[javascript] how to dynamically add options to an existing select in vanilla javascript

I'd like to add option to a select dynamically using plain javascript. Everything I could find involves JQuery or tries to create the select dynamically as well. The closest thing I could find was Dynamically add input type select with options in Javascript which does the latter and was the only I found that doesn't involve JQuery. Although I did try and use it like so:

daySelect = document.getElementById('daySelect');
daySelect.innerHTML += "<option'>Hello world</option'>";
alert(daySelect.innerHTML)

After I did this there was no change to the select and the alert gave me

HELLO WORLD</option'>

I apologize if this is simple but I'm very new at javascript and web programming in general. Thank you for any assistance.

EDIT: So I tried the given suggestions like so.

daySelect = document.getElementById('daySelect');
myOption = document.createElement("option");
myOption.text = "Hello World";
myOption.value = "Hello World";
daySelect.appendChild(myOption);

This has not changed the select in the page at all. Any idea why? And I did check that the code was being run with an alert.

EDIT: The idea did work, it just turned out that it wasn't displaying a value in the dropdown. I don't know why but I can figure that one out I think.

I guess something like this would do the job.

var option = document.createElement("option");
option.text = "Text";
option.value = "myvalue";
var select = document.getElementById("daySelect");
select.appendChild(option);

.add() also works.

var daySelect = document.getElementById("myDaySelect");
var myOption = document.createElement("option");
myOption.text = "test";
myOption.value = "value";
daySelect.add(option);

W3 School - try

Use the document.createElement function and then add it as a child of your select.

var newOption = document.createElement("option");
newOption.text = 'the options text';
newOption.value = 'some value if you want it';
daySelect.appendChild(newOption);

The simplest way is:

selectElement.add(new Option('Text', 'value'));

Yes, that simple. And it works even in IE8. And has other optional parameters.

See docs:

• HTMLSelect?Element?.add(item[, before])
  
• new Option(text, value, defaultSelected, selected);

Try this;

   var data = "";
   data = "<option value = Some value> Some Option </option>";         
   options = [];
   options.push(data);
   select = document.getElementById("drop_down_id");
   select.innerHTML = optionsHTML.join('\n');