JSFiddle - React, Tailwind, and code Playground

HTML

<form>
<input class="test" type="text" />
</form>

JavaScript

function commaSeparateNumber(val){
    val = val.replace(',', '');
    var array = val.split('');
    var index = -3;
    while (array.length + index > 0) {
        array.splice(index, 0, ',');
        // Decrement by 4 since we just added another unit to the array.
        index -= 4;
    }
    return array.join('');
};    

$(document).on('keyup', '.test', function() {
    var value = $(this).val();
    value = commaSeparateNumber(value);
    $(this).val(value);
});