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HTML

              
                <!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <!-- <meta name="viewport" content="width=device-width, initial-scale=1.0"> -->
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Javascript Calculator</title>

    <script src="https://unpkg.com/babel-standalone@6/babel.min.js"></script>
    <script src="https://unpkg.com/react@16/umd/react.development.js" crossorigin></script>
    <script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js" crossorigin></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/redux/4.0.4/redux.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react-redux/7.1.0/react-redux.js"></script>

    
    <link href="https://fonts.googleapis.com/css?family=Comfortaa|Graduate&display=swap" rel="stylesheet"> 
    <script src="https://kit.fontawesome.com/3431e6ad2c.js"></script>
    <link rel="stylesheet" href="styles.css">
</head>

<body>
    <!-- <script src="https://cdn.freecodecamp.org/testable-projects-fcc/v1/bundle.js"></script> -->
    <script src="src/App.js" type="text/babel"></script>
    <div class="root-container" id = "root"></div>
    <footer>
        <p class="footer-text">Created by Lucas Ezequiel Ojeda for the "<a class="footer-link"
                href="https://learn.freecodecamp.org/front-end-libraries/front-end-libraries-projects/build-a-javascript-calculator"
                title="Build a JavaScript calculator in FreeCodeCamp">Build a JavaScript Calculator
                challenge</a>" at FreeCodeCamp (August, 2019)</p>

        <a class="footer-link" href="https://github.com/lezojeda" target="_blank" title="Link to my GitHub profile">
            <i class="fab fa-github fa-3x profile-icon"></i></a>
        <a class="footer-link" href="https://www.freecodecamp.org/lezojeda" target="_blank"
            title="Link to my FreeCodeCamp profile">
            <i class="fab fa-free-code-camp fa-3x profile-icon"></i></a>
    </footer>
</body>

</html>
              
            
!

CSS

              
                * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }

body {
    background: #203055;
    display:flex;
    font-family: 'Comfortaa', cursive;
    justify-content: center;
}

.root-container {
    background:#EDEEF1;
    border: 2px solid black;
    border-radius:5px;
    display:flex;
    flex-direction:column;
    padding: 10px;
    position: absolute;
    top: 42.5%;
    transform: translateY(-50%);
}

.display-container {
    display:flex;
    flex-direction: column;

    background: #6b778d;
    cursor: default;
    font-family: 'Graduate', cursive;
    font-size: 1.2em;
    text-align: right;
    margin-bottom: 3px;
    padding: 2px 5px 2px 0;
    width:223px;
}

.display {
    display:flex;
    justify-content: flex-end;
    overflow-x:auto;
}

.keypad-container {
    display:flex;
    width: 220px;   
}

.keypad > button {
    border:none;
    cursor:pointer;
    font-size: 1em;
    font-family: 'Comfortaa', cursive;
    transition: 0.15s;
}

.keypad > button:hover {
    background: #EDEEF1;
}

.numpad > button:active {
    background: #ff6768;
}

.operators > button:active {
    background: #6b778d;
}

.numpad {
    align-items: center;
    display:flex;
    flex-wrap:wrap;
    justify-content:center;
    
    background: #ff6768;
    margin-right: 2px;
    width: 180px;
}

.numpad > button {
    background: #ff6768;
    height: 55px;
    width: 55px;
}

button[value="0"] {
    width: 62%;
}

.operators {
    align-items:center;
    display:flex;
    flex-direction:column;
    justify-content:center;

    background: #6b778d;
    width: 40px;
}

.operators > button {
    background: #6b778d;
    font-weight: bold;
    height: 45px;
    width: 45px;
}

footer {
    background: #EDEEF1;
    bottom:0;   
    font-size: 0.9em;
    left:0;
    padding: 10px 0;
    position:absolute;
    right:0;
    text-align:center
}

.footer-link {
    text-decoration: none;
}

.footer-link:hover {
    color: gray;
}

.footer-text {
    display:none;
    padding-bottom: 10px;
}

.fab {
    color: black;
}

.fab:hover {
    color: gray
}

@media only screen and (min-width : 640px) and (min-height: 600px) {
    .display-container {
        height: 70px;
        width: 268px;
    }
    .display {
        align-items:center;
        font-size: 1.2em;
        height:40px;
    }
    .keypad > button {
        font-size: 1.2em;
    }
    .numpad {
        margin-right: 10px;
        width: 210px;
    }
    .numpad > button {
        height: 70px;
        width: 70px;
    }
    .operators > button {
        height: 55px;
        width: 55px;
    }
    .keypad-container {
        width: 270px;
    }
    .footer-text {
        display:block;
    }
}

@media only screen and (min-width : 640px) and (min-height: 900px) {
    .display-container {
        height: 100px;
        width: 378px;
    }
    .display {
        align-items:center;
        font-size: 1.5em;
        height:50px;
    }
    .keypad > button {
        font-size: 1.4em;
    }
    .numpad {
        margin-right: 20px;
        width: 300px;
    }
    .numpad > button {
        height: 100px;
        width: 100px;
    }
    .operators > button {
        height: 75px;
        width: 75px;
    }
    .keypad-container {
        width: 380px;
    }
    .footer-text {
        display:block;
    }
}

              
            
!

JS

              
                //COMPONENTS

//DISPLAY
const Display = (props) => {
    return (
        <div className="display-container" id="display-container">
            <div id="display-input" className="display">
                {props.input}
            </div>
            <div id="display" className="display">
                {props.operation}
            </div>
        </div>
    )
}

var contains_decimal = (/\./)

//NUMPAD
const NumPad = (props) => {
    function handleNumPadClick(e) {
        if (e.target.value == "." && !contains_decimal.test(props.input)) {
            props.addNewDecimal()
        } else if (e.target.value == "0") {
            props.addNewZero()
        } else if (props.input.length == 1 && props.input == 0) {
            props.addFirstNumber_dispatched(e.target.value)
        } else if((/[0-9]/).test(e.target.value)){ 
            props.addNewNumber(e.target.value)
        }
    }


    return (
        <div className="keypad numpad" onClick={handleNumPadClick} tabIndex="1">
            <button id="one" value="1">1</button>
            <button id="two" value="2">2</button>
            <button id="three" value="3">3</button>
            <button id="four" value="4">4</button>
            <button id="five" value="5">5</button>
            <button id="six" value="6">6</button>
            <button id="seven" value="7">7</button>
            <button id="eight" value="8">8</button>
            <button id="nine" value="9">9</button>
            <button id="zero" value="0">0</button>
            <button id="decimal" tabIndex="0" value=".">.</button>
        </div>
    )
}

//OPERATOR PAD
const Operators = (props) => {
    function handleOperatorsClick(e) {
        if (e.target.value == "=") {
            props.evaluateOperation_dispatched(props.operation)
        } else if (e.target.value == "clear") {
            props.clearResultAndOperation_dispatched()
        } else {
            props.addNewOperator(e.target.value)
        }
    }

    return (
        <div className="keypad operators" onClick={handleOperatorsClick} tabIndex="1">
            <button id="add" value="+">+</button>
            <button id="subtract" value="-">-</button>
            <button id="multiply" value="*">x</button>
            <button id="divide" value="/">/</button>
            <button id="equals" value="=">=</button>
            <button id="clear" value="clear">AC</button>
        </div>
    )
}

//PRESENTATIONAL COMPONENT

class Presentational extends React.Component {
    constructor(props) {
        super(props);
        this.handleKeyDown = this.handleKeyDown.bind(this)
    }

    handleKeyDown(e) {
        if (e.key == "." && !contains_decimal.test(this.props.input)) {
            this.props.addNewDecimal()
        } else if (e.key == "0") {
            this.props.addNewZero()
        } else if (this.props.input.length == 1 && this.props.input == 0) {
            this.props.addFirstNumber_dispatched(e.key)
        } else if ((/[0-9]/).test(e.key)){
            this.props.addNewNumber(e.key)
        } else if (e.key == "=") {
            this.props.evaluateOperation_dispatched(this.props.operation)
        } else if(e.key =="C" || e.key == "c") {
            this.props.clearResultAndOperation_dispatched()
        } else if ((/\/|\*|\-|\+/).test(e.key)){
            this.props.addNewOperator(e.key)
        }
    }

    componentDidMount(){
        document.addEventListener("keydown", this.handleKeyDown, false);
      }

    componentWillUnmount(){
        document.removeEventListener("keydown", this.handleKeyDown, false);
    }
    

    render() {
        return (
            <div className="container">
                <Display
                    input={this.props.input}
                    operation={this.props.operation}
                    result={this.props.result} />
                <div className="keypad-container">
                    <NumPad
                        input={this.props.input} addFirstNumber_dispatched={this.props.addFirstNumber_dispatched} addNewNumber={this.props.addNewNumber} addNewDecimal={this.props.addNewDecimal} addNewZero={this.props.addNewZero} />

                    <Operators
                        operation={this.props.operation}
                        addNewOperator={this.props.addNewOperator}
                        evaluateOperation_dispatched={this.props.evaluateOperation_dispatched}
                        result={this.props.result}
                        clearResultAndOperation_dispatched={this.props.clearResultAndOperation_dispatched} />
                </div>
            </div >
        )
    }
}

////////////////////////////////////////////////
// REDUX

const FIRST_NUMBER = 'FIRST_NUMBER'
const ADD_NUMBER = 'ADD_NUMBER'
const ADD_DECIMAL = 'ADD_DECIMAL'
const ADD_ZERO = 'ADD_ZERO'
const ADD_OPERATOR = 'ADD_OPERATOR'
const EVALUATE = 'EVALUATE'
const CLEAR = 'CLEAR'

//ACTION CREATOR
const addFirstNumber = (number) => {
    return {
        type: FIRST_NUMBER,
        number: number
    }
}

const addNumber = (number) => {
    return {
        type: ADD_NUMBER,
        number: number
    }
}

const addDecimal = () => {
    return {
        type: ADD_DECIMAL
    }
}

const addZero = (number) => {
    return {
        type: ADD_ZERO,
        number: number
    }
}

const addOperator = (operator) => {
    return {
        type: ADD_OPERATOR,
        operator: operator
    }
}

const evaluateOperation = (operation) => {
    return {
        type: EVALUATE,
        operation: operation
    }
}

const clearResultAndOperation = () => {
    return {
        type: CLEAR,
    }
}

//REDUCER     

const inputReducer = (state = '0', action) => {
    switch (action.type) {
        case FIRST_NUMBER:
            return eval(parseInt(state + action.number))
        case ADD_NUMBER:
            return state + action.number
        case ADD_DECIMAL:
            if (state.toString().length == 1) {
                console.log("asd")
                return state + '.'//if it's first number just add the decimal
            } else if (state[state.length - 1].indexOf('.') == -1) {
                return state + '.'//if it's not the first number, check that the last character is not a decimal to avoid repetition
            }
            return state
        case ADD_ZERO:
            if (contains_decimal.test(state) || state != "0") {
                return state + "0"
            }
            return state
        case ADD_OPERATOR:
            return action.operator
        case EVALUATE:
            try {
                return eval(state)
            } catch (err) {
                return state
            }
        case CLEAR:
            state = '0'
        default:
            return state
    }
}

const outputReducer = (state = '0', action) => {
    switch (action.type) {
        case FIRST_NUMBER:
            return eval(parseInt(state + action.number))
        case ADD_NUMBER:
            return state + action.number
        case ADD_DECIMAL:
            if (state.toString().length == 1) {
                return state + '.' 
            } else if (state[state.length - 1].indexOf('.') == -1) {
                return state + '.'
            }
            return state
        case ADD_ZERO:
            if (contains_decimal.test(state) || state != "0") {
                return state + "0"
            }
            return state
        case ADD_OPERATOR:
            var is_operator = (/\/|\*|\-|\+/)
            var is_number = (/[0-9]/)
            var two_operators = /(\/|\*|\-|\+){2}/

            if (two_operators.test(state.toString().slice(-2))) {
                return state.replace(state.toString().slice(-2), action.operator)
            } else if (is_number.test(state) && state.toString().length == 1 || is_number.test(state[state.length - 1])) {
                return state + action.operator

            } else if (is_operator.test(state[state.length - 1]) && action.operator != "-") {
                return state.substring(0, state.length - 1) + action.operator
            } else if (is_operator.test(state[state.length - 1]) && state[state.length - 1] != "-") {
                return state + action.operator
            }
            return state + action.operator

        case EVALUATE:
            try {
                return eval(state)
            } catch (err) {
                alert("Please finish the operation or clear input (AC)")
                return state
            }
        case CLEAR:
            state = '0'
        default:
            return state
    }
}

//STORE

const rootReducer = Redux.combineReducers({ input: inputReducer, operation: outputReducer });

const store = Redux.createStore(rootReducer);

//CONECTO REACT-REDUX
const Provider = ReactRedux.Provider;

//MAP STATE AND PROPS
function mapStateToProps(state) {
    return ({
        input: state.input,
        operation: state.operation,
    })
}

function mapDispatchToProps(dispatch) {
    return ({
        addFirstNumber_dispatched: (number) => {
            dispatch(addFirstNumber(number))
        },
        addNewNumber: (number) => {
            dispatch(addNumber(number))
        },
        addNewDecimal: () => {
            dispatch(addDecimal())
        },
        addNewZero: (number) => {
            dispatch(addZero(number))
        },
        addNewOperator: (operator) => {
            dispatch(addOperator(operator))
        },
        evaluateOperation_dispatched: (operation) => {
            dispatch(evaluateOperation(operation))
        },
        clearResultAndOperation_dispatched: () => {
            dispatch(clearResultAndOperation())
        }
    })
}

const Container = ReactRedux.connect(mapStateToProps, mapDispatchToProps)(Presentational)

class AppWrapper extends React.Component {
    render() {
        return (
            <Provider store={store}>
                <Container />
            </Provider>
        )
    }
}

ReactDOM.render(<AppWrapper />, document.getElementById("root"))
              
            
!
999px

Console