Search in Select Multi Drop Down List

Search & only display matching options in a Select multi list.

HTML

Search:
<input type="text" size="25" ID="searchFor" onkeyup="searchSelect(this.id,'searchIn')">
<br />
<select size="3" multiple="multiple" name="searchIn" id="searchIn">
    <option value="abc">Option abc</option>
    <option value="123">Option 123</option>
    <option value="abc123">Option abc123</option>
    <option value="xyz">Option xyz</option>
    <option value="789">Option 789</option>
    <option value="xyz789">Option xyz789</option>
</select>

JavaScript

function searchSelect(searchForID, searchInID) {
    var input = document.getElementById(searchFor).value.toLowerCase();
    var output = document.getElementById(searchIn).options;

    for (var i = 0; i < output.length; i++) {
        if (output[i].text.toLowerCase().indexOf(input) < 0) {
            output[i].style.display = "none";
            output[i].setAttribute('style', 'display:none');

        } else {
            output[i].style.display = "";
            output[i].setAttribute('style', 'display:');
        }
    }
}