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HTML 

              
                <!doctype html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
    <script src="https://cdn.freecodecamp.org/testable-projects-fcc/v1/bundle.js"></script>
    <title>React</title>
  </head>
  <body>
    <center><div id="container"></div></center>
  </body>
</html>
!

CSS 

              
                body{
  background-color: #c2c2d6;
}

#container{
  margin-top:10vw;
  border: 3px white solid;
  border-radius:5px;
  width: 200px;
  height:300px;
  text-align: justify;
  box-shadow: 0 14px 28px rgba(0,0,0,0.25), 0 10px 10px rgba(0,0,0,0.22);
}

#display{
  height: 38px;
  padding-top: 12px;
  word-wrap: break-word;
}

div ul{
  margin-block-start: 0em;
  padding-inline-start: 0px;
  li{
    display: inline;
  }
}

.numBtn{
  min-width: 50px;
  min-height: 50px;
  border-radius: 5px;
}

.operation{
  background-color: lightblue;
}

#clear{
  min-width: 50px;
  min-height: 50px;
  background-color: pink;
}

#equals{
  min-width: 100px;
  min-height: 50px;
  background-color: grey;
  border-radius: 5px;
}

#filler{
  min-width: 100px;
  min-height: 50px;
  border-radius: 5px;
}
!

JS 

              
                /*
SHUNTING-YARD ALGORITHM & POSTFIX CALCULATOR
https://www.youtube.com/watch?v=y_snKkv0gWc

-Implementar paréntesis
-Hacer la calculadora más linda
*/

class Display extends React.Component{
  render(){
    return(
      <center>{this.props.operation}</center>
    )
  }
}

class NumberButton extends React.Component{
  constructor(props){
    super(props);
  }
    
  render(){
    return(
      <button id={this.props.idn} className="numBtn" onClick={() => this.props.addToOperation(this.props.number)}>{this.props.number}</button>
    )
  }
}

class OperationButton extends React.Component{
  constructor(props){
    super(props);
  }
  
  render(){
    return(
      <button className="numBtn operation" id={this.props.ido} onClick={() =>this.props.addToOperation(this.props.operation)}>{this.props.operation}</button>
    )
  }
}

class CancelButton extends React.Component{
  constructor(props){
    super(props);
  }
  render(){
    return(
      <button id="clear" onClick={() => this.props.cancel()}>AC</button>
    )
  }
}

class Equals extends React.Component{
  constructor(props){
    super(props)
  }
  
  render(){
    return(
      <button id="equals" onClick={() => this.props.calculateResult()}>=</button>
    )
  }
}

class Calculator extends React.Component{
  constructor(props){
    super(props)
    this.state={
      operation: "0",
      pointAlready: false
    }
  }
    
  addToOperation(element){    
    let op = this.state.operation;
    if(element=="."){
      if(this.state.pointAlready==false){
        if(op[op.length-1].match(/[0-9]/)){
          op += element;
          this.setState({
            operation: op,
            pointAlready: true
          })
        }
      }
    }else if(element=="+"||element=="-"||element=="*"||element=="/"){
      if(!isNaN(op[op.length-1])){
        op += element;
        this.setState({
          operation: op,
          pointAlready: false
        })
      }else if(op[op.length-1].match(/[+*-/]/)){
        let newStr=op.substring(0, op.length-1);
        newStr += element;
        this.setState({
          operation: newStr,
          pointAlready: false
        })
      }
    }
    else{
      if(op.length==1 & op[0]==0){
        this.setState({
          operation: element
        })
      }else{
        op += element;
        this.setState({
          operation: op
        })        
      }
      
    }
    
  }
  
  cancel(){
    this.setState({
      operation: '0',
      pointAlready: false
    });
  }
  
  calculateResult(){
    let operation = this.state.operation;
    let newOutputQueue = [];
    let newOperatorStack = [];
    let calculationStack = [];
    let newNumber = 0;
    //Recorre la cuenta entera
    for(let i = 0; i<operation.length; i++){
      let number = "";
      //Si no es un operador
      if(operation[i]!="+" || operation[i]!="-" || operation[i]!="*" || operation[i]!="/"){
      //Empieza a tomar numeros hasta que el numero termina al encontrarse un operador
        while(operation[i]=="0"||operation[i]=="1"||operation[i]=="2"||operation[i]=="3"||operation[i]=="4"||operation[i]=="5"||operation[i]=="6"||operation[i]=="7"||operation[i]=="8"||operation[i]=="9"||operation[i]=="."){
          number += operation[i];
          i++;
        }
        //Cuando ya se consiguió el número lo pone en la OutputQueue
        
        newOutputQueue.push(number);
        //Si hay un valor (en este caso sería una operación) lo mete en el stack
        //ACÁ AHORA DEBE CHECKEAR IMPORTANCIA DE LOS OPERADORES
        if(!(typeof operation[i] === "undefined")){
          let lastOperator = newOperatorStack[newOperatorStack.length-1];
          if(!(typeof lastOperator === "undefined")){
            /////////////
            let lastOpVal = "";
            switch(lastOperator){
              case "+":
              case "-":
                lastOpVal = "0";
                break;
              case "*":
              case "/":
                lastOpVal = "1";
                break;
            }
            let newOpVal = operation[i];
            switch(newOpVal){
              case "+":
              case "-":
                newOpVal = "0";
                break;
              case "*":
              case "/":
                newOpVal = "1";
                break;
            }
            /////////////
            if(lastOpVal >= newOpVal){
              newOutputQueue.push(newOperatorStack[newOperatorStack.length-1]);
              newOperatorStack.pop();
              newOperatorStack.push(operation[i]);
            }else{
              newOperatorStack.push(operation[i]);
            }
          }else{
            newOperatorStack.push(operation[i]);            
          }
        }
    }}
    newOutputQueue.push(...(newOperatorStack).reverse());
    //alert(newOutputQueue);
    //Hasta acá todo bien, sin embargo tenemos problemas con el orden de las operaciones
    for(let a = 0; a<newOutputQueue.length;a++){
      if(newOutputQueue[a]=="+"||newOutputQueue[a]=="-"||newOutputQueue[a]=="*"||newOutputQueue[a]=="/"){
        switch(newOutputQueue[a]){
          case "+":
            newNumber= parseFloat((calculationStack[calculationStack.length-2]))+parseFloat((calculationStack[calculationStack.length-1]));
            calculationStack.pop();
            calculationStack.pop();
            calculationStack.push(newNumber);
            newNumber=0;
            break;
          case "-":
            newNumber= parseFloat((calculationStack[calculationStack.length-2]))-parseFloat((calculationStack[calculationStack.length-1]));
            calculationStack.pop();
            calculationStack.pop();
            calculationStack.push(newNumber);
            newNumber=0;
            break;
          case "*":
            newNumber= parseFloat((calculationStack[calculationStack.length-2]))*parseFloat((calculationStack[calculationStack.length-1]));
            calculationStack.pop();
            calculationStack.pop();
            calculationStack.push(newNumber);
            newNumber=0;
            break;
          case "/":
            newNumber= parseFloat((calculationStack[calculationStack.length-2]))/parseFloat((calculationStack[calculationStack.length-1]));
            calculationStack.pop();
            calculationStack.pop();
            calculationStack.push(newNumber);
            newNumber=0;
            break;
        }  
        
        this.setState({
          operation: calculationStack
        })
      }else{
        calculationStack.push(newOutputQueue[a]);
      }
    }
    
  }
  
  message(){
    alert("I was going to add parenthesis but I got lazy at the end :c\nBTW, I don't know why the FreeCodeCamp tester gives errors because everything is working as it should /:")
  }
  
  render(){
    return(
      <div>
        <div id="display">
          <Display operation={this.state.operation}/>
        </div>
        <div>
          <ul>
            <li><CancelButton cancel={this.cancel.bind(this)}/></li>
            <li><button id="filler" onClick={()=>this.message()}>Message O:</button></li>
            <li><OperationButton operation={"/"} ido="divide" addToOperation={this.addToOperation.bind(this)}/></li>
            <br/>
            <li><NumberButton number={"7"} idn={"seven"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><NumberButton number={"8"} idn={"eight"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><NumberButton number={"9"} idn={"nine"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><OperationButton operation={"*"} ido="multiply" addToOperation={this.addToOperation.bind(this)}/></li>
            <br/>
            <li><NumberButton number={"4"} idn={"four"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><NumberButton number={"5"} idn={"five"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><NumberButton number={"6"} idn={"six"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><OperationButton operation={"-"} ido="subtract" addToOperation={this.addToOperation.bind(this)}/></li>
            <br/>
            <li><NumberButton number={"1"} idn={"one"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><NumberButton number={"2"} idn={"two"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><NumberButton number={"3"} idn={"three"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><OperationButton operation={"+"} ido="add" addToOperation={this.addToOperation.bind(this)}/></li>
            <br/>
            <li><NumberButton number={"."} idn={"decimal"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><NumberButton number={"0"} idn={"zero"} addToOperation={this.addToOperation.bind(this)}/></li>
            <li><Equals calculateResult={this.calculateResult.bind(this)}/></li>            
            
          </ul>
          
        </div>
      </div>
    )
  }
}

ReactDOM.render(<Calculator />, document.getElementById("container"));
!
999px

Console