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HTML 

              
                

<div class="calculator card">

    <input type="text" class="calculator-screen z-depth-1" value="" disabled />

    <div class="calculator-keys">

<button type="button" class="operator btn btn-info" value="+">+</button>
      <button type="button" class="operator btn btn-info" value="-">-</button>
      <button type="button" class="operator btn btn-info" value="*">&times;</button>
      <button type="button" class="operator btn btn-info" value="/">&divide;</button>

      <button type="button" value="7" class="btn btn-light waves-effect">7</button>
      <button type="button" value="8" class="btn btn-light waves-effect">8</button>
      <button type="button" value="9" class="btn btn-light waves-effect">9</button>


      <button type="button" value="4" class="btn btn-light waves-effect">4</button>
      <button type="button" value="5" class="btn btn-light waves-effect">5</button>
      <button type="button" value="6" class="btn btn-light waves-effect">6</button>


      <button type="button" value="1" class="btn btn-light waves-effect">1</button>
      <button type="button" value="2" class="btn btn-light waves-effect">2</button>
      <button type="button" value="3" class="btn btn-light waves-effect">3</button>


      <button type="button" value="0" class="btn btn-light waves-effect">0</button>
      <button type="button" class="decimal function btn btn-secondary" value=".">.</button>
      <button type="button" class="all-clear function btn btn-danger btn-sm" value="all-clear">DEL</button>

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

    </div>
  </div>
</div>
!

CSS 

              
                 html {
      font-size: 80%;
      box-sizing: border-box;
      background-color: #FFF;
    }

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

    .calculator {
      border: 5px solid #137dc1;
      border-radius: 5px;
      position: absolute;
      top: 55%;
      left: 55%;
      transform: translate(-50%, -50%);
      width: 400px;
    }

    .calculator-screen {
      width: 100%;
      height: 80px;
      border: none;
      background-color: #137dc1;
      color: #fff;
      text-align: right;
      padding-right: 20px;
      padding-left: 10px;
      font-size: 4rem;
    }

    button {
      height: 60px;
      font-size: 2rem!important;
    }

    .equal-sign {
      height: 50%;
      grid-area: 2 / 4 / 6 / 5;
    }

    .calculator-keys {
      display: grid;
      grid-template-columns: repeat(4, 1fr);
      grid-gap: 10px;
      padding: 40px;
    }
!

JS 

              
                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 the `displayValue` does not contain a decimal point
  if (!calculator.displayValue.includes(dot)) {
    // Append the decimal point
    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) {
    calculator.firstOperand = inputValue;
  } else if (operator) {
    const currentValue = firstOperand || 0;
    const result = performCalculation[operator](currentValue, inputValue);

    calculator.displayValue = String(result);
    calculator.firstOperand = result;
  }

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

const performCalculation = {
  '/': (firstOperand, secondOperand) => firstOperand / secondOperand,

  '*': (firstOperand, secondOperand) => firstOperand * secondOperand,

  '+': (firstOperand, secondOperand) => firstOperand + secondOperand,

  '-': (firstOperand, secondOperand) => firstOperand - secondOperand,

  '=': (firstOperand, secondOperand) => secondOperand
};

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

function updateDisplay() {
  const display = document.querySelector('.calculator-screen');
  display.value = calculator.displayValue;
}

updateDisplay();

const keys = document.querySelector('.calculator-keys');
keys.addEventListener('click', (event) => {
  const { target } = event;
  if (!target.matches('button')) {
    return;
  }

  if (target.classList.contains('operator')) {
    handleOperator(target.value);
		updateDisplay();
    return;
  }

  if (target.classList.contains('decimal')) {
    inputDecimal(target.value);
		updateDisplay();
    return;
  }

  if (target.classList.contains('all-clear')) {
    resetCalculator();
		updateDisplay();
    return;
  }

  inputDigit(target.value);
  updateDisplay();
});
!
999px

Console