// highlight text in an input element
element.select();
// highlight a portion of an element
// element.setSelectionRange(start, end, ?direction)
// start, end - start and end indices of the new selection
// direction (optional) - "forward", "backward", or "none" (default)
element.setSelectionRange(3, 5);<select id="box1" onChange="myNewFunction(this);">
<option value="98">dog</option>
<option value="7122">cat</option>
<option value="142">bird</option>
</select>
<script>
function myNewFunction(sel) {
alert(sel.options[sel.selectedIndex].text);
}
</script>// Select text in element 'elm'
const elm = document.getElementById('elm');
// Clear any current selection
const selection = window.getSelection();
selection.removeAllRanges();
// Select paragraph
const range = document.createRange();
range.selectNodeContents(elm);
selection.addRange(range);