JSFiddle - React, Tailwind, and code Playground

HTML

<select id="attributeNames" name="attributeNames" size="5" multiple="multiple" onchange="getUniqueValues.call(this)">
    <option value="Apple"> Apple </option>
    <option value="Mango"> Mango </option>
    <option value="Orange"> Orange </option>                                            <option value="Banana"> Banana </option>
</select>

JavaScript

function getUniqueValues(){
    for(var i = 0; i < this.options.length; i++){
        if(this.options[i].selected)
            alert(this.options[i].value);
    }
}