Js Validation If Dropdown Value Selected Or Not

I am trying to write a validation block inside my JS to check if the user has selected a value or not and pop up a message if the value is not selected. function validate(form) {

Solution 1:

If no option is selected, the select element's selectedIndex will be -1. It's recommended that one option is always set to selected, that way you know what the default selected option is (user agents may make the first option selected by default if you don't).

So the test:

if (form.dropdown.selectedIndex ==  0 )  {  

will only be true if the first option has the selected attribute. So either test against -1 or make the first option selected by default, e.g.

<select name="dropdown" ...>
  <option selected value="default">Please select an option
  ...
</select>