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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

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CSS

            
              html, body {
  background-color: #1D1E22;
  color: #fff;
  font-family: 'Roboto', sans-serif;
  font-size: 24px;
  height: 100%;
  margin: 0;
  width: 100%;
}

#app {
  align-items: center;
  display: flex;
  justify-content: center;
  height: 100%;
}

#calc {
  background-color: rgba(255, 255, 255, .05);
  border-radius: 2px;
  padding: 15px;
}

#display {
  background-color: rgba(255, 255, 255, 0.1);
  border-radius: 2px;
  padding: 30px;
  margin-bottom: 15px;
  min-height: 27px;
  text-align: right;
}

#buttons {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr 1fr;
}

#buttons button {
  align-items: center;
  background-color: rgba(255, 255, 255, 0.29);
  border: none;
  border-radius: 2px;
  cursor: pointer;
  display: flex;
  justify-content: center;
  padding: 30px 35px;
  margin: 4px;
  outline: none;
}

.primaryBtn {
  background-color: rgba(255, 255, 255, 0.5) !important;
}
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JS

            
              const App = () => (
  <Calc />
)

class Calc extends React.Component {
  state = {
    display: '0',
    expression: [],
    currentNum: '0'
  }
  handleClickButton = e => {
    switch (e.target.id) {
      case 'clear':
        this.clearDisplay()
        break
      case 'divide':
        this.handleOperation('/')
        break
      case 'multiply':
        this.handleOperation('*')
        break
      case 'seven':
        this.handleNumber(7)
        break
      case 'eight':
        this.handleNumber(8)
        break
      case 'nine':
        this.handleNumber(9)
        break
      case 'subtract':
        this.handleOperation('-')
        break
      case 'four':
        this.handleNumber(4)
        break
      case 'five':
        this.handleNumber(5)
        break
      case 'six':
        this.handleNumber(6)
        break
      case 'add':
        this.handleOperation('+')
        break
      case 'one':
        this.handleNumber(1)
        break
      case 'two':
        this.handleNumber(2)
        break
      case 'three':
        this.handleNumber(3)
        break
      case 'equals':
        this.handleCalc()
        break
      case 'zero':
        this.handleNumber(0)
        break
      case 'decimal':
        this.handleDecimal()
        break
    }
  }
  clearDisplay = () => {
    this.setState(() => ({
      display: '0',
      expression: [],
      currentNum: '0',
      prevCalc: undefined
    }))
  }
  handleNumber = num => {
    const { prevCalc } = this.state
    if (prevCalc !== undefined) {
      this.setState(prevState => {
        return {
          display: num.toString(),
          expression: [],
          currentNum: num.toString(),
          prevCalc: undefined
        }
      })
    } else {
      this.setState(prevState => {
        const { display, expression, currentNum } = prevState
        return {
          display: display === '0' ? num.toString() : display + num.toString(),
          currentNum: currentNum === '0' ? num.toString() : currentNum + num.toString()
        }
      })
    }
  }
  handleDecimal = () => {
    this.setState(prevState => {
      const { display, expression, currentNum } = prevState
      const update = currentNum.indexOf('.') > -1
      return {
        display: update ? display : display + '.',
        currentNum: update ? currentNum : currentNum + '.',
        prevCalc: undefined
      }
    })
  }
  handleOperation = op => {
    const { expression, currentNum } = this.state
    if (currentNum === '') {
      this.setState(prevState => {
        const { expression, currentNum } = prevState
        let newExpression = [...expression]
        newExpression.splice(-1, 1)
        return {
          expression: newExpression.concat([op]),
          currentNum: '',
          prevCalc: undefined
        }
      })
      this.updateDisplay()
    } else {
      this.setState(prevState => {
        const { display, expression, currentNum } = prevState
        return {
          expression: expression.concat([currentNum]).concat([op]),
          currentNum: '',
          prevCalc: undefined
        }
      })
      this.updateDisplay()
    }
  }
  updateDisplay = () => {
    this.setState(prevState => {
      const { expression, currentNum } = prevState
      return {
        display: expression.join('')
      }
    })
  }
  handleCalc = () => {
    const { display } = this.state
    if (!/[*/+-]/.test(display.charAt(display.length - 1))) {
      this.setState(prevState => {
        const { display, expression, currentNum } = prevState
        return {
          display: eval(this.state.expression.join('') + this.state.currentNum).toString(),
          expression: [],
          currentNum: eval(this.state.expression.join('') + this.state.currentNum).toString(),
          prevCalc: eval(this.state.expression.join('') + this.state.currentNum).toString()
        }
      })
    }
  }
  render () {
    return (
      <div id='calc'>
        <Display display={this.state.display.substring(0, 23)} />
        <div id='buttons'>
          <button id='clear' style={{ gridColumnStart: 1, gridColumnEnd: 3 }} onClick={this.handleClickButton}>AC</button>
          <button id='divide' onClick={this.handleClickButton}>/</button>
          <button id='multiply' onClick={this.handleClickButton}>x</button>
          <button id='seven' className='primaryBtn' onClick={this.handleClickButton}>7</button>
          <button id='eight' className='primaryBtn' onClick={this.handleClickButton}>8</button>
          <button id='nine' className='primaryBtn' onClick={this.handleClickButton}>9</button>
          <button id='subtract' onClick={this.handleClickButton}>-</button>
          <button id='four' className='primaryBtn' onClick={this.handleClickButton}>4</button>
          <button id='five' className='primaryBtn' onClick={this.handleClickButton}>5</button>
          <button id='six' className='primaryBtn' onClick={this.handleClickButton}>6</button>
          <button id='add' onClick={this.handleClickButton}>+</button>
          <button id='one' className='primaryBtn' onClick={this.handleClickButton}>1</button>
          <button id='two' className='primaryBtn' onClick={this.handleClickButton}>2</button>
          <button id='three' className='primaryBtn' onClick={this.handleClickButton}>3</button>
          <button id='equals' style={{ gridColumnStart: 4, gridRowStart: 4, gridRowEnd: 6 }} onClick={this.handleClickButton}>=</button>
          <button id='zero' style={{ gridColumnStart: 1, gridColumnEnd: 3 }} onClick={this.handleClickButton}>0</button>
          <button id='decimal' style={{  }} onClick={this.handleClickButton}>.</button>
        </div>
      </div>
    )
  }
}

const Display = props => (
  <div id="display">{props.display}</div>
)

ReactDOM.render(<Calc />, document.getElementById('app'))
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