Why does d3.nest.map prepend a prefix?

[dayvidwhy]

Hi there 👋

We're long time users of d3 and recently looked at upgrading from d3 v3.5.8.

C3 have bumped their dependency on this project so we're wanting to make use of the latest d3 if we're going to update.

One thing I noticed right away is that since quite a while ago you introduced a prefix to keys when using d3.map;

https://github.com/d3/d3-collection/blame/5aad1d67f264c0b3b6a9f3e9833021497f03beeb/src/map.js#L1

Some very common code from us might look like the following;

let nestedData = d3.nest()
  .key((b) => b["Name"])
  .rollup((b) => {
    // return some aggregation on buckets of data by name
  })
  .map(data);

let names = Object.keys(nestedData);

So now in getting all of the unique "Name" elements we now get, for example ["$David", "$Jason"] if that makes sense, instead of ["David", "Jason"].

Now I'm not saying there's anything wrong with this, I imagine this update was part of a breaking change version bump, I'm just curious as to the reasoning?

Also I noticed that you export the prefix, did you have a snippet of code as an example of setting the prefix to nothing, if that's possible?

[dayvidwhy]

@mbostock if you get a chance would you be able to reply to this?

[mbostock]

http://2ality.com/2012/01/objects-as-maps.html

You should use ES Map instead of Object.

[dayvidwhy]

@mbostock correct me if I'm wrong, but you're asking me to just splice off the $ from the start of the keys?

edit:
I just saw this line;
https://github.com/d3/d3-collection/blame/5aad1d67f264c0b3b6a9f3e9833021497f03beeb/src/map.js#L26
Sorry I wasn't familiar with the new data structure, thanks for the information that's really interesting.

[mbostock]

No, you shouldn’t be iterating over the properties on a d3.map instance, as those are private; you should be using map.keys.