jQuery add blank option to top of list and make se

问题:

So I have a dropdown list

<select id="theSelectId">
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

This is what I would like

<select id="theSelectId">
  <option value="" selected="selected"></option>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

Trying to add a blank option before and set it to this, want to enforce the user to select a value from the original list but would like it to be blank when they see the option they have to choose.

Trying this but not working

// Add blank option
var blankOption = {optVal : ''};
$.each(blankOption, function(optVal, text) {
   $('<option></option>').val(optVal).html(text).preprendTo('#theSelectId');
});

and I have tried this but is clears out the other values

$('#theSelectId option').prependTo('<option value=""></option>');

回答1:

This worked:

$("#theSelectId").prepend("<option value='' selected='selected'></option>");

Firebug Output:

<select id="theSelectId">
  <option selected="selected" value=""/>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

You could also use .prependTo if you wanted to reverse the order:

​$("<option>", { value: '', selected: true }).prependTo("#theSelectId");​​​​​​​​​​​

回答2:

Solution native Javascript :

document.getElementById("theSelectId").insertBefore(new Option('', ''), document.getElementById("theSelectId").firstChild);

example : http://codepen.io/anon/pen/GprybL