The HTMLSelectElement.selectedIndex is a long that reflects the index of the first selected <option> element. The value -1 indicates that no element is selected.
Syntax
var index = selectElem.selectedIndex;
selectElem.selectedIndex = index;
Example
HTML
<p id="p">selectedIndex: 0</p> <select id="select"> <option selected>Option A</option> <option>Option B</option> <option>Option C</option> <option>Option D</option> <option>Option E</option> </select>
JavaScript
var selectElem = document.getElementById('select')
var pElem = document.getElementById('p')
// When a new <option> is selected
selectElem.addEventListener('change', function() {
  var index = selectElem.selectedIndex;
  // Add that data to the <p>
  pElem.innerHTML = 'selectedIndex: ' + index;
})
Specifications
| Specification | Status | Comment | 
|---|---|---|
| WHATWG HTML Living Standard The definition of 'HTMLSelectElement' in that specification. | Living Standard | No change since the latest snapshot, HTML5. | 
| HTML5 The definition of 'HTMLSelectElement' in that specification. | Recommendation | Initial definition, snapshot of WHATWG HTML Living Standard. | 
Browser compatibility
| Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) | 
|---|---|---|---|---|---|
| Basic support | ? | ? | ? | ? | ? | 
| Feature | Android | Chrome | Firefox Mobile (Gecko) | Firefox OS | IE Phone | Opera Mobile | Safari Mobile | 
|---|---|---|---|---|---|---|---|
| Basic support | ? | ? | ? | ? | ? | ? | ? |