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HTML

              
                <div id="react-root"></div>
              
            
!

CSS

              
                body { font: 100 14px 'Roboto'; }
              
            
!

JS

              
                // React Native version of https://codepen.io/mjijackson/pen/xOzyGX

const { AppRegistry, StyleSheet, Text, TouchableHighlight, UIManager, View } = ReactNative

/**
 * KeyboardInput
 */
class KeyboardInput extends React.Component {
  handleKeyDown(event) {
    if (this.props.onKeyDown)
      this.props.onKeyDown(event)
  }
  
  componentDidMount() {
    // Ah, auto-binding. I hardly knew ye.
    this.handleKeyDown = this.handleKeyDown.bind(this)
    document.addEventListener('keydown', this.handleKeyDown)
  }
  
  componentWillUnmount() {
    document.removeEventListener('keydown', this.handleKeyDown)
  }
  
  render() {
    return null
  }
}

let counter = 0
/**
 * CalculatorDisplay
 */
class CalculatorDisplay extends React.Component {
  constructor(props, context) {
    super(props, context)
    
    this.state = {
      scale: 1
    }
  }
  
  handleTextLayout = (e) => {
    const { scale } = this.state
    const { width, x } = e.nativeEvent.layout
    const maxWidth = width + x
    const actualWidth = width / scale
    const newScale = maxWidth / actualWidth
    if (x < 0) {
      this.setState({ scale: newScale  })
    } else if (x > width) {
      this.setState({ scale: 1 })
    }
  };

  render() {
    const { value } = this.props
    const { scale } = this.state
    
    const language = navigator.language || 'en-US'
    let formattedValue = parseFloat(value).toLocaleString(language, {
      useGrouping: true,
      maximumFractionDigits: 6
    })
    
    const trailingZeros = value.match(/\.0*$/)
    
    if (trailingZeros)
      formattedValue += trailingZeros
    
    return (
      <View style={calculatorDisplayStyles.root}>
        <Text
          children={formattedValue}
          onLayout={this.handleTextLayout}
          style={[
            calculatorDisplayStyles.text,
            { transform: [ { scale }  ] }
          ]}
        />
      </View>
    )
  }
}

const calculatorDisplayStyles = StyleSheet.create({
  root: {
    backgroundColor: '#1c191c',
    flex: 1,
    justifyContent: 'center'
  },
  text: {
    alignSelf: 'flex-end',
    color: 'white',
    //lineHeight: 130,
    fontSize: '6em',
    paddingHorizontal: 30,
    //position: 'absolute',
    right: 0,
    transformOrigin: 'right'
  }
})

/**
 * CalculatorKey
 */
class CalculatorKey extends React.Component {
  render() {
    const { children, onPress, style, textStyle } = this.props
    
    return (
      <TouchableHighlight
        accessibilityRole="button"
        onPress={onPress}
        style={[
          calculatorKeyStyles.root,
          style
        ]}
        underlayColor='rgba(0,0,0,0.25)'
      >
        <Text
          children={children}
          style={[
            calculatorKeyStyles.text,
            textStyle
          ]}
        />
      </TouchableHighlight>
    )
  }
}

const calculatorKeyStyles = StyleSheet.create({
  root: {
    width: 80,
    height: 80,
    borderTopWidth: 1,
    borderTopColor: '#777',
    borderTopStyle: 'solid',
    borderRightWidth: 1,
    borderRightColor: '#666',
    borderTopStyle: 'solid',
    outline: 'none'
  },
  text: {
    lineHeight: 80,
    textAlign: 'center'
  }
})

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

/**
 * Calculator
 */
class Calculator extends React.Component {
  constructor(props, context) {
    super(props, context)
    
    this.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()
      }
    }
  }

  render() {
    const { displayValue } = this.state
    
    const clearDisplay = displayValue !== '0'
    const clearText = clearDisplay ? 'C' : 'AC'
    
    return (
      <View style={calculatorStyles.root}>
        <KeyboardInput onKeyDown={event => this.handleKeyDown(event)}/>
        <CalculatorDisplay value={displayValue}/>
        <View style={calculatorStyles.keypad}>
          <View style={calculatorStyles.inputKeys}>
            <View style={calculatorStyles.functionKeys}>
              <FunctionKey onPress={() => clearDisplay ? this.clearDisplay() : this.clearAll()}>{clearText}</FunctionKey>
              <FunctionKey onPress={() => this.toggleSign()} style={calculatorStyles.keySign}>±</FunctionKey>
              <FunctionKey onPress={() => this.inputPercent()} style={calculatorStyles.keyPercent}>%</FunctionKey>
            </View>
            <View style={calculatorStyles.digitKeys}>
              <DigitKey onPress={() => this.inputDigit(0)} style={calculatorStyles.key0} textStyle={{ textAlign: 'left' }}>0</DigitKey>
              <DigitKey onPress={() => this.inputDot()} style={calculatorStyles.keyDot} textStyle={calculatorStyles.keyDotText}>.</DigitKey>
              <DigitKey onPress={() => this.inputDigit(1)}>1</DigitKey>
              <DigitKey onPress={() => this.inputDigit(2)}>2</DigitKey>
              <DigitKey onPress={() => this.inputDigit(3)}>3</DigitKey>
              <DigitKey onPress={() => this.inputDigit(4)}>4</DigitKey>
              <DigitKey onPress={() => this.inputDigit(5)}>5</DigitKey>
              <DigitKey onPress={() => this.inputDigit(6)}>6</DigitKey>
              <DigitKey onPress={() => this.inputDigit(7)}>7</DigitKey>
              <DigitKey onPress={() => this.inputDigit(8)}>8</DigitKey>
              <DigitKey onPress={() => this.inputDigit(9)}>9</DigitKey>
            </View>
          </View>
          <View style={calculatorStyles.operatorKeys}>
            <OperatorKey onPress={() => this.performOperation('/')}>÷</OperatorKey>
            <OperatorKey onPress={() => this.performOperation('*')}>×</OperatorKey>
            <OperatorKey onPress={() => this.performOperation('-')}>−</OperatorKey>
            <OperatorKey onPress={() => this.performOperation('+')}>+</OperatorKey>
            <OperatorKey onPress={() => this.performOperation('=')}>=</OperatorKey>
          </View>
        </View>
      </View>
    )
  }
}

const DigitKey = (props) => (
  <CalculatorKey
    {...props}
    style={[calculatorStyles.digitKey, props.style]}
    textStyle={[calculatorStyles.digitKeyText, props.textStyle]}
  />
)

const FunctionKey = (props) => (
  <CalculatorKey
    {...props}
    style={[calculatorStyles.functionKey, props.style]}
    textStyle={[calculatorStyles.functionKeyText, props.textStyle]}
  />
)

const OperatorKey = (props) => (
  <CalculatorKey
    {...props}
    style={[calculatorStyles.operatorKey, props.style]}
    textStyle={[calculatorStyles.operatorKeyText, props.textStyle]}
  />
)

const calculatorStyles = StyleSheet.create({
  root: {
    width: 320,
    height: 520,
    backgroundColor: 'black',
    boxShadow: '0px 0px 20px 0px #aaa'
  },
  keypad: {
    height: 400,
    flexDirection: 'row'
  },
  inputKeys: {
    width: 240
  },
  calculatorKeyText: {
    fontSize: '2em'
  },
  functionKeys: {
    backgroundImage: 'linear-gradient(to bottom, rgba(202,202,204,1) 0%, rgba(196,194,204,1) 100%)',
    flexDirection: 'row'
  },
  functionKey: {
    fontSize: '2em'
  },
  digitKeys: {
    backgroundColor: '#e0e0e7',
    flexDirection: 'row',
    flexWrap: 'wrap-reverse'
  },
  digitKeyText: {
    fontSize: '2.25em'                                 
  },
  operatorKeys: {
    backgroundImage: 'linear-gradient(to bottom, rgba(252,156,23,1) 0%, rgba(247,126,27,1) 100%)'
  },
  operatorKey: {
    borderRightWidth: 0
  },
  operatorKeyText: {
    color: 'white',
    fontSize: '3em'
  },
  keyMultiplyText: {
    lineHeight: 50
  },
  key0: {
    paddingLeft: 32,
    width: 160
  },
  keyDot: {
    overflow: 'hidden'
  },
  keyDotText: {
    fontSize: '5em',
    marginTop: -10
  }
})

const CalculatorWrapper = () => (
  <View style={{
    alignItems: 'center',
    justifyContent: 'center',
    flex: 1
  }}>
    <Calculator />
  </View>
)

AppRegistry.registerComponent('Calculator', () => CalculatorWrapper)
AppRegistry.runApplication(
  'Calculator',
  { rootTag: document.getElementById('react-root') }
)
              
            
!
999px

Console