<div className="container">
<div className="timeDisplay">
{formatWithLeadingZero(hours)} : {formatWithLeadingZero(minutes)} :{" "}
{formatWithLeadingZero(seconds)} : {formatWithLeadingZero(miliSeconds)}
</div>
<div className="buttons">
<button
className="btn"
onClick={handleStart}
disabled={isRunning}
style={{ cursor: isRunning ? "not-allowed" : "pointer" }}
>
Start
</button>
<button className="btn" onClick={handlePause}>
Pause
</button>
<button className="btn" onClick={handleLap}>
Lap
</button>
<button className="btn" onClick={handleReset}>
Reset
</button>
</div>
<Lap laps={laps} />
</div>
);
}
// Lap.js
import React from "react";
export default function Laps({ laps }) {
const calculateTimeDifference = (timeString1, timeString2) => {
const time1 = new Date(`1970-01-01T${timeString1}Z`);
const time2 = new Date(`1970-01-01T${timeString2}Z`);
const timeDiff = Math.abs(time2 - time1);
const hours = Math.floor(timeDiff / 3600000).toString().padStart(2, '0');
const minutes = Math.floor((timeDiff % 3600000) / 60000).toString().padStart(2, '0');
const seconds = Math.floor((timeDiff % 60000) / 1000).toString().padStart(2, '0');
const milliseconds = ((timeDiff % 1000) / 10).toString().padStart(2, '0');
const resultString = `${hours}:${minutes}:${seconds}.${milliseconds}`;
return resultString;
}
return (
<div className="laps" style={{ display: (laps.length) ? 'block' : 'none' }}>
<h3><span>Lap</span><span>Time</span><span>Total Time</span></h3>
<ul>
{laps.map((lap, index) => (
<li key={index}>
<span>{`Lap ${index + 1}`}</span>
<span>{calculateTimeDifference(lap, (index !== 0) ? laps[index - 1] : "00:00:00.00")}</span>
<span>{lap}</span>
</li>
))}
</ul>
</div>
);
};
To start the application run the following command
npm start
Output
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