Getting wrong order switch case statement in JavaScript

So I’m trying to do a switch – case based on months. If a user picks 1 then it prints january.. I have all the months in a array so arr[0] = January but the picking 1 in the dropdown would print "january". In my code I keep getting the wrong month, it’s always one behind. I’ve read some documentation on the switch – case but I think I’ve misunderstood the "expression". I’m trying to use case switch to avoid having a bunch of if-statements.

function months() {
    monthText = document.getElementById("month");
    var monthNumber = document.getElementById("month_number").value;
    var monthsArr = ["January", "February", "March", "April", "May", "June", "July", "August", "September", " October", "November", "December"];

    switch (monthNumber) {
        //Januari
        case monthNumber == "1": 
        monthText = monthsArr[0];
        break;
        //Februari
        case monthNumber == "2": 
        monthText = monthsArr[1];
        break;
        //Mars
        case monthNumber == "3": 
        monthText = monthsArr[2];
        break;
        //April
        case monthNumber == "4": 
        monthText = monthsArr[3];
        break;
        //Maj
        case monthNumber == "5": 
        monthText = monthsArr[4]
        break;
        //Juni
        case monthNumber == "6": 
        monthText = monthsArr[5];
        break;
        //Juli
        case monthNumber == "7": 
        monthText = monthsArr[6];
        break;
        //Augusti
        case monthNumber == "8": 
        monthText = monthsArr[7];
        break;
        //Septembar
        case monthNumber == "9": 
        monthText = monthsArr[8];
        break;
        //Oktober
        case monthNumber == "10": 
        monthText = monthsArr[9];
        break;
        //November
        case monthNumber == "11": 
        monthText = monthsArr[10];
        break;
        //December
        case monthNumber == "12": 
        monthText = monthsArr[11];
        break;
      }

      monthText.value = monthsArr[monthNumber];
      console.log(monthsArr[monthNumber]);
}

<form>

<form>
  <fieldset>
    <select id="month_number" onchange="months()">
      <option value="1">1</option>
      <option value="2">2</option>
      <option value="3">3</option>
      <option value="4">4</option>
      <option value="5">5</option>
      <option value="6">6</option>
      <option value="7">7</option>
      <option value="8">8</option>
      <option value="9">9</option>
      <option value="10">10</option>
      <option value="11">11</option>
      <option value="12">12</option>
    </select>
    <input type="text" id="month" value="" size="15" />

>Solution :

You seem to be using it in wrong way.
In switch part the month number should go and in case part you should not be comparing like you are doing. The correct code would be something like:

switch (monthNumber) {
    //January
    case 1: 
     monthText = monthsArr[0];
     break;
   
   // February
   case 2: 
    monthText = monthsArr[1];
    break;

  // ... so on
}

You can see a similar example here: https://www.w3schools.com/js/js_switch.asp

Also note that this problem can be solved in better ways as well. Eg:

monthText = monthsArr[monthNumber - 1]; 
/* Let's say user selects 1, so this statement would evaluate monthNumber - 
 1 first which would be 1 - 1 = 0 and returns monthsArr[0] which is actually January.*/