如何在没有jQuery的情况下检查select字段是否具有某个选项值

Question

How do you check if an option with a particular value exists in a select field?

<select>
  <option value="o1">Option 1</option>
  <option value="o2">Option 2</option>
</select>

If select has value "o1" = true

If select has value "o4" = false

Answer 1

If you'll add an id field to the select, say id="selector" you can do:

var x = document.getElementById("selector").options;
for(var i=0; i<x.length; i++){
    if (x[i].value == "o1"){
    ...

etc.

Answer 2

You can loop over the options and check their values:

var select = document.getElementById('selectID') || 
             document.forms[formName or index].elements[selectName or index];
var options = select.options

for (var i=0, iLen=options.length; i<iLen; i++) {
  if (options[i].value == 'foo') {
    // found option with value 'foo'
  }
}