JSFiddle - React, Tailwind, and code Playground

HTML

<select multiple>
	<option val="1">1</option>
	<option val="2">2</option>
	<option val="3">3</option>
	<option val="4">4</option>
	<option val="5">5</option>
	<option val="6">6</option>
</select>

CSS

select option {
    padding:3px 15px;
}

JavaScript

// Underscore.js isArray()
var isArray = Array.isArray || function(obj) {
    return toString.call(obj) == '[object Array]';
};

$('select').on('change', function(e) {
    var $this = $(this),
        values = $this.val(),
        summ = 0;
    if (isArray(values)) {
        for (var i = 0, length = values.length; i < length; i++) {
            summ += parseInt(values[i], 10);
        }
    } else {
        summ = values;
    }
    alert(summ); // <- result
}).triggerHandler('change'); // 'triggerHandler'! Not 'change()' or "trigger('change')"!