Using the dataset property

The new dataset property allows access (for both reading and writing) to all data attributes data-* on any element.

<p>Countries:</p>
<ul>
  <li id="C1" onclick="showDetails(this)" data-id="US" data-dial-code="1">USA</li>
  <li id="C2" onclick="showDetails(this)" data-id="CA" data-dial-code="1">Canada</li>
  <li id="C3" onclick="showDetails(this)" data-id="FF" data-dial-code="3">France</li>
</ul>
<button type="button" onclick="correctDetails()">Correct Country Details</button>
<script>
function showDetails(item) {
    var msg = item.innerHTML
            + "\r\nISO ID: " + item.dataset.id
            + "\r\nDial Code: " + item.dataset.dialCode;
    alert(msg);
}

function correctDetails(item) {
    var item = document.getEmementById("C3");
    item.dataset.id = "FR";
    item.dataset.dialCode = "33";
}
</script>

Note: The dataset property is only supported in modern browsers and it's slightly slower than the getAttribute and setAttribute methods which are supported by all browsers.

Using the getAttribute & setAttribute methods

If you want to support the older browsers before HTML5, you can use the getAttribute and setAttribute methods which are used to access any attribute including the data attributes. The two functions in the example above can be written this way:

<script>
function showDetails(item) {
    var msg = item.innerHTML
            + "\r\nISO ID: " + item.getAttribute("data-id")
            + "\r\nDial Code: " + item.getAttribute("data-dial-code");
    alert(msg);
}

function correctDetails(item) {
    var item = document.getEmementById("C3");
    item.setAttribute("id", "FR");
    item.setAttribute("data-dial-code", "33");
}
</script>