JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" name="example[]" value="288" checked/>
<input type="checkbox" name="example[]" value="289" checked/>
<input type="checkbox" name="example[]" value="290" />
<input type="checkbox" name="example1[]" value="289" checked/>

JavaScript

var output = jQuery.map($(':checkbox[name=example\\[\\]]:checked'), function (n, i) {
    return n.value;
}).join(',');

alert(output);