JS Calculator Errors

JavaScript
1

I’ve been trying many things to figure out how to solve the two problems I’ve encountered.

The problems are:

  1. When trying to input “0”, subMath shows 00 - 6
  2. Equation doesn’t work properly. For example, I entered 1 + 2 and then press “=”, showing 3, but when I try to click on “+” afterward, it add 3 to the subMath, showing 1 + 23. Therefore, after “=” sign, mainMath should be equal to the final answer.

My codepen is: https://codepen.io/kikibres/pen/MEQvqv.

I did several solutions but I couldn’t find the right solution that’ll solve both problems…

My Javascript code is:

$(document).ready(function() {
  
  var mainMath = "0";
  var subMath = "0";
  var finalset = "";
  var nextanswer = "";
  update();
  
  $("button").click(function(){
    calculate($(this).attr("value"));
  });
  
  function calculate(keyitem) {
    switch(keyitem) {
      case "clear":
        clearScreen();
        break;
      case "plusminus":
        plusminusScreen();
        break;
      case "%":
        percentageScreen();
        break;
      case "/":
      case "*":
      case "+":
      case "-":
        addOperator(keyitem);
        break;
      case "0":
      case "1":
      case "2":
      case "3":
      case "4":
      case "5":
      case "6":
      case "7":
      case "8":
      case "9":
        addNumber(keyitem);
        break;
      case ".":
        addDecimal(keyitem);
        break;
      case "=":
        solveEqual();
        break;
    }
    update();
    };
 
  function clearScreen() {
     mainMath = "0";
     subMath = "0";
    if(mainMath.length > 0){
      $(".entry").css("font-size", "4em");
    }
  };
  
  function plusminusScreen() {
     mainMath = -1 * mainMath;
    finalset = mainMath;
  };
  
  function addNumber(keyitem) {
    if (mainMath == "0"){
      mainMath = keyitem;
      finalset = mainMath;
      return;
    } 
    mainMath+=keyitem;
      /*subMath += "";*/
    if(mainMath.length > 8){
      $(".entry").css("font-size", "1.5em");
    }
    
  };
  
  function addOperator(keyitem){
    if(mainMath == "0"){
    subMath += "0";
    }
    if(subMath == "0"){
      subMath = "";
    }
    /*subMath += "";*/
    addNumber(keyitem);
    subMath += mainMath;
    mainMath = "0";
  };
  
  function addDecimal(keyitem){
    /*if (keyitem == "."){*/
    if (mainMath.indexOf(keyitem) === -1){
      if(mainMath == "0") {
        mainMath = "0" + keyitem;
        subMath = "0";
        return;
      }
    } else {
        return;
      }
    /*}*/
    addNumber(keyitem);
  };
  
  function solveEqual() {
    mainMath = eval(subMath+mainMath);
    subMath += finalset;
    var finalresult = mainMath.toString();
    if(finalresult.length > 8){
      $(".entry").css("font-size", "1.5em");
    }
  };
  
  function update(){
  $("#answer").html(mainMath);
  $("#history").html(subMath);
};
  
});