JSFiddle - React, Tailwind, and code Playground

HTML

<select id="myselect" multiple="multiple" name="myselect">
	<option value="1">Option 1</option>
	<option value="2">Option 2</option>
	<option value="3">Option 3</option>
	<option value="4">Option 4</option>
	<option value="5">Option 5</option>
	<option value="6">Option 6</option>
</select>

<br />
<a href="javascript:;" onclick="selectAll();">Select All</a> | <a href="javascript:;" onclick="deSelectAll();">De-Select All</a>

JavaScript

// coded by Alauddin Ansari (http://google.com/+AlauddinAnsari)
// Tested in Firefox, Chrome, IE

var select_ids = [];
$(document).ready(function(e) {
	$('select#myselect option').each(function(index, element) {
		select_ids.push($(this).val());
	})
});

function selectAll()
{
	$('select#myselect').val(select_ids);
}

function deSelectAll()
{
	$('select#myselect').val('');
}