Undocumented third argument to event listener callbacks in v3?

[Anna Whitney]

I am in the process of updating some code from D3 v3 to v4, and the original v3 code does something like this:

selection.on('mouseenter', function(datum, unused, sIndex) {

   // more code that uses datum and sIndex

})

I can't figure out how to replicate the usage of the mysterious "sIndex" in D3 v4, since the v3 docs say event listener callbacks receive two arguments: the datum and the element index. What is this number being passed as the third argument, and how do I get at it in v4?

[Anna Whitney]

In case anyone else is wondering, this is the group index (http://stackoverflow.com/questions/32507388/d3-on-sends-3-arguments-to-a-callback-function).

[Mike Tahani]

The group index is no longer exposed in v4 (the third argument is the group itself). Instead, you can "push that parent data down into the child data, replicating it for each child. Or you can use selection.each and create a closure that exposes parent and child data simultaneously" (source).

Bostock includes an example in the above Github issue that illustrates that concept.

MT

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