Answers for "how to can i get custom data attribute value in javascript"

js get data attribute

var element = document.querySelector('.element');
var dataAttribute = element.getAttribute('data-name');
// replace "data-name" with your data attribute name

console.log(dataAttribute);

dataset js

for javascript : 
webappData.getAttribute('data-id');
document.querySelector('#elementId').dataset.id = 'value';

for jquery : 
$(element).attr('data-key', 'value');

how to can i get custom data attribute value in javascript

example <button data-keyname = "a'></button>

let variable = element.dataset.keyname;
or
let varibale = element.['keyname'];
or
let variable = element.getAttribute('data-keyname');

data attribute html

<!-- data-* attritubes can be used on any html element -->
<div data-my-custom-data="true"></div>