JSFiddle - React, Tailwind, and code Playground

HTML

<select id="multiselect" size="5" multiple>
    <option value="1" selected>Option 1</option>
    <option value="2" selected>Option 2</option>
    <option value="3">Option 3</option>
    <option value="4">Option 4</option>
    <option value="5">Option 5</option>
</select>

JavaScript

$(function(){
    $("#multiselect option").prop("selected", false);
    if ($("#multiselect option:selected").length) {
        alert("FAILED: did not unselect all #multiselect options");
    }
    $("#multiselect option").prop("selected", true);
    if ($("#multiselect option:selected").length != $("#multiselect option").length) {
        alert("FAILED: did not select all #multiselect options");
    }
    $("#multiselect option").prop("selected", false);
    if ($("#multiselect option:selected").length) {
        alert("FAILED: did not unselect all #multiselect options");
    }
});