JavaScript Calculator Tutorial Starter Fiddle

Article: https://freshman.tech/javascript-calculator

by Shrikrishna Gupta

HTML

<div class="calculator">

  <input type="text" class="calculator-screen" value="" disabled />
  
  <div class="calculator-keys">
    
    <button type="button" class="operator" value="+">+</button>
    <button type="button" class="operator" value="-">-</button>
    <button type="button" class="operator" value="*">&times;</button>
    <button type="button" class="operator" value="/">&divide;</button>

    <button type="button" value="7">7</button>
    <button type="button" value="8">8</button>
    <button type="button" value="9">9</button>


    <button type="button" value="4">4</button>
    <button type="button" value="5">5</button>
    <button type="button" value="6">6</button>


    <button type="button" value="1">1</button>
    <button type="button" value="2">2</button>
    <button type="button" value="3">3</button>


    <button type="button" value="0">0</button>
    <button type="button" class="decimal" value=".">.</button>
    <button type="button" class="all-clear" value="all-clear">AC</button>

    <button type="button" class="equal-sign operator" value="=">=</button>

  </div>
</div>

CSS

html {
  font-size: 62.5%;
  box-sizing: border-box;
}

*, *::before, *::after {
  margin: 0;
  padding: 0;
  box-sizing: inherit;
}

.calculator {
  border: 1px solid #ccc;
  border-radius: 5px;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 400px;
}

.calculator-screen {
  width: 100%;
  font-size: 5rem;
  height: 80px;
  border: none;
  background-color: #252525;
  color: #fff;
  text-align: right;
  padding-right: 20px;
  padding-left: 10px;
}

button {
  height: 60px;
  background-color: #fff;
  border-radius: 3px;
  border: 1px solid #c4c4c4;
  background-color: transparent;
  font-size: 2rem;
  color: #333;
  background-image: linear-gradient(to bottom,transparent,transparent 50%,rgba(0,0,0,.04));
  box-shadow: inset 0 0 0 1px rgba(255,255,255,.05), inset 0 1px 0 0 rgba(255,255,255,.45), inset 0 -1px 0 0 rgba(255,255,255,.15), 0 1px 0 0 rgba(255,255,255,.15);
  text-shadow: 0 1px rgba(255,255,255,.4);
}

button:hover {
  background-color: #eaeaea;
}

.operator {
  color: #337cac;
}

.all-clear {
  background-color: #f0595f;
  border-color: #b0353a;
  color: #fff;
}

.all-clear:hover {
  background-color: #f17377;
}

.equal-sign {
  background-color: #2e86c0;
  border-color: #337cac;
  color: #fff;
  height: 100%;
  grid-area: 2 / 4 / 6 / 5;
}

.equal-sign:hover {
  background-color: #4e9ed4;
}

.calculator-keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-gap: 20px;
  padding: 20px;
}

JavaScript

const calculator = {
  displayValue: '0',
  firstOperand: null,
  waitingForSecondOperand: false,
  operator: null,
};

function inputDigit(digit) {
  const { displayValue, waitingForSecondOperand } = calculator;

  if (waitingForSecondOperand === true) {
    calculator.displayValue = digit;
    calculator.waitingForSecondOperand = false;
  } else {
    calculator.displayValue = displayValue === '0' ? digit : displayValue + digit;
  }
}

function inputDecimal(dot) {
  if (calculator.waitingForSecondOperand === true) {
  	calculator.displayValue = "0."
    calculator.waitingForSecondOperand = false;
    return
  }

  if (!calculator.displayValue.includes(dot)) {
    calculator.displayValue += dot;
  }
}

function handleOperator(nextOperator) {
  const { firstOperand, displayValue, operator } = calculator
  const inputValue = parseFloat(displayValue);
  
  if (operator && calculator.waitingForSecondOperand)  {
    calculator.operator = nextOperator;
    return;
  }


  if (firstOperand == null && !isNaN(inputValue)) {
    calculator.firstOperand = inputValue;
  } else if (operator) {
    const result = calculate(firstOperand, inputValue, operator);

    calculator.displayValue = `${parseFloat(result.toFixed(7))}`;
    calculator.firstOperand = result;
  }

  calculator.waitingForSecondOperand = true;
  calculator.operator = nextOperator;
}

function calculate(firstOperand, secondOperand, operator) {
  if (operator === '+') {
    return firstOperand + secondOperand;
  } else if (operator === '-') {
    return firstOperand - secondOperand;
  } else if (operator === '*') {
    return firstOperand * secondOperand;
  } else if (operator === '/') {
    return firstOperand / secondOperand;
  }

  return secondOperand;
}

function resetCalculator() {
  calculator.displayValue = '0';
  calculator.firstOperand = null;
  calculator.waitingForSecondOperand = false;
  calculator.operator = null;
}

function updateDisplay() {
  const display =...