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HTML 

              
                <div class="container">
  <div class="message">
    <h1>Simple React Calculator</h1>
    <p><strong>Continous calculation</strong> and <strong>keyboard inputs</strong> are working. Esc is <strong>AC</strong>, Enter does not execute, but does <strong>'click' on focused button</strong>.</p>
  </div>
  <div class="calculator">
    <div id="root"></div>
  </div>
</div>
!

CSS 

              
                $font-family: 'Roboto', sans-serif;
$white: #ffffff;
$grey20: #f9f9f9;
$grey80: #555555;
$grey40: #333;
$gradient-orange: linear-gradient(0.35turn, #FBAB7E, #F7CE68);

body {
  font-family: $font-family;
  height: 100vh;
  min-height: 800px;
  background: $grey40;
  overflow: auto;
}
button {
  background: $white;
  color: $grey80;
  font-family: $font-family;
  font-size: 16px;
  border: none;
  padding: 15px;
  flex-grow: 1;
  width: 50px;
  &:focus {
    outline: none;
  }
}
.message {
  text-align: center;
  color: $white;
  margin-bottom: 30px;
  min-width: 300px;
  max-width: 40%;
  h1 {
    font-family: 'Roboto Slab';
    font-size: 2rem;
    margin-bottom: 20px;
    line-height: 2.5rem;
  }
  p {
    font-size: 0.8rem;
    line-height: 1.2rem;
  }
  strong {
    font-weight: bold;
  }
}
.container {
  height: 100%;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}
.calculator {
  border-radius: 7px;
  overflow: hidden;
  box-shadow: 0 0 50px #202020;
}
.display {
  font-size: 3rem;
  font-weight: 300;
  padding: 20px 10px;
  color: $white;
  background: $gradient-orange;
  &-text {
    flex-grow: 5;
    text-align: right;
  }
  &-overall {
    font-size: 12px;
    line-height: 10px;
    height: 20px;
    margin-bottom: 10px;
    color: rgba(255,255,255,0.6);
  }
}
.row {
  display: flex;
}
.inputs {
  display: flex;
}
.sides {
  display: flex;
  flex-direction: column;
  .ac {
    flex-grow: 1;
  }
  .equal {
    flex-grow: 2;
    color: $white;
    background: $gradient-orange;
  }
}
.operator,
.sides {
  button {
    background: $grey20;
  }
}
.operator button {
  &.active {
    background: $gradient-orange;
    color: $white;
  }
}
!

JS 

              
                class CalcApp extends React.Component {
  state = {
  	firstVal: '',
    secondVal: '',
    operator: '',
    display: '0',
  }

  componentDidMount() {
    const { keypressHandler } = this
    document.addEventListener('keyup', ev => {
      keypressHandler(ev)
    })
  }

  keypressHandler = ev => {
    const {
      setNumberValue,
      setOperatorValue,
      equalHandler,
      allClear,
      deleteChar,
    } = this
    const { 
      operator,
      secondVal,
    } = this.state
    const numRegex = /^([0-9]|\.)*$/g;
    const opRegex =  /[+|\-|:|*]/g;
    const eqRegex =  /(=)/g;
    const delRegex = /(Backspace|Delete)/g;
    const acRegex = /(Escape)/g;
    const key = ev.key
   
    const isNumber = !!numRegex.exec(key)
    const isOperator = !!opRegex.exec(key)
    const isEqual = !!eqRegex.exec(key)
    const isDel = !!delRegex.exec(key)
    const isAc = !!acRegex.exec(key)
    
    if (key && isNumber) {
      setNumberValue(key + '')
    }
    
    if (key && isOperator) {
      setOperatorValue(key + '')
    }
    
    if (key && isEqual) {
      equalHandler()
    }
    
    if (key && isDel) {
      deleteChar()
    }
    
    if (key && isAc) {
      allClear()
    }
  }

  resetState = resetAll => {
  	if (resetAll) {
  		this.setState({
  			firstVal: '',
   			secondVal: '',
   			operator: '',
        display: '0',
   	 })
    } else {
  		this.setState({
  			firstVal: '',
   			secondVal: '',
   			operator: '',
   	 })
    }
  } 
  
  hasPoint = (value) => {
  	return value.indexOf('.') > -1
  }
  
  setNumberValue = value => {
    const {
  	  firstVal,
  	  secondVal,
  	  operator,
  	} = this.state
    const { 
      fixNumberString,
      setDisplay,
    } = this
    let total
    
    // if point is pressed, check if we already have it in current states
    if (value === '.') {
    	if (!operator && !this.hasPoint(firstVal)) {     	
        total = fixNumberString(firstVal + value)
				this.setState({
   				firstVal: total
    		})
      }
    	if (!!operator && !this.hasPoint(secondVal)) {
        total = fixNumberString(secondVal + value)
				this.setState({
   				secondVal: total
    		})
      }
      if (total) {
        setDisplay(total + '')
      }
      return
    }
    
    // if input is a number, check if it's first or second number
    if (!operator) {
      total = fixNumberString(firstVal + value)
			this.setState({
   			firstVal: total
   		})
    } else {
      total = fixNumberString(secondVal + value)
    	this.setState({
    		secondVal: total
    	})
    }
    setDisplay(total + '')
  }
  
  getOverall = () => {
    const { 
      firstVal,
      secondVal,
      operator,
    } = this.state
    return firstVal + ' ' + operator + ' ' + secondVal
  }
  
  setDisplay = value => {
    const { 
      firstVal,
      secondVal,
    } = this.state
    
  	this.setState({
    	display: value,
    })
  }
  
  getCurrentTargetValue = () => {
    const {
      firstVal,
      secondVal,
      operator,
    } = this.state
    return !operator ? firstVal : secondVal
  }
  
  numberClickHandler = (e) => {
    const value = e.target.innerHTML
    if (value) {
  		this.setNumberValue(value)
    }
  }
  
  setOperatorValue = operatorInput => {
  	const { 
      firstVal,
      secondVal,
      operator,
      display,
    } = this.state
    const { 
      fixNumberString,
      calculate,
      setDisplay,
    } = this
    const fixedNumber = fixNumberString(firstVal, false)
    
    if (firstVal && !secondVal) {
      this.setState({
        operator: operatorInput,
        display: fixedNumber,
      })
    } else if (firstVal && operator && secondVal) {
     const total = calculate()
     this.setState({
       operator: operatorInput,
       firstVal: total + '',
       secondVal: '',
     })
     setDisplay(total + '')
    } else {
      this.setState({
        operator: operatorInput,
        firstVal: fixNumberString(display, false),
      })
    }
  }
  
  operatorClickHandler = (e) => {
    const { setOperatorValue } = this
    const operatorInput = e.target.innerHTML
    
    setOperatorValue(operatorInput)
  }
  
  allClear = () => {
  	this.resetState(true)
  }
  
  deleteChar = () => {
    const {
      firstVal,
      secondVal,
      operator,
    } = this.state
    const opRegex =  /[+|\-|:|*]/g;
    
    if (!operator) {
      const newVal = firstVal.slice(0, -1)
      this.setState({
        firstVal: newVal,
        display: newVal ? newVal : '0',
      })
    } else if (operator && !secondVal) {
      this.setState({
        display: firstVal,
        operator: '',
      })
    } else {
      const newVal = secondVal.slice(0, -1)
      this.setState({
        secondVal: newVal,
        display: newVal ? newVal : '0',
      })
    }
  }
  
  removeZeroAtStart = value => {
    return value.indexOf('0') === 0 ? value.substring(1) : value
  }
  
  fixNumberString = (value, finalize = false) => {
    // if input has hanging point e.g. '0.'/'1.', add trailing zero
    // should only run when moving to second value / begin calculation
    if (finalize && value.indexOf('.') === value.length - 1 && value.length > 1) { 
      return value + '0'
    }
    // if value has zero prefix but not a decimal value, e.g. '01'/'03', remove zero
    if (value.indexOf('0') === 0 && !value.indexOf('0.') === 0) {
      return value.substring(1)
    }
    // if value is a first point input '.', add zero prefix
    if (value.indexOf('.') === 0 && value.length === 1) {
      return '0.'
    }
    if (!value) {
      return '0'
    }
    return value
  }
  
  calculate = () => {
  	const {
    	firstVal,
      secondVal,
      operator,
    } = this.state
    const {
      fixNumberString,
    } = this
    
    const vfirstVal = fixNumberString(firstVal, true)
    const vsecondVal = fixNumberString(secondVal, true)
    let total = '0';
    
    switch (operator) {
    	case '-' :
      	total = parseFloat(vfirstVal) - parseFloat(vsecondVal)
      	break;
      case '*':
      	total = parseFloat(vfirstVal) * parseFloat(vsecondVal)
      	break;
    	case ':' :
      	total = parseFloat(vfirstVal) / parseFloat(vsecondVal)
      	break;
    	case '+' :
      default:
      	total = parseFloat(vfirstVal) + parseFloat(vsecondVal)
      	break;
    }
    
    return total.toLocaleString()
  }
  
  equalHandler = () => {
    const {
      firstVal,
      secondVal,
      operator,
    } = this.state
    const {
      setDisplay,
      resetState,
      calculate,
    } = this
    
    if (firstVal && secondVal && operator) {
      let total = calculate()
      setDisplay(total + '')
      resetState()
    }
  }
  
  render() {
    const { 
      display,
      operator,
    } = this.state
    const {
      operatorClickHandler,
      numberClickHandler,
      deleteChar,
      allClear,
      equalHandler,
      getOverall,
    } = this
    const activeOperator = name => {
      return operator === name ? 'active' : ''
    }
    return (
      <div>
        <div className="display">
          <p className="display-overall">{ getOverall().trim() }</p>
          <p className="display-text">{ display }</p>
        </div>
        <div className="inputs">
          <div className="column main">
            <div className="operator">
              <div className="row">
  	            <button 
                  className={activeOperator('+')}
                  onClick={operatorClickHandler}>+</button>
  	            <button 
                  className={activeOperator('-')}
                  onClick={operatorClickHandler}>-</button>
  	            <button 
                  className={activeOperator(':')}
                  onClick={operatorClickHandler}>:</button>
  	            <button 
                  className={activeOperator('*')}
                  onClick={operatorClickHandler}>*</button>
              </div>
            </div> { /* operator */ }
            <div className="numbers">
              <div className="row">
  	            <button onClick={numberClickHandler}>1</button>
  	            <button onClick={numberClickHandler}>2</button>
  	            <button onClick={numberClickHandler}>3</button>
              </div>
              <div className="row">
  	            <button onClick={numberClickHandler}>4</button>
  	            <button onClick={numberClickHandler}>5</button>
  	            <button onClick={numberClickHandler}>6</button>
              </div>
              <div className="row">
                <button onClick={numberClickHandler}>7</button>
                <button onClick={numberClickHandler}>8</button>
                <button onClick={numberClickHandler}>9</button>
              </div>
              <div className="row">
  	            <button onClick={numberClickHandler}>.</button>
  	            <button onClick={numberClickHandler}>0</button>
  	            <button onClick={deleteChar}>C</button>
              </div>
            </div> { /* numbers */ }
          </div> { /* main */ }
          <div className="column sides">
  	        <button className="ac" onClick={allClear}>AC</button>
  	        <button className="equal" onClick={equalHandler}>=</button>
          </div> { /* sides */ }
        </div> { /* inputs */ }
      </div>
    )
  }
}

ReactDOM.render(
  <CalcApp />,
  document.getElementById('root')
);
!
999px

Console