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HTML 

            
              <html>
  <body>
    <div id='frame' class="container-fluid">
      <div id='numPad'class="row">
        <p id='display' style='font-size:62px;'></p>
      </div>
      <div class='row text-center'>
    
    <div id='row1' class='row' style='margin: 0;'>
     <button class="button allClear" style='width:111px;'>Clear</button>
      <button class="button clearEntry" style='width:111px;'>CE</button>
      <button class="button op_key">/</button>
    </div>

    <div id='row2' class='row' style='margin:0;'>
      <button class="button digit">7</button>
      <button class="button digit">8</button>
      <button class="button digit">9</button>
      <button class="button op_key">x</button>
     </div>

    <div id='row3' class='row' style='margin:0;'>
      <button class="button digit">4</button>
      <button class="button digit">5</button>
      <button class="button digit">6</button>
      <button class="button op_key">-</button>
    </div>

    <div id='row4' class='row' style='margin:0;'>
      <button class="button digit">1</button>
      <button class="button digit">2</button>
      <button class="button digit">3</button>
      <button class="button op_key">+</button>
    </div>

    <div id='row6' class='row' style='margin:0;'>
      <button class="button digit">0</button>
      <button class="button point" >.</button>
      <button class="button equals" style='width:148px; background: #71BCC6;'>=</button>
    </div>

    </div>
  </div>
</body>
</html>
!

CSS 

            
              body{
  background: #9ae6f9;
}
#frame{
  position: absolute;
  margin: auto;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  width: 300px;
  height: 350px;
  border: 2px solid #9B150B;
}
#numPad{
  border-bottom: 1px solid #9B150B;
  width: auto;
  height: 100px;
  background: #FF9890;

}
.col-centered{
  float: none;
  margin: 0 auto;
}



.button {
  background-color: #E84D41;
  color: white;
  padding: 15px 15px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 16px;
  cursor: pointer;
  border: none;
  float: left;
  width: 74px;
  height: 49.2px;
  outline: 0;
}
.button:hover{
  background-color:#FF9890;
  color: black;
}
p{
  color: black;
  float: right;
}
!

JS 

            
              alert("Thanks for checking out my OOP JavaScript Calculator. Enjoy!");

var kclass = {
  //number
  NUM: 1,
  //decimal
  DECI: 2,
  //operator
  OP: 3,
  //clear entry
  CE: 4,
  //equal
  EQ: 5
}

//different states
var states = {
  //default state
  START: 1,
  //when the first number is added
  FIRST_ARG: 2,
  //if the first num starts with a decimal or contains a decimal
  FIRST_ARG_FLOAT: 3,
  //operator
  OP: 4,
  //when the second num is added
  SECOND_ARG: 5,
  //if the 2nd num has a decimal other than the beginning
  SEC_ARG_FLOAT: 6,
  //if 2nd num start with a decimal
  SEC_ARG_DECI: 7,
  //equals
  EQ: 8
}

//default state of calculator
  $("#display").text(0);

//contains variables and calc functions
var calc = {
  //monitors current state
  state: states.START,
  //monitors current operator
  op: "",
  //monitors what is currently displayed on the display
  disp: "",
  //stores the first number for further operations
  acc1: "",
  //stores the 2nd number for further operations
  acc2: "",
  //function that controls transitions between states using switch
  //two arguments - key_class(key category), key(specific key)
  doStep: function(key_class, key){
      switch (this.state){
         case states.START:
            if(key_class === kclass.NUM){
                //state action
                this.dispSet(key);
                //move to the next state
                this.state = states.FIRST_ARG;
            }
            if(key_class === kclass.DECI){
                this.dispSet("0.");
                this.state = states.FIRST_ARG_FLOAT;
            }
            break;
        case states.FIRST_ARG:
            if(key_class === kclass.NUM){
                this.dispAppend(key);
                this.state = states.FIRST_ARG;
            }
            if(key_class === kclass.OP){
                this.op = key;
                //store value of the disp in a acc1 variable in order to be able to store second number in the disp
                this.acc1 = this.disp;
                this.state = states.OP;
            }
            if(key_class === kclass.DECI){
                this.dispAppend(key);
                this.state = states.FIRST_ARG_FLOAT;
            }
            if(key_class === kclass.CE){
                this.dispSet("0");
                calc.state = states.START
            }
            break;
        case states.FIRST_ARG_FLOAT:
            if(key_class === kclass.NUM){
                this.dispAppend(key);
                this.state = states.FIRST_ARG_FLOAT;
            }
            if(key_class === kclass.OP){
                this.op = key;
                //store value of the disp in a acc1 variable in order to be able to store second number in the disp
                this.acc1 = this.disp;
                this.state = states.OP;
            }
            if(key_class === kclass.CE){
                this.dispSet("0");
                calc.state = states.START;
            }
            break;
        case states.OP:
            if(key_class === kclass.NUM){
                this.dispSet(key);
                this.state = states.SECOND_ARG;
            }
            if(key_class === kclass.DECI){
                this.dispSet("0.");
                this.state = states.SEC_ARG_DECI;
            }
            break;
        case states.SECOND_ARG:
            if(key_class === kclass.DECI){
                this.dispAppend(key);
                this.state = states.SEC_ARG_FLOAT;
            }
            if(key_class === kclass.NUM){
                this.dispAppend(key);
                this.state = states.SECOND_ARG;
            }
            if(key_class === kclass.EQ){
                //store the second number in the acc2 variable so that we can use it if the equal sign is pressed more than once
                this.acc2 = this.disp;
                //calculate the result
                this.operation(this.acc1, this.disp);
                this.displayUpdate(this.disp);
                this.state = states.EQ;
             }
            if(key_class === kclass.OP){
                //calculate the result
                this.operation(this.acc1, this.disp);
                this.op = key;
                //store the result of the operation in the acc1 in order to be used in the next operation
                this.acc1 = this.disp;
                this.displayUpdate(this.disp);
                this.state = states.OP;
            }
            if(key_class === kclass.CE){
                this.dispSet("0");
                calc.state = states.OP;
            }
            break;
        case states.SEC_ARG_FLOAT:
            if(key_class === kclass.NUM){
                this.dispAppend(key);
                this.state = states.SEC_ARG_FLOAT;
            }
            if(key_class === kclass.EQ){
                this.acc2 = this.disp;
                this.operation(this.acc1, this.disp);
                this.displayUpdate(this.disp);
                this.state = states.EQ;
             }
             if(key_class === kclass.OP){
                 this.operation(this.acc1, this.disp);
                 this.op = key;
                 this.acc1 = this.disp;
                 this.displayUpdate(this.disp);
                 this.state = states.OP;
             }
             if(key_class === kclass.CE){
                 this.dispSet("0");
                 calc.state = states.OP;
             }
            break;
        case states.SEC_ARG_DECI:
            if(key_class === kclass.NUM){
                this.dispAppend(key);
                this.state = states.SEC_ARG_FLOAT;
            }
            if(key_class === kclass.CE){
                this.dispSet("0");
                calc.state = states.OP;
            }
            break;
        case states.EQ:
            if(key_class === kclass.EQ){
                this.operation(this.disp, this.acc2);
                this.displayUpdate(this.disp);
                this.state = states.EQ;
            }
            if(key_class === kclass.NUM){
                this.dispSet(key);
                this.state = states.FIRST_ARG;
            }
            if(key_class === kclass.OP){
                this.op = key;
                this.acc1 = this.disp;
                this.state = states.OP;
            }
            if(key_class === kclass.DOT){
                this.dispSet("0.");
                this.state = states.FIRST_ARG_FLOAT;
            }
            if(key_class === kclass.CE){
                this.dispSet("0");
                this.clearer();
            }
            break;
      }
  },
  //does all the arithmetical operations
  operation: function(first, sec){
      if(this.op === "/"){
          this.disp = first / sec;
      }
      if(this.op === "-"){
          this.disp = first - sec;
      }
      if(this.op === "x"){
          this.disp = first * sec;
      }
      if(this.op === "+"){
          this.disp = Number(first) + Number(sec);
      }
      if(this.op === "%"){
          this.disp = first % sec;
      }
  },
  //restarts the calculator to the default state
  clearer: function(){
      $("#display").text(0);
      this.state = states.START;
      this.op;
      this.disp;
      this.acc1;
      this.acc2;
  },
  //appends a display var
  dispAppend: function(key){
      this.disp += key;
      this.displayUpdate(this.disp);
  },
  //set a new value to the display var
  dispSet: function(key){
      this.disp = key;
      this.displayUpdate(this.disp);
  },
  //dipsplay display var
  displayUpdate: function(dispText){
      $("#display").text(dispText);
  }
}


//Button Click Functions

//when the number is clicked
$(".digit").on("click", function(){
  calc.doStep(kclass.NUM, $(this).html());
})
//when the operator is clicked
$(".op_key").on("click", function(){
  calc.doStep(kclass.OP, $(this).html());
})
//when the equal sign is clicked
$(".equals").on("click", function(){
  calc.doStep(kclass.EQ, $(this).html());
})
//when the dot is clicked
$(".point").on("click", function(){
  calc.doStep(kclass.DECI, $(this).html());
})
//when AC clicked - clear all variable values
$(".allClear").on("click", function(){
  calc.clearer();
})
//when CE clicked - erase the last entered input
$(".clearEntry").on("click", function(){
  calc.doStep(kclass.CE, $(this).html());
})
!
999px

Console