Calculator Project using HTML, CSS, and JavaScript

Welcome to this step-by-step guide on creating a basic yet fully functional calculator using plain JavaScript, HTML, and CSS. This project is designed to strengthen your understanding of web development fundamentals by implementing an interactive web application.

Project Setup

Create a Project Directory: Name it as a calculator. This will be your workspace, containing all project files.

File Structure: Inside the calculator directory, create three files:
  • index.html: The structure of your calculator.
  • style.css: The styling to make your calculator look appealing.
  • script.js: The JavaScript logic that makes your calculator work.

Development Steps

1. HTML Structure (index.html)

Start with a basic HTML skeleton. Include links to your CSS file and JavaScript file.

Create a div with the class calculator-grid, which will serve as the container for your calculator.

Inside this container, create another div to display the output (previous and current operands).

Add buttons for digits (0-9), operations (+, -, *, /), all-clear (AC), delete (DEL), and equals (=).

Let's open the index.html and add the following code to it:
<!DOCTYPE html>
<html lang="en">
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link href="style.css" rel="stylesheet">
  <script src="script.js" defer></script>
  <div class="calculator-grid">
    <div class="output">
      <div data-previous-operand class="previous-operand"></div>
      <div data-current-operand class="current-operand"></div>
    <button data-all-clear class="span-two">AC</button>
    <button data-delete>DEL</button>
    <button data-operation>รท</button>
    <button data-number>1</button>
    <button data-number>2</button>
    <button data-number>3</button>
    <button data-operation>*</button>
    <button data-number>4</button>
    <button data-number>5</button>
    <button data-number>6</button>
    <button data-operation>+</button>
    <button data-number>7</button>
    <button data-number>8</button>
    <button data-number>9</button>
    <button data-operation>-</button>
    <button data-number>.</button>
    <button data-number>0</button>
    <button data-equals class="span-two">=</button>

2. Styling (style.css) 

Reset the default browser styling to ensure consistency across different browsers. 

Design the calculator-grid with CSS Grid to arrange buttons and display in a neat layout. 

Apply styles to buttons and display for a clean, modern look. 

Use :hover effects for better user interaction. 

Ensure the calculator is responsive and looks good on desktop and mobile devices.

Let's open the style.css file and add the following code to it:
*, *::before, *::after {
  box-sizing: border-box;
  font-family: Gotham Rounded, sans-serif;
  font-weight: normal;

body {
  padding: 0;
  margin: 0;
  background: linear-gradient(to right, #00AAFF, #00FF6C);

.calculator-grid {
  display: grid;
  justify-content: center;
  align-content: center;
  min-height: 100vh;
  grid-template-columns: repeat(4, 100px);
  grid-template-rows: minmax(120px, auto) repeat(5, 100px);

.calculator-grid > button {
  cursor: pointer;
  font-size: 2rem;
  border: 1px solid white;
  outline: none;
  background-color: rgba(255, 255, 255, .75);

.calculator-grid > button:hover {
  background-color: rgba(255, 255, 255, .9);

.span-two {
  grid-column: span 2;

.output {
  grid-column: 1 / -1;
  background-color: rgba(0, 0, 0, .75);
  display: flex;
  align-items: flex-end;
  justify-content: space-around;
  flex-direction: column;
  padding: 10px;
  word-wrap: break-word;
  word-break: break-all;

.output .previous-operand {
  color: rgba(255, 255, 255, .75);
  font-size: 1.5rem;

.output .current-operand {
  color: white;
  font-size: 2.5rem;

3. JavaScript Logic (script.js) 

Core Functionality: 

  • Implement a Calculator class with methods for clearing the output, deleting a digit, appending a number, choosing an operation, and computing the result. 
  • Initialize calculator operation by clearing previous and current operands and setting the default operation. 
  • Append digits to the current operand when a number button is clicked. If the decimal point is entered more than once, prevent it from being added again. 
  • On the operation button, click, choose the operation and prepare for the next operand. 
  • Compute the result when the equals button is clicked and update the display accordingly. 
  • Implement the all-clear and delete functionality to manage calculator inputs. 

Event Listeners: 

  • Attach event listeners to number buttons, operation buttons, and equals buttons to invoke calculator methods on click. 
  • Ensure the display is updated each time a button is pressed to reflect the current state of the calculation. 

Updating the Display: 

  • Format numbers to be displayed correctly, including the handling of decimal places. 
  • Display the current and previous operands, as well as the operation symbol, in the output area. 
Let's open a JavaScript file named script.js and add the following JavaScript code to it:
class Calculator {
  constructor(previousOperandTextElement, currentOperandTextElement) {
    this.previousOperandTextElement = previousOperandTextElement
    this.currentOperandTextElement = currentOperandTextElement

  clear() {
    this.currentOperand = ''
    this.previousOperand = ''
    this.operation = undefined

  delete() {
    this.currentOperand = this.currentOperand.toString().slice(0, -1)

  appendNumber(number) {
    if (number === '.' && this.currentOperand.includes('.')) return
    this.currentOperand = this.currentOperand.toString() + number.toString()

  chooseOperation(operation) {
    if (this.currentOperand === '') return
    if (this.previousOperand !== '') {
    this.operation = operation
    this.previousOperand = this.currentOperand
    this.currentOperand = ''

  compute() {
    let computation
    const prev = parseFloat(this.previousOperand)
    const current = parseFloat(this.currentOperand)
    if (isNaN(prev) || isNaN(current)) return
    switch (this.operation) {
      case '+':
        computation = prev + current
      case '-':
        computation = prev - current
      case '*':
        computation = prev * current
      case 'รท':
        computation = prev / current
    this.currentOperand = computation
    this.operation = undefined
    this.previousOperand = ''

  getDisplayNumber(number) {
    const stringNumber = number.toString()
    const integerDigits = parseFloat(stringNumber.split('.')[0])
    const decimalDigits = stringNumber.split('.')[1]
    let integerDisplay
    if (isNaN(integerDigits)) {
      integerDisplay = ''
    } else {
      integerDisplay = integerDigits.toLocaleString('en', { maximumFractionDigits: 0 })
    if (decimalDigits != null) {
      return `${integerDisplay}.${decimalDigits}`
    } else {
      return integerDisplay

  updateDisplay() {
    this.currentOperandTextElement.innerText =
    if (this.operation != null) {
      this.previousOperandTextElement.innerText =
        `${this.getDisplayNumber(this.previousOperand)} ${this.operation}`
    } else {
      this.previousOperandTextElement.innerText = ''

const numberButtons = document.querySelectorAll('[data-number]')
const operationButtons = document.querySelectorAll('[data-operation]')
const equalsButton = document.querySelector('[data-equals]')
const deleteButton = document.querySelector('[data-delete]')
const allClearButton = document.querySelector('[data-all-clear]')
const previousOperandTextElement = document.querySelector('[data-previous-operand]')
const currentOperandTextElement = document.querySelector('[data-current-operand]')

const calculator = new Calculator(previousOperandTextElement, currentOperandTextElement)

numberButtons.forEach(button => {
  button.addEventListener('click', () => {

operationButtons.forEach(button => {
  button.addEventListener('click', () => {

equalsButton.addEventListener('click', button => {

allClearButton.addEventListener('click', button => {

deleteButton.addEventListener('click', button => {

Testing - Open index.html in Browser 

Let's open the index.html file in the browser, and you will be able to see the following screen:


By following these steps, you've created a basic calculator that can perform essential arithmetic operations. This project demonstrates the power of combining HTML, CSS, and JavaScript to build interactive web applications. It also provides a solid foundation in front-end web development, which you can build upon with more complex projects and functionalities in the future.