JSFiddle - React, Tailwind, and code Playground
HTML
<input type="checkbox" name="buy-item" class="checkbox-buy" value="value 1" checked="true">
<input type="checkbox" name="buy-item" class="checkbox-buy" value="value 2">
<input type="checkbox" name="buy-item" class="checkbox-buy" value="value 3">
JavaScript
var a = $(".checkbox-buy:checked").map(function(){
return $(this).val();
}).toArray();
console.log(a)