Get list of data-* attributes using javascript / jQuery

Actually, if you're working with jQuery, as of version 1.4.3 1.4.4 (because of the bug as mentioned in the comments below), data-* attributes are supported through .data():

As of jQuery 1.4.3 HTML 5 data- attributes will be automatically pulled in to jQuery's data object.

Note that strings are left intact while JavaScript values are converted to their associated value (this includes booleans, numbers, objects, arrays, and null). The data- attributes are pulled in the first time the data property is accessed and then are no longer accessed or mutated (all data values are then stored internally in jQuery).

The jQuery.fn.data function will return all of the data- attribute inside an object as key-value pairs, with the key being the part of the attribute name after data- and the value being the value of that attribute after being converted following the rules stated above.

I've also created a simple demo if that doesn't convince you: http://jsfiddle.net/yijiang/WVfSg/

A pure JavaScript solution ought to be offered as well, as the solution is not difficult:

var a = [].filter.call(el.attributes, function(at) { return /^data-/.test(at.name); });

This gives an array of attribute objects, which have name and value properties:

if (a.length) {
    var firstAttributeName = a[0].name;
    var firstAttributeValue = a[0].value;
}

Edit: To take it a step further, you can get a dictionary by iterating the attributes and populating a data object:

var data = {};
[].forEach.call(el.attributes, function(attr) {
    if (/^data-/.test(attr.name)) {
        var camelCaseName = attr.name.substr(5).replace(/-(.)/g, function ($0, $1) {
            return $1.toUpperCase();
        });
        data[camelCaseName] = attr.value;
    }
});

You could then access the value of, for example, data-my-value="2" as data.myValue;

jsfiddle.net/3KFYf/33

Edit: If you wanted to set data attributes on your element programmatically from an object, you could:

Object.keys(data).forEach(function(key) {
    var attrName = "data-" + key.replace(/[A-Z]/g, function($0) {
        return "-" + $0.toLowerCase();
    });
    el.setAttribute(attrName, data[key]);
});

jsfiddle.net/3KFYf/34

EDIT: If you are using babel or TypeScript, or coding only for es6 browsers, this is a nice place to use es6 arrow functions, and shorten the code a bit:

var a = [].filter.call(el.attributes, at => /^data-/.test(at.name));

Have a look here:

If the browser also supports the HTML5 JavaScript API, you should be able to get the data with:

var attributes = element.dataset

or

var cat = element.dataset.cat

Oh, but I also read:

Unfortunately, the new dataset property has not yet been implemented in any browser, so in the meantime it’s best to use getAttribute and setAttribute as demonstrated earlier.

It is from May 2010.

If you use jQuery anyway, you might want to have a look at the customdata plugin. I have no experience with it though.

As mentioned above modern browsers have the The HTMLElement.dataset API.
That API gives you a DOMStringMap, and you can retrieve the list of data-* attributes simply doing:

var dataset = el.dataset; // as you asked in the question

you can also retrieve a array with the data- property's key names like

var data = Object.keys(el.dataset);

or map its values by

Object.keys(el.dataset).map(function(key){ return el.dataset[key];});
// or the ES6 way: Object.keys(el.dataset).map(key=>{ return el.dataset[key];});

and like this you can iterate those and use them without the need of filtering between all attributes of the element like we needed to do before.

or convert gilly3's excellent answer to a jQuery method:

$.fn.info = function () {
    var data = {};
    [].forEach.call(this.get(0).attributes, function (attr) {
        if (/^data-/.test(attr.name)) {
            var camelCaseName = attr.name.substr(5).replace(/-(.)/g, function ($0, $1) {
                return $1.toUpperCase();
            });
            data[camelCaseName] = attr.value;
        }
    });
    return data;
}

Using: $('.foo').info();