First, always coerce the tween target value (the value of the style or attribute
at the end of the transition) to a string. This guarantees that the correct
interpolator is chosen, and avoids a null dereference.
Second, default color channels to zero rather than undefined, so as to avoid NaN
channel values if there is an error parsing the color. This can occur, say, if
the background color of a div is unset and defaults to "rgba(0, 0, 0, 0)".
The data join is now specified as a single function of data, as with all other
properties. This allows the key to be computed on the previously-bound data,
rather than requiring the key to be serialized into the DOM (say, as an
attribute). In the case that there is no previously-bound data, it is still
possible to access the associated node as the `this` context.
The `enter` operator no longer performs an append. For symmetry with the `exit`
operator, you must call `append` after obtaining the entering selection. This
requires a tiny bit more code, but should make the code more clear. Also, it
provides an opportunity to use a different instantiation operator, such as the
new `insert` operator. This takes a second argument, which is a selector for the
insert-before reference element. For example, the selector ":first-child" will
prepend nodes.
The `empty` operator allows you to query whether a selection is empty (i.e.,
contains zero matching nodes).
It's a lot more predictable if we always use selectAll to determine the parent
node for a given group, rather than trying to infer parent nodes per node
during select.
Jason Davies
Michael Bostock