Build a Calculator using JavaScript, HTML and CSS in 2022

Whether you are learning a new programming language or a development framework, there comes a time when you have to work on real-world projects. As you are advancing through the subject, you should test your skills and understanding of the language. Building a project using newly learned technology or framework is a good idea. There are plenty of ideas available online as to what you should create with your knowledge.

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The most commonly found suggestions include a Calculator in JavaScript and To-do List. While they seem easy, it can be quite a task when you start working on these projects. Here, we are going to make it easy and logical as well as eventful for you. 

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Logic Before Everything Else

When you start working on a project, whether it is a calculator or a to-do list, the first thing you need to identify is the logic.

How does the calculator work?

  1. You add two or more numbers (any number of digits) using the number keys given on the calculator.
  2. You then perform fundamental arithmetic functions, i.e. addition, subtraction, multiplication and division on these numbers.
  3. The calculator should be able to calculate and give out the correct solutions.

This is the premise of your calculator tool. So you need a keypad, a display unit and function elements. 

The “equals to” button will evaluate the answer while the “clear” button will ensure the removal of all the inputs on the calculator display. 

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Tackling the Project: Key to Building the Code

Instead of heading to the code right away, you need to learn how to tackle a new project. You have two things ready for you: the logic and a basic understanding of the front-end technologies, i.e. JavaScript, HTML, and CSS. 

Before you begin building your calculator using JavaScript, you need the following:

  1. An Integrated Development Environment that will help you build your project using all three technologies. 
  2. A local server that will help you test your codes and remove the bugs. As a result, you will be able to launch the application faster and with greater agility. 

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#1 Getting Started with HTML

The first step is to get your hands a bit dirty with HTML so that you have built the outline for your calculator. 

There are ten buttons on the calculator, ranging from 0 to 9. HTML is responsible for building the keys for each digit.

Apart from this, you will need to use HTML to create separate keys for the different arithmetic functions as well when building a calculator using JavaScript.

You can use HTML to add the button to display the entered digits or the results as well as to clear the display.

The visual unit of your calculator is entirely dependent on HTML and CSS, of which the buttons and their IDs need to be created using HTML.

 The Boilerplate Code

<!DOCTYPE html>

<html lang=”en”>


  <meta charset=”UTF-8″>

  <meta name=”viewport” content=”width=device-width, initial-scale=1.0″>

  <link rel=”stylesheet” type=”text/css” href=”css/styles.css”>




The Actual Code for Creating the Calculator Outline


  <!– calculator –>

  <div class=”calculator”>


    <!– display –>

    <input type=”text” class=”display” disabled>

    <!– keys –>

    <div class=”keys”>

      <!– 4 rows of keys –>

      <div class=”row”>

        <button value=”7″>7</button>

        <button value=”8″>8</button>

        <button value=”9″>9</button>

        <button value=”+” class=”operator”>+</button>


      <div class=”row”>

        <button value=”4″>4</button>

        <button value=”5″>5</button>

        <button value=”6″>6</button>

        <button value=”-” class=”operator”>-</button>


      <div class=”row”>

        <button value=”1″>1</button>

        <button value=”2″>2</button>

        <button value=”3″>3</button>

        <button value=”*” class=”operator”>*</button>


      <div class=”row”>

        <button value=”C” class=”operator”>C</button>

        <button value=”0″>0</button>

        <button value=”/” class=”operator”>/</button>

        <button value=”=” class=”operator”>=</button>




  <!– calculator body ends –>

  <script type=”text/javascript” src=”js/script.js”></script>



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#2 Style with CSS

Once you have defined the structure for the calculator, you will use the inline CSS elements to style your calculator and make it visually appealing and easy-to-use. 

While inline CSS is excellent when it comes to designing and styling your app’s appearance, it may not be SEO friendly. Apart from this, you can also use external CSS elements by tagging them to the same root directory. You can even use internal or embedded CSS elements that are present within the <style></style> tags within the head section.

Here, we will take you through the code that you can use to style your calculator. 

(You can create variations to this code if you want.)

/* common styles */

* {

  padding: 0;

  margin: 0;


body {

  width: 100vw;

  height: 100vh;

  overflow: hidden;

  display: flex;

  justify-content: center;

  align-items: center;

  background-color: #222831;

  font-family: sans-serif;


/* common styles end */

/* calculator */

.calculator {

  width: 300px;

  padding-bottom: 15px;

  border-radius: 7px;

  background-color: #000;

  box-shadow: 5px 8px 8px -2px rgba(0, 0, 0, 0.61);


/* calculator style end */

/* display */

.display {

  width: 100%;

  height: 80px;

  border: none;

  box-sizing: border-box;

  padding: 10px;

  font-size: 2rem;

  background-color: #00ff44;

  color: #000;

  text-align: right;

 border-top-left-radius: 7px;

 border-top-right-radius: 7px;


/* display style end */

/* row */

.row {

  display: flex;

  justify-content: space-between;


/* row style end */

/* button */

button {

  width: 50px;

  height: 50px;

  border-radius: 50%;

  border: none;

  outline: none;

  font-size: 1.5rem;

  background-color: #222;

  color: #fff;

  margin: 10px;


button:hover {

  cursor: pointer;


/* button style end */

/* operator */

.operator {

  background-color: #00ff44;

  color: #000;


/* operator style end */

In-Demand Software Development Skills

#3 Adding Functionality with JavaScript

We have now discussed in detail the visible aspects of building a calculator using JavaScript. Let’s do some scripting to help make the calculator functional and perform the calculation tasks. We will understand the few steps that are involved in building the JS code, which will help you develop the code faster. 

  • You will need to call the HTML tags that you have created for individual functions using JavaScript. You will use selectors to call each of these inputs and store them in a variable. For each function, you will need a variable. For instance, there is a separate variable for display, the digits, clearing function as well as equal to function. The getElementbyID() function can be used in this case.
  • Let’s say you want to perform a simple calculation of 20+30, you will call 20 first using the method mentioned above. Before calling 30 and the arithmetic function, you will need to store the previously called 20 somewhere. Your code should make space for the temporary storage of the numbers to make calculations easy. An empty array will hold the number as a string. Once you have added the other number and the arithmetic function, using eval(), you can evaluate the numbers and get the answer. 
  • When you click on the buttons in the calculator, you want some results, either they are displayed, or they are evaluated. For this purpose, you should add event listeners to your code. This way, the buttons will listen to the click. When you add functions to the event listeners, they will automatically perform the task on the click sound.
  • The display unit should return the value of the button that you click. You can use object, as it will return the value of the element on which the event has occurred. For instance, you want the number “1” displayed as soon as you click on it. For this, you will need to store the number in the form of a variable. Using the “if” function, you will compare the existing variable to identify if it is zero or not. In case it is a zero, then you will append it to an empty string so that no number begins with a zero. Lastly, you will add the contents of the key “1” to the display variable. As a result, as soon as the number key is triggered, you will see the contents in the display box.
  • Lastly, you need to create the function that will help apply mathematical operations to the displayed numbers. Make sure your calculator in JavaScript can perform the mathematical calculations with ease. To enable this functionality, you can use a combination of syntaxes and methods. We have specified the code for the JavaScript elements below for your perusal.

// select all the buttons

const buttons = document.querySelectorAll(‘button’);

// select the <input type=”text” class=”display” disabled> element

const display = document.querySelector(‘.display’);

// add eventListener to each button

buttons.forEach(function(button) {

  button.addEventListener(‘click’, calculate);


// calculate function

function calculate(event) {

  // current clicked buttons value

  const clickedButtonValue =;

  if (clickedButtonValue === ‘=’) {

    // check if the display is not empty then only do the calculation

    if (display.value !== ”) {

      // calculate and show the answer to display

      display.value = eval(display.value);


  } else if (clickedButtonValue === ‘C’) {

    // clear everything on display

    display.value = ”;

  } else {

    // otherwise concatenate it to the display

    display.value += clickedButtonValue;



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Summing Up

Whether you are building a calculator in JavaScript or working on a more complex project, the idea is to break down the contents in a logical and construed manner. This will help you derive the exact syntax and will lead to a fluid web application. Instead of starting with coding directly, try to understand what is the real purpose of the technology you are using, and it will lead you to the code.  

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