<div id="wrapper">
  <div id="app"></div>
</div>
html {
  box-sizing: border-box;
}
*, *:before, *:after {
  box-sizing: inherit;
}

body {
  margin: 0;
  font: 100 14px 'Roboto';
}

button {
  display: block;
  background: none;
  border: none;
  padding: 0;
  font-family: inherit;
  user-select: none;
  cursor: pointer;
  outline: none;
  
  -webkit-tap-highlight-color: rgba(0,0,0,0);
}

button:active {
  box-shadow: inset 0px 0px 80px 0px rgba(0,0,0,0.25);
}

#wrapper {
  height: 100vh;
  
  display: flex;
  align-items: center;
  justify-content: center;
}

#app {
  width: 320px;
  height: 520px;
  position: relative;
}

.calculator {
  width: 100%;
  height: 100%;
  background: black;
  
  display: flex;
  flex-direction: column;
}

#wrapper .calculator {
  box-shadow: 0px 0px 20px 0px #aaa;
}

.calculator-display {
  color: white;
  background: #1c191c;
  line-height: 130px;
  font-size: 6em;
  
  flex: 1;
}

.auto-scaling-text {
  display: inline-block;
}

.calculator-display .auto-scaling-text {
  padding: 0 30px;
  position: absolute;
  right: 0;
  transform-origin: right;
}

.calculator-keypad {
  height: 400px;
  
  display: flex;
}

.calculator .input-keys {
  width: 240px;
}

.calculator .function-keys {
  display: flex;
}

.calculator .digit-keys {
  background: #e0e0e7;
  
  display: flex;
  flex-direction: row;
  flex-wrap: wrap-reverse;
}

.calculator-key {
  width: 80px;
  height: 80px;
  border-top: 1px solid #777;
  border-right: 1px solid #666;  
  text-align: center;
  line-height: 80px;
}
.calculator .function-keys .calculator-key {
  font-size: 2em;
}
.calculator .function-keys .key-multiply {
  line-height: 50px;
}
.calculator .digit-keys .calculator-key {
  font-size: 2.25em;
}
.calculator .digit-keys .key-0 {
  width: 160px;
  text-align: left;
  padding-left: 32px;
}
.calculator .digit-keys .key-dot {
  padding-top: 1em;
  font-size: 0.75em;
}
.calculator .operator-keys .calculator-key {
  color: white;
  border-right: 0;
  font-size: 3em;
}

.calculator .function-keys {
  background: linear-gradient(to bottom, rgba(202,202,204,1) 0%, rgba(196,194,204,1) 100%);
}
.calculator .operator-keys {
  background:  linear-gradient(to bottom, rgba(252,156,23,1) 0%, rgba(247,126,27,1) 100%);
}
const PointTarget = ReactPoint.PointTarget

class AutoScalingText extends React.Component {
  state = {
    scale: 1
  };
  
  componentDidUpdate() {
    const { scale } = this.state
    
    const node = this.node
    const parentNode = node.parentNode
    
    const availableWidth = parentNode.offsetWidth
    const actualWidth = node.offsetWidth
    const actualScale = availableWidth / actualWidth
    
    if (scale === actualScale)
      return
    
    if (actualScale < 1) {
      this.setState({ scale: actualScale })
    } else if (scale < 1) {
      this.setState({ scale: 1 })
    }
  }
  
  render() {
    const { scale } = this.state
    
    return (
      <div
        className="auto-scaling-text"
        style={{ transform: `scale(${scale},${scale})` }}
        ref={node => this.node = node}
      >{this.props.children}</div>
    )
  }
}

class CalculatorDisplay extends React.Component {
  render() {
    const { value, ...props } = this.props
    
    const language = navigator.language || 'en-US'
    let formattedValue = parseFloat(value).toLocaleString(language, {
      useGrouping: true,
      maximumFractionDigits: 6
    })
    
    // Add back missing .0 in e.g. 12.0
    const match = value.match(/\.\d*?(0*)$/)
    
    if (match)
      formattedValue += (/[1-9]/).test(match[0]) ? match[1] : match[0]
    
    return (
      <div {...props} className="calculator-display">
        <AutoScalingText>{formattedValue}</AutoScalingText>
      </div>
    )
  }
}

class CalculatorKey extends React.Component {
  render() {
    const { onPress, className, ...props } = this.props
    
    return (
      <PointTarget onPoint={onPress}>
        <button className={`calculator-key ${className}`} {...props}/>
      </PointTarget>
    )
  }
}

const CalculatorOperations = {
  '/': (prevValue, nextValue) => prevValue / nextValue,
  '*': (prevValue, nextValue) => prevValue * nextValue,
  '+': (prevValue, nextValue) => prevValue + nextValue,
  '-': (prevValue, nextValue) => prevValue - nextValue,
  '=': (prevValue, nextValue) => nextValue
}

class Calculator extends React.Component {
  state = {
    value: null,
    displayValue: '0',
    operator: null,
    waitingForOperand: false
  };
  
  clearAll() {
    this.setState({
      value: null,
      displayValue: '0',
      operator: null,
      waitingForOperand: false
    })
  }

  clearDisplay() {
    this.setState({
      displayValue: '0'
    })
  }
  
  clearLastChar() {
    const { displayValue } = this.state
    
    this.setState({
      displayValue: displayValue.substring(0, displayValue.length - 1) || '0'
    })
  }
  
  toggleSign() {
    const { displayValue } = this.state
    const newValue = parseFloat(displayValue) * -1
    
    this.setState({
      displayValue: String(newValue)
    })
  }
  
  inputPercent() {
    const { displayValue } = this.state
    const currentValue = parseFloat(displayValue)
    
    if (currentValue === 0)
      return
    
    const fixedDigits = displayValue.replace(/^-?\d*\.?/, '')
    const newValue = parseFloat(displayValue) / 100
    
    this.setState({
      displayValue: String(newValue.toFixed(fixedDigits.length + 2))
    })
  }
  
  inputDot() {
    const { displayValue } = this.state
    
    if (!(/\./).test(displayValue)) {
      this.setState({
        displayValue: displayValue + '.',
        waitingForOperand: false
      })
    }
  }
  
  inputDigit(digit) {
    const { displayValue, waitingForOperand } = this.state
    
    if (waitingForOperand) {
      this.setState({
        displayValue: String(digit),
        waitingForOperand: false
      })
    } else {
      this.setState({
        displayValue: displayValue === '0' ? String(digit) : displayValue + digit
      })
    }
  }
  
  performOperation(nextOperator) {    
    const { value, displayValue, operator } = this.state
    const inputValue = parseFloat(displayValue)
    
    if (value == null) {
      this.setState({
        value: inputValue
      })
    } else if (operator) {
      const currentValue = value || 0
      const newValue = CalculatorOperations[operator](currentValue, inputValue)
      
      this.setState({
        value: newValue,
        displayValue: String(newValue)
      })
    }
    
    this.setState({
      waitingForOperand: true,
      operator: nextOperator
    })
  }
  
  handleKeyDown = (event) => {
    let { key } = event
    
    if (key === 'Enter')
      key = '='
    
    if ((/\d/).test(key)) {
      event.preventDefault()
      this.inputDigit(parseInt(key, 10))
    } else if (key in CalculatorOperations) {
      event.preventDefault()
      this.performOperation(key)
    } else if (key === '.') {
      event.preventDefault()
      this.inputDot()
    } else if (key === '%') {
      event.preventDefault()
      this.inputPercent()
    } else if (key === 'Backspace') {
      event.preventDefault()
      this.clearLastChar()
    } else if (key === 'Clear') {
      event.preventDefault()
      
      if (this.state.displayValue !== '0') {
        this.clearDisplay()
      } else {
        this.clearAll()
      }
    }
  };
  
  componentDidMount() {
    document.addEventListener('keydown', this.handleKeyDown)
  }
  
  componentWillUnmount() {
    document.removeEventListener('keydown', this.handleKeyDown)
  }
  
  render() {
    const { displayValue } = this.state
    
    const clearDisplay = displayValue !== '0'
    const clearText = clearDisplay ? 'C' : 'AC'
    
    return (
      <div className="calculator">
        <CalculatorDisplay value={displayValue}/>
        <div className="calculator-keypad">
          <div className="input-keys">
            <div className="function-keys">
              <CalculatorKey className="key-clear" onPress={() => clearDisplay ? this.clearDisplay() : this.clearAll()}>{clearText}</CalculatorKey>
              <CalculatorKey className="key-sign" onPress={() => this.toggleSign()}>±</CalculatorKey>
              <CalculatorKey className="key-percent" onPress={() => this.inputPercent()}>%</CalculatorKey>
            </div>
            <div className="digit-keys">
              <CalculatorKey className="key-0" onPress={() => this.inputDigit(0)}>0</CalculatorKey>
              <CalculatorKey className="key-dot" onPress={() => this.inputDot()}>●</CalculatorKey>
              <CalculatorKey className="key-1" onPress={() => this.inputDigit(1)}>1</CalculatorKey>
              <CalculatorKey className="key-2" onPress={() => this.inputDigit(2)}>2</CalculatorKey>
              <CalculatorKey className="key-3" onPress={() => this.inputDigit(3)}>3</CalculatorKey>
              <CalculatorKey className="key-4" onPress={() => this.inputDigit(4)}>4</CalculatorKey>
              <CalculatorKey className="key-5" onPress={() => this.inputDigit(5)}>5</CalculatorKey>
              <CalculatorKey className="key-6" onPress={() => this.inputDigit(6)}>6</CalculatorKey>
              <CalculatorKey className="key-7" onPress={() => this.inputDigit(7)}>7</CalculatorKey>
              <CalculatorKey className="key-8" onPress={() => this.inputDigit(8)}>8</CalculatorKey>
              <CalculatorKey className="key-9" onPress={() => this.inputDigit(9)}>9</CalculatorKey>
            </div>
          </div>
          <div className="operator-keys">
            <CalculatorKey className="key-divide" onPress={() => this.performOperation('/')}>÷</CalculatorKey>
            <CalculatorKey className="key-multiply" onPress={() => this.performOperation('*')}>×</CalculatorKey>
            <CalculatorKey className="key-subtract" onPress={() => this.performOperation('-')}>−</CalculatorKey>
            <CalculatorKey className="key-add" onPress={() => this.performOperation('+')}>+</CalculatorKey>
            <CalculatorKey className="key-equals" onPress={() => this.performOperation('=')}>=</CalculatorKey>
          </div>
        </div>
      </div>
    )
  }
}

ReactDOM.render(
  <Calculator/>,
  document.getElementById('app')
)
View Compiled
Run Pen

External CSS

  1. https://fonts.googleapis.com/css?family=Roboto:100

External JavaScript

  1. https://npmcdn.com/react@15.2.1/dist/react.min.js
  2. https://npmcdn.com/react-dom@15.2.1/dist/react-dom.min.js
  3. https://npmcdn.com/number-to-locale-string@1.0.1/polyfill.number.toLocaleString.js
  4. https://npmcdn.com/react-point@2.0.0/umd/react-point.min.js