Create a Simple calculator using vanilla JavaScript (Step-by-Step)

This is a simple calculator created only with HTML, CSS and JS. It will enhance your basics of JavaScript which is more important than learning new technologies.

Prerequisite Knowledge

Basics of HTML, CSS, JavaScript and desire to create something new.

What you’ll learn

CSS Flexbox

Grid Layout

JS Array Methods

Logical Thinking

DOM Manipulations

Let's Start

As in our to do list we will create the sketch of calculator and then convert it into prototype.

1. Brainstorming

2. Create Prototype

3. Create Project Directory

First create a folder called Simple Calculator.

Create another folder called Designs, where you can save all your project * assets.

Now create HTML file called index.html

Create a CSS file called style.css

Create a JavaScript file called script.js

4. Make HTML Structure

Open index.html into your favorite text editor and start creating the structure of calculator.

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
      <title>Calcualtor :: By Karan Kumar</title>
    </head>

    <body>
        <div class="container">
            <section id="Display-area">
                <div class="currentInput"></div>
                <div class="answerScreen">0</div>
            </section>
            <section class="keypad-btns">
                <button type="button" class="fun_btn" id="clear" value="">
                    C
                </button>
                <button type="button" class="fun_btn" id="erase" value="">
                    &#11013;
                </button>
                <button type="button" class="fun_btn" value="/">/</button>
                <button type="button" class="fun_btn" value="%">%</button>
                <button type="button" class="num_btn" value="7">7</button>
                <button type="button" class="num_btn" value="8">8</button>
                <button type="button" class="num_btn" value="9">9</button>
                <button type="button" class="fun_btn" value="*">x</button>
                <button type="button" class="num_btn" value="4">4</button>
                <button type="button" class="num_btn" value="5">5</button>
                <button type="button" class="num_btn" value="6">6</button>
                <button type="button" class="fun_btn" value="-">-</button>
                <button type="button" class="num_btn" value="1">1</button>
                <button type="button" class="num_btn" value="2">2</button>
                <button type="button" class="num_btn" value="3">3</button>
                <button type="button" class="fun_btn" value="+">+</button>
                <button type="button" class="num_btn" value="00">00</button>
                <button type="button" class="num_btn" value="0">0</button>
                <button type="button" class="num_btn" value=".">.</button>
                <button type="button" class="fun_btn" id="evaluate" value="">=</button>
            </section>
        </div>
    </body>
</html>

Output -

5. Add CSS Styling

In order to add CSS styling first we have to link our style.css with index.html file.

<head>
<link rel="stylesheet" href="style.css" />
</head>

And we can easily do that using link tag inside <head> tag of index.html file
Now open the style.css file and write this code to add styling.

* {
    font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
}

body {
    background-color: #f5f5f5;
    min-height: 90vh;
    display: flex;
    justify-content: center;
    align-items: center;
}

.container {
    background-color: #131313;
    max-width: 300px;
    height: auto;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    padding: 25px 35px;
    border-radius: 10px;
    user-select: none;
}

#Display-area {
    width: 100%;
    margin: 3vh 0;
    display: grid;
    grid-template-columns: 100%;
    grid-template-rows: repeat(2, minmax(40px, auto));
    word-wrap: break-word;
    padding-bottom: 20px;
    border-bottom: 1px rgba(128, 128, 128, 0.116) solid;
}

#Display-area .currentInput {
    /* display: flex; */
    text-align: right;
    height: 8vh;
    color: white;
    font-size: xx-large;
    line-height: 2;
    word-wrap: break-word;
}

#Display-area .answerScreen {
    text-align: right;
    color: rgba(150, 150, 150, 0.87);
    height: 7px;
    line-height: 3;
    font-size: larger;
}

.keypad-btns {
    display: grid;
    grid: repeat(5, 70px) / repeat(4, 70px);
    grid-row-gap: 20px;
    grid-column-gap: 20px;
}

.keypad-btns button {
    outline: none;
    border: none;
    border-radius: 10px;
    background-color: #131313;
    color: white;
    font-size: x-large;
}

.keypad-btns .fun_btn {
    color: #ff8800;
}

.num_btn:hover,.fun_btn:hover {
    background-color: rgba(29, 29, 29, 0.979);
}

Output -

6. Make it responsive

To make our calculator responsive, we have to use media queries.

@media only screen and (max-width: 600px) {
    body {
        background-color: #131313;
        align-items: flex-start;
    }
}

This code is saying that, When maximum width of body is 600px, then change the background color and align the items.

7. JavaScript Functionality

Before we start any JS code, first we have to plan the working of the calculator, like what should happen whenever user press a button.

First we have to link our script.js file with index.html file

<head>
        <script src="script.js" defer></script>
</head>

Here defer means, we are telling the browser to first parse all the files and then start executing scripts.

Now open the script .js file and add write this code in your script file.

// Variables
let currentInput = document.querySelector('.currentInput');
let answerScreen = document.querySelector('.answerScreen');
let buttons = document.querySelectorAll('button');
let erasebtn = document.querySelector('#erase');
let clearbtn = document.querySelector('#clear');
let evaluate = document.querySelector('#evaluate');



// Calculator Display
let realTimeScreenValue = []

// To Clear

clearbtn.addEventListener("click", () => {

    realTimeScreenValue = [''];
    answerScreen.innerHTML = 0;
    currentInput.className = 'currentInput'
    answerScreen.className = 'answerScreen';
    answerScreen.style.color = " rgba(150, 150, 150, 0.87)";
})

// Get value of any button clicked and display to the screen

buttons.forEach((btn) => {


    btn.addEventListener("click", () => {
        // when clicked button is not erased button 
        if (!btn.id.match('erase')) {
            // To display value on btn press
            realTimeScreenValue.push(btn.value)
            currentInput.innerHTML = realTimeScreenValue.join('');

            // To evaluate answer in real time
            if (btn.classList.contains('num_btn')) {

                answerScreen.innerHTML = eval(realTimeScreenValue.join(''));

            }

        }

        // When erase button is clicked
        if (btn.id.match('erase')) {
            realTimeScreenValue.pop();
            currentInput.innerHTML = realTimeScreenValue.join('');
            answerScreen.innerHTML = eval(realTimeScreenValue.join(''));
        }

        // When clicked button is evaluate button
        if (btn.id.match('evaluate')) {
            currentInput.className = 'answerScreen';
            answerScreen.className = 'currentInput';
            answerScreen.style.color = "white";
        }

        // To prevent undefined error in screen
        if (typeof eval(realTimeScreenValue.join('')) == 'undefined') {
            answerScreen.innerHTML = 0
        }

    })
})