Simple implementation of JS calculator function

Functions of this calculator:
1. Verification: decimal point, repeated calculations, and a large number of operations that are more in line with the user experience.
2. Ability to input from keyboard.

html code:

css code:

@CHARSET "UTF-8"; 
.button { 
  width: 73px; 
  height: 50px; 
  margin-bottom: 5px; 
  border: 1px solid #ddd; 
  border-radius: 1px; 
  font-weight: bold; 
  color: #1e395b; 
  background: aqua; 
  font-size:20px; 
  border-radius: 3px; 
} 
.text { 
  width: 300px; 
  border: 1px solid #c2c2c2; 
  height: 50px; 
  border-radius: 5px; 
  font-size: 24px; 
  text-align: right; 
  background: #ffffff; 
  margin-top: 10px; 
  margin-bottom: 10px; 
} 
.textShow{ 
  width: 300px; 
  border: 1px solid #c2c2c2; 
  height: 40px; 
  border-radius: 5px; 
  font-size: 18px; 
  text-align: right; 
  background: #ffffff; 
  margin-top: 10px; 
  margin-bottom: 10px; 
} 
.cal{ 
margin-left:200px; 
border:2px solid #99ccff; 
margin:100px auto; 
position:relative; 
width:330px; 
height:420px; 
border-radius: 10px; 
} 
.cont{ 
 margin-left: 10px; 
} 
.funcBtn{ 
padding-left: 580px; 
border-radius: 10px; 
font-size: 20px; 
position: fixed; 
}

js code:

/** 
 * 
 */
    var checkEqual = false;//Avoid repetition calcu 
    var opFlag = false; //Check repetition add operation 
    var upText = document.getElementById("upText"); 
    var cal = document.getElementById("cal").style; 
    var downText = document.getElementById("downText"); 
      
    function show() { 
      cal.display = "block"; 
    } 
      
    function hide() { 
      cal.display = "none"; 
    } 
      
    function clearAllText() { 
      upText.value = ""; 
      downText.value = "0"; 
      opFlag = false; 
      checkEqual = false; 
    } 
      
    function oppositeOp(){ 
      downText.value = -downText.value;     
    } 
      
    function deleteOneDigit() { 
      downText.value = downText.value.substring(0, downText.value.length - 1); 
      if (downText.value == "" || (downText.value.charAt(0) == "-" && downText.value.charAt(1) == "")) { 
        downText.value = "0"; 
        return downText.value; 
      } 
      return downText.value; 
    } 
      
    function clickNum(num) { 
      if(num == ".") {         
        checkIfAddPoint(num); 
      } else { 
        checkIfAddNum(num); 
      } 
      checkEqual = false; 
    } 
      
    function checkIfAddPoint(num) { 
      if((num=="." && downText.value == "0") || opFlag == true) { 
        downText.value="0."; 
        opFlag = false; 
      } else if(num == "." && downText.value.indexOf(".") > -1) { 
        downText.value; 
      } else { 
        downText.value += num; 
      } 
    } 
      
    function checkIfAddNum(num) { 
       if((num != "." && downText.value == "0" && downText.value[1] != ".")  
           || opFlag == true 
           || downText.value == "Infinity"
           || checkEqual == true) { 
        downText.value = num; 
        opFlag = false; 
      } else { 
        downText.value += num; 
      } 
    } 
      
    function clickOperation(op){ 
      checkEqual = false; 
      downText.value = checkdownTextValid(downText.value); 
      downText.value = checkZero(upText.value,downText.value); 
      switch(op){ 
      case "+":{    
        upText.value = appendupTextValue(upText.value,downText.value,"+"); 
        opFlag = true; 
       }break; 
      case "-":{ 
        upText.value = appendupTextValue(upText.value,downText.value,"-"); 
        opFlag = true; 
       }break; 
      case "x":{ 
        upText.value = appendupTextValue(upText.value,downText.value,"*"); 
        opFlag = true; 
       }break; 
      case "/":{  
        upText.value = appendupTextValue(upText.value,downText.value,"/"); 
        opFlag = true; 
       }break; 
      } 
    } 
      
    function checkdownTextValid(downText){ 
      if(downText.charAt(downText.length-1) == "."){ 
        return downText.substring(0,downText.length-1); 
      }  
      return downText; 
    } 
      
    function checkZero(upText,downText){ 
      if(upText.charAt(upText.length-1) == "/" && downText == "0"){ 
        return "Error"; 
      } 
      return downText; 
    } 
      
    function appendupTextValue(oldvalue,newvalue,operation){ 
      if(oldvalue == ""){ 
         return oldvalue = newvalue+operation; 
      } else if(opFlag!=true){ 
         return oldvaluee = oldvalue+newvalue+operation;  
      } else{ 
        return oldvalue.substring(0, oldvalue.length-1)+operation; 
      } 
    } 
      
    function calcu() { 
      if(!checkEqual){   
      downText.value = eval(upText.value+downText.value); 
      upText.value = ""; 
      checkEqual = true; 
      } 
    } 
      
    window.document.onkeydown = chooseKey; 
    function chooseKey(evt){ 
      if(evt.keyCode == 13){alert("=");} 
      else if(evt.keyCode == 8){deleteOneDigit();} 
      else if(evt.keyCode == 27){clearAllText();} 
      else if(evt.keyCode == 48){clickNum('0');} 
      else if(evt.keyCode == 49){clickNum('1');} 
      else if(evt.keyCode == 50){clickNum('2');} 
      else if(evt.keyCode == 51){clickNum('3');} 
      else if(evt.keyCode == 52){clickNum('4');} 
      else if(evt.keyCode == 53){clickNum('5');} 
      else if(evt.keyCode == 54){clickNum('6');} 
      else if(evt.keyCode == 55){clickNum('7');} 
      else if(evt.keyCode == 56){clickNum('8');} 
      else if(evt.keyCode == 57){clickNum('9');} 
      else if(evt.keyCode == 96){clickNum('0');} 
      else if(evt.keyCode == 97){clickNum('1');} 
      else if(evt.keyCode == 98){clickNum('2');} 
      else if(evt.keyCode == 99){clickNum('3');} 
      else if(evt.keyCode == 100){clickNum('4');} 
      else if(evt.keyCode == 101){clickNum('5');} 
      else if(evt.keyCode == 102){clickNum('6');} 
      else if(evt.keyCode == 103){clickNum('7');} 
      else if(evt.keyCode == 104){clickNum('8');} 
      else if(evt.keyCode == 105){clickNum('9');} 
      else if(evt.keyCode == 110){clickNum('.');} 
      else if(evt.keyCode == 106){clickOperation('x');} 
      else if(evt.keyCode == 107){clickOperation('+');} 
      else if(evt.keyCode == 111){clickOperation('÷');} 
      else if(evt.keyCode == 109){clickOperation('-');}

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