I used data-mykey_1 as key, with a value ‘abcd’ . But it was showing undefined only.
It took me some time to figure out , and then I removed the underscore to data-mykey1, to make it work. I am not sure if it’s an issue or intentional though. 
Underscores are ok..
- The name of a custom data attribute in HTML begins with
data-. It must contain only letters, numbers and the following characters: dash (-), dot (.), colon (:), underscore (_) -- but NOT any ASCII capital letters (AtoZ).
And works fine my end

let someElement_ = hypeDocument.getElementById('someElement')
console.log(someElement_.dataset.mykey_1);
1 Like