selection.datum()

selection.datum([value])

Gets or sets the bound data for each selected element. Unlike selection.data, this method does not compute a join and does not affect indexes or the enter and exit selections.

If a value is specified, sets the element’s bound data to the specified value on all selected elements. If the value is a constant, all elements are given the same datum; otherwise, if the value is a function, then the function is evaluated for each selected element, in order, being passed the current datum (d), the current index (i), and the current group (nodes), with this as the current DOM element. The function is then used to set each element’s new data. A null value will delete the bound data.

If a value is not specified, returns the bound datum for the first (non-null) element in the selection. This is generally useful only if you know the selection contains exactly one element.

This method is useful for accessing HTML5 custom data attributes. For example, given the following elements:

<ul id="list">
  <li data-username="shawnbot">Shawn Allen</li>
  <li data-username="mbostock">Mike Bostock</li>
</ul>

You can expose the custom data attributes by setting each element’s data as the built-in dataset property:

selection.datum(function() { return this.dataset; })