Issue resetting total value with select and input fields using JS

Change the onchange() event handler of the select list in the HTML to:

onchange="getTax(this.value);
getTicketCost(ticketTime.value);"

and you also need to initialize tax in the Javascript in case the user selects the ticket type before selecting a state (or if they come from Alabama!).

var tax = 0.08;

//create array to hold values for dropdown
var states = [
      'Alabama', 'Alaska', 'Arizona', 'Arkansas', 'California', 'Colorado', 'Connecticut',
      'Delaware', 'Florida', 'Georgia', 'Hawaii', 'Idaho', 'Illinois', 'Indiana', 'Iowa', 'Kansas',
      'Kentucky', 'Louisiana', 'Maine', 'Maryland', 'Massachusetts', 'Michigan', 'Minnesota', 
      'Mississippi', 'Missouri', 'Montana', 'Nebraska', 'Nevada', 'New Hampshire', 'New Jersey', 
      'New Mexico', 'New York', 'North Carolina', 'North Dakota', 'Ohio', 'Oklahoma', 'Oregon',
      'Pennsylvania', 'Rhode Island', 'South Carolina', 'South Dakota', 'Tennessee', 'Texas', 'Utah', 
      'Vermont', 'Virginia', 'Washington', 'West Virginia', 'Wisconsin', 'Wyoming'
      ];

var selectState = document.getElementById('selectState');
var contents;
var tax = 0.08;

//for loop append option to select tag
for (let i = 0; i < states.length; i++) {
   contents += '<option>' + states[i] + '</option>';
}

selectState.innerHTML = contents;

//function to get tax per state
 function getTax(state) {
   if (state === 'California') {
      tax = 0.06;
   } else {
       tax = 0.08;
   }
   return tax;
 }

 //function to calculate the cost of ticket
  function getTicketCost(ticketTime) {
    var morningTicket = 20;
    var afternoonTicket = 25;
    var nightTicket = 15;
    var ticketCost = 'Total price of ticket ';
    
    if (ticketTime === 'Morning') {
          totalCost = morningTicket * tax + morningTicket;
    } else if (ticketTime === 'Afternoon') {
          totalCost = afternoonTicket * tax + afternoonTicket;
    } else {
          totalCost = nightTicket * tax + nightTicket;
    }

    document.getElementById('priceOfTicket').innerHTML =
          ticketCost + '$' + totalCost;
  }

<form action="">
      <label for="states">Choose your state:</label>
      <select
        name="states"
        id="selectState"
        onchange="getTax(this.value);
        getTicketCost(ticketTime.value);"
      ></select
      ><br />
      <br />

      <label for="ticketTime">Select your ticket:</label>
      <br />
      <input
        type="radio"
        name="ticketTime"
        onclick="getTicketCost(this.value)"
        value="Morning"
      />Morning<br />
      <input
        type="radio"
        name="ticketTime"
        onclick="getTicketCost(this.value)"
        value="Afternoon"
      />Afternoon<br />
      <input
        type="radio"
        name="ticketTime"
        onclick="getTicketCost(this.value)"
        value="Night"
      />Night<br />
    </form>
    <p id="priceOfTicket"></p>

  

the problem is that Tax doesn't update when you change the state! You can use this code, I test it and it works well.

Html:

<form action="">
    <label for="states">Choose your state:</label>
    <select name="states" id="selectState" onchange="getTicketCost()"></select><br />
    <br />
    <label for="ticketTime">Select your ticket:</label>
    <br />
    <input type="radio" name="ticketTime" onclick="getTicketCost()" value="Morning" />Morning<br />
    <input type="radio" name="ticketTime" onclick="getTicketCost()" value="Afternoon" />Afternoon<br />
    <input type="radio" name="ticketTime" onclick="getTicketCost()" value="Night" />Night<br />
</form>
<p id="priceOfTicket"></p>

JS:

//create array to hold values for dropdown
    var states = [
        'Alabama', 'Alaska', 'Arizona', 'Arkansas', 'California', 'Colorado', 'Connecticut',
        'Delaware', 'Florida', 'Georgia', 'Hawaii', 'Idaho', 'Illinois', 'Indiana', 'Iowa', 'Kansas',
        'Kentucky', 'Louisiana', 'Maine', 'Maryland', 'Massachusetts', 'Michigan', 'Minnesota',
        'Mississippi', 'Missouri', 'Montana', 'Nebraska', 'Nevada', 'New Hampshire', 'New Jersey',
        'New Mexico', 'New York', 'North Carolina', 'North Dakota', 'Ohio', 'Oklahoma', 'Oregon',
        'Pennsylvania', 'Rhode Island', 'South Carolina', 'South Dakota', 'Tennessee', 'Texas', 'Utah',
        'Vermont', 'Virginia', 'Washington', 'West Virginia', 'Wisconsin', 'Wyoming'
    ];

    var selectState = document.getElementById('selectState');
    var contents;

    //for loop append option to select tag
    for (let i = 0; i < states.length; i++) {
        contents += '<option>' + states[i] + '</option>';
    }

    selectState.innerHTML = contents;

    //function to get tax per state
    function getTax(state) {
        if (state === 'California') {
            tax = 0.06;
        } else {
            tax = 0.08;
        }
        return tax;
    }

    function getRadioButtonValue() {
        var radios = document.getElementsByName('ticketTime');
        for (var i = 0, length = radios.length; i < length; i++) {
            if (radios[i].checked) {
                return radios[i].value;
            }
        }
    }
    
    //function to calculate the cost of ticket
    function getTicketCost() {
        var morningTicket = 20;
        var afternoonTicket = 25;
        var nightTicket = 15;
        var ticketCost = 'Total price of ticket ';
        var currentTax = getTax(document.getElementById("selectState").value);
        var ticketTime = getRadioButtonValue();
        if (ticketTime == undefined) {
            return;
        }
        if (ticketTime === 'Morning') {
            totalCost = morningTicket * currentTax + morningTicket;
        } else if (ticketTime === 'Afternoon') {
            totalCost = afternoonTicket * currentTax + afternoonTicket;
        } else {
            totalCost = nightTicket * currentTax + nightTicket;
        }
        document.getElementById('priceOfTicket').innerHTML =
            ticketCost + '$' + totalCost;
    }