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In vanilla-JS, assuming spanEl is a reference to the DOM node

spanEl.dataset['complexKey'] 

will work using the camelCase notation (see http://jsbin.com/oduguw/3/edit) when your data attribute contains hypens (-) and also

spanEl.getAttribute('data-complex-key')

will work fine as you already noticed. As a side note, in jQuery you can access to that data attribute with

$(spanEl).data("complex-key")

In vanilla-JS, assuming spanEl is a reference to the DOM node

spanEl.dataset.complexKey

will work using the camelCase notation (see http://jsbin.com/oduguw/3/edit) when your data attribute contains hypens (-) and also

spanEl.getAttribute('data-complex-key')

will work fine as you already noticed. As a side note, in jQuery you can access to that data attribute with

$(spanEl).data("complex-key")

in jQuery you can access to that data attribute with

$(spanEl).data("complex-key")

(assuming spanEl is a reference to the DOM node). In straight javascript

spanEl.getAttribute('data-complex-key')

will work fine as you already noticed and also

spanEl.dataset['complexKey'] 

will work using the camelCase notation (see http://jsbin.com/oduguw/3/edit) when your data attribute contains hypens (-)

In vanilla-JS, assuming spanEl is a reference to the DOM node

spanEl.dataset['complexKey'] 

will work using the camelCase notation (see http://jsbin.com/oduguw/3/edit) when your data attribute contains hypens (-) and also

spanEl.getAttribute('data-complex-key')

will work fine as you already noticed. As a side note, in jQuery you can access to that data attribute with

$(spanEl).data("complex-key")

in jQuery you can access to that data attribute with

$(spanEl).data("complex-key")

(assuming spanEl is a reference to the DOM node). In straight javascript

spanEl.getAttribute('data-complex-key')

will work fine as you already noticed and also

spanEl.dataset['complexKey'] 

using the camelCase notation (see http://jsbin.com/oduguw/3/edit)

in jQuery you can access to that data attribute with

$(spanEl).data("complex-key")

(assuming spanEl is a reference to the DOM node). In straight javascript

spanEl.getAttribute('data-complex-key')

will work fine as you already noticed and also

spanEl.dataset['complexKey'] 

will work using the camelCase notation (see http://jsbin.com/oduguw/3/edit) when your data attribute contains hypens (-)