selection.attr()

selection.attr(name[, value])

If a value is specified, sets the attribute with the specified name to the specified value on the selected elements and returns this selection. If the value is a constant, all elements are given the same attribute value; otherwise, if the value is a function, 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’s return value is then used to set each element’s attribute. A null value will remove the specified attribute.

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

The specified name may have a namespace prefix, such as xlink:href to specify the href attribute in the XLink namespace. See namespaces for the map of supported namespaces; additional namespaces can be registered by adding to the map.