JSFiddle - React, Tailwind, and code Playground

HTML

<div data-foo='1'></div>
<div data-foo='2'></div>
<div data-foo='3'></div>
<div data-foo='4'></div>

JavaScript

display("Showing original values:");
jQuery("div").attr("data-foo", function(index, value) {
    display("" + index + ": " + value);
});
display("Modifying the odd ones.");
jQuery("div").attr("data-foo", function(index, value) {
    if (value % 2 == 1) {
        return -value;
    }
});
display("Showing updated values:");
jQuery("div").attr("data-foo", function(index, value) {
    display("" + index + ": " + value);
});

function display(msg) {
    $("<p>").html(msg).appendTo(document.body);
}