The data function confuses a lot of people, it’s not just you. 🙂
data manages jQuery’s internal data object for the element, not data-* attributes. data only uses data-* attributes to set initial values, and more, it guesses at what type you want those to be based on what they look like (so something that looks like a number is converted to a number; something that looks like JSON gets converted to an object). The data method never sets data-* attributes on elements, it only sets the data on its internal data object. That means the two (the internal data object and the attribute) get out of sync:
const t = $("#target");
let value;
// Getting the attribute always gets a string
value = t.attr("data-height");
console.log(`${value} (${typeof value})`); // 1 (string)
// Using `.data`, jQuery will guess that because the attribute looks like a number,
// you want it converted to a number
value = t.data("height");
console.log(`${value} (${typeof value})`); // 1 (number)
// `data` only sets the internal data object properties, not the attribute...
t.data("height", 2);
// ...so the attribute still has `"1"`
value = t.attr("data-height");
console.log(`${value} (${typeof value})`); // 1 (string)
// ...even though the data object has 2
value = t.data("height");
console.log(`${value} (${typeof value})`); // 2 (number)<div id="target" data-height="1"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>If you want to actually set a data-* attribute, use attr:
$(this).attr("data-height", h);
const t = $("#target");
let value;
value = t.attr("data-height");
console.log(`${value} (${typeof value})`); // 1 (string)
// `attr` converts whatever you give it to string
t.attr("data-height", 2);
value = t.attr("data-height");
console.log(`${value} (${typeof value})`); // 2 (string)<div id="target" data-height="1"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>But if you only want this information for future use, data is fine (assuming you’re okay with its automatic type conversion), just don’t expect to see it in the DOM inspector, because jQuery doesn’t write this information to the DOM.