将用户输入添加到数组并更新选项列表

// find elements
var banner = $("#banner-message")
var button = $("button")

var select = document.getElementById("selectFood");
var options = ["Pizza", "Hambruger", "Salad"];

for (var i = 0; i < options.length; i++) {
  var opt = options[i];
  var el = document.createElement("option");
  el.textContent = opt;
  el.value = opt;
  select.appendChild(el);
}

function foodInput() {
  boxvalue = document.getElementById('addedFood').value;
  options.push(boxvalue);
}

function addFunction() {
  var x = document.getElementById("myDIV");
  if (x.style.display === "block") {
    x.style.display = "none";
  } else {
    x.style.display = "block";
  }
}

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#banner-message {
  background: #778899;
  border-radius: 4px;
  padding: 20px;
  font-size: 25px;
  text-align: center;
  transition: all 0.2s;
  margin: 0 auto;
  width: 300px;
}


button {
  background: #A52A2A;
  border: none;
  border-radius: 5px;
  padding: 8px 14px;
  font-size: 15px;
  color: #fff;
}

.addButton {
  background: #A52A2A;
  color: white;
}

#banner-message.alt {
  background: #0084ff;
  color: #fff;
  margin-top: 40px;
  width: 200px;
}

#banner-message.alt button {
  background: #fff;
  color: #000;
}

.hide {
  display: none;
}

.show {
  display: block;
}

.demo {
  background: #A52A2A;
  border: none;
  border-radius: 5px;
  padding: 8px 14px;
  font-size: 15px;
  color: #fff;
}

#myDIV {
  background: #778899;
  padding: 20px;
  font-size: 25px;
  text-align: center;
  transition: all 0.2s;
  margin: 0 auto;
  width: 300px;
  display: none;
}

input[type=submit] {
  background: #A52A2A;
  border: none;
  border-radius: 5px;
  padding: 8px 14px;
  font-size: 15px;
  color: #fff;

}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="banner-message">
  <p>My favorite food</p>
  <select id="selectFood">
    <option>Select food</option>
  </select>
  <button>Choose</button>
  <br>
  <button id="addButton" onclick="addFunction();">
    Add new food
  </button>
</div>

<div id="myDIV">
  <form onsubmit="return foodInput()">
    <input type="text" id="addedFood" placeholder="Type food">
    <input type="submit" value="Submit" onclick="addFunction()">
  </form>
</div>