Simple Calculator in HTML Code

Simple Calculator in HTML Code
Code Snippet:JavaScript Calculator
Author: Vikas Lalwani
Published: January 11, 2024
Last Updated: January 22, 2024
Downloads: 12,448
License: MIT
Edit Code online: View on CodePen
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This code snippet helps you to create a simple calculator in HTML. It comes with a flat UI design with all basic calculation methods. Users can easily do addition, subtraction, multiplication, and division operations on numbers.

This simple calculator comes with a virtual Numpad. It doesn’t support keyboard keys to enter numbers. However, you can use the mouse (or tap) to enter numbers to perform mathematics operations. Moreover, you can highly customize it with additional CSS.

How to Create Simple HTML Calculator

1. First of all, create the HTML structure for the calculator as follows:

<div class="calculator">
  <div class="input" id="input"></div>
  <div class="buttons">
    <div class="operators">
      <div>+</div>
      <div>-</div>
      <div>×</div>
      <div>÷</div>
    </div>
    <div class="leftPanel">
      <div class="numbers">
        <div>7</div>
        <div>8</div>
        <div>9</div>
      </div>
      <div class="numbers">
        <div>4</div>
        <div>5</div>
        <div>6</div>
      </div>
      <div class="numbers">
        <div>1</div>
        <div>2</div>
        <div>3</div>
      </div>
      <div class="numbers">
        <div>0</div>
        <div>.</div>
        <div id="clear">C</div>
      </div>
    </div>
    <div class="equal" id="result">=</div>
  </div>
</div>

2. After that, style the calculator interface using the following CSS.

.calculator {
  padding: 20px;
  max-width: 440px;
  margin: 10px auto;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-radius: 1px;
}
.input {
  border: 1px solid #ddd;
  border-radius: 1px;
  height: 60px;
  padding-right: 15px;
  padding-top: 10px;
  text-align: right;
  margin-right: 6px;
  font-size: 2.5rem;
  overflow-x: auto;
  transition: all .2s ease-in-out;
}
.input:hover {
  border: 1px solid #bbb;
  -webkit-box-shadow: inset 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: inset 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
}
.operators div {
  display: inline-block;
  border: 1px solid #bbb;
  border-radius: 1px;
  width: 80px;
  text-align: center;
  padding: 10px;
  margin: 20px 4px 10px 0;
  cursor: pointer;
  background-color: #ddd;
  transition: border-color .2s ease-in-out, background-color .2s, box-shadow .2s;
}
.operators div:hover {
  background-color: #ddd;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-color: #aaa;
}
.operators div:active {
  font-weight: bold;
}
.leftPanel {
  display: inline-block;
}
.numbers div {
  display: inline-block;
  border: 1px solid #ddd;
  border-radius: 1px;
  width: 80px;
  text-align: center;
  padding: 10px;
  margin: 10px 4px 10px 0;
  cursor: pointer;
  background-color: #f9f9f9;
  transition: border-color .2s ease-in-out, background-color .2s, box-shadow .2s;
}
.numbers div:hover {
  background-color: #f1f1f1;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-color: #bbb;
}
.numbers div:active {
  font-weight: bold;
}
div.equal {
  display: inline-block;
  border: 1px solid #3079ED;
  border-radius: 1px;
  width: 17%;
  text-align: center;
  padding: 127px 10px;
  margin: 10px 6px 10px 0;
  vertical-align: top;
  cursor: pointer;
  color: #FFF;
  background-color: #4d90fe;
  transition: all .2s ease-in-out;
}
div.equal:hover {
  background-color: #307CF9;
  -webkit-box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  box-shadow: 0px 1px 4px 0px rgba(0, 0, 0, 0.2);
  border-color: #1857BB;
}
div.equal:active {
  font-weight: bold;
}

3. Finally, add the following JavaScript code before closing the body tag and done.

"use strict";

var input = document.getElementById('input'), // input/output button
  number = document.querySelectorAll('.numbers div'), // number buttons
  operator = document.querySelectorAll('.operators div'), // operator buttons
  result = document.getElementById('result'), // equal button
  clear = document.getElementById('clear'), // clear button
  resultDisplayed = false; // flag to keep an eye on what output is displayed

// adding click handlers to number buttons
for (var i = 0; i < number.length; i++) {
  number[i].addEventListener("click", function(e) {

    // storing current input string and its last character in variables - used later
    var currentString = input.innerHTML;
    var lastChar = currentString[currentString.length - 1];

    // if result is not diplayed, just keep adding
    if (resultDisplayed === false) {
      input.innerHTML += e.target.innerHTML;
    } else if (resultDisplayed === true && lastChar === "+" || lastChar === "-" || lastChar === "×" || lastChar === "÷") {
      // if result is currently displayed and user pressed an operator
      // we need to keep on adding to the string for next operation
      resultDisplayed = false;
      input.innerHTML += e.target.innerHTML;
    } else {
      // if result is currently displayed and user pressed a number
      // we need clear the input string and add the new input to start the new opration
      resultDisplayed = false;
      input.innerHTML = "";
      input.innerHTML += e.target.innerHTML;
    }

  });
}

// adding click handlers to number buttons
for (var i = 0; i < operator.length; i++) {
  operator[i].addEventListener("click", function(e) {

    // storing current input string and its last character in variables - used later
    var currentString = input.innerHTML;
    var lastChar = currentString[currentString.length - 1];

    // if last character entered is an operator, replace it with the currently pressed one
    if (lastChar === "+" || lastChar === "-" || lastChar === "×" || lastChar === "÷") {
      var newString = currentString.substring(0, currentString.length - 1) + e.target.innerHTML;
      input.innerHTML = newString;
    } else if (currentString.length == 0) {
      // if first key pressed is an opearator, don't do anything
      console.log("enter a number first");
    } else {
      // else just add the operator pressed to the input
      input.innerHTML += e.target.innerHTML;
    }

  });
}

// on click of 'equal' button
result.addEventListener("click", function() {

  // this is the string that we will be processing eg. -10+26+33-56*34/23
  var inputString = input.innerHTML;

  // forming an array of numbers. eg for above string it will be: numbers = ["10", "26", "33", "56", "34", "23"]
  var numbers = inputString.split(/\+|\-|\×|\÷/g);

  // forming an array of operators. for above string it will be: operators = ["+", "+", "-", "*", "/"]
  // first we replace all the numbers and dot with empty string and then split
  var operators = inputString.replace(/[0-9]|\./g, "").split("");

  console.log(inputString);
  console.log(operators);
  console.log(numbers);
  console.log("----------------------------");

  // now we are looping through the array and doing one operation at a time.
  // first divide, then multiply, then subtraction and then addition
  // as we move we are alterning the original numbers and operators array
  // the final element remaining in the array will be the output

  var divide = operators.indexOf("÷");
  while (divide != -1) {
    numbers.splice(divide, 2, numbers[divide] / numbers[divide + 1]);
    operators.splice(divide, 1);
    divide = operators.indexOf("÷");
  }

  var multiply = operators.indexOf("×");
  while (multiply != -1) {
    numbers.splice(multiply, 2, numbers[multiply] * numbers[multiply + 1]);
    operators.splice(multiply, 1);
    multiply = operators.indexOf("×");
  }

  var subtract = operators.indexOf("-");
  while (subtract != -1) {
    numbers.splice(subtract, 2, numbers[subtract] - numbers[subtract + 1]);
    operators.splice(subtract, 1);
    subtract = operators.indexOf("-");
  }

  var add = operators.indexOf("+");
  while (add != -1) {
    // using parseFloat is necessary, otherwise it will result in string concatenation 🙂
    numbers.splice(add, 2, parseFloat(numbers[add]) + parseFloat(numbers[add + 1]));
    operators.splice(add, 1);
    add = operators.indexOf("+");
  }

  input.innerHTML = numbers[0]; // displaying the output

  resultDisplayed = true; // turning flag if result is displayed
});

// clearing the input on press of clear
clear.addEventListener("click", function() {
  input.innerHTML = "";
})

That’s all! hopefully, you have successfully created a simple calculator using this HTML code snippet. If you have any questions or suggestions, let me know by comment below.

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