d3.group, d3.groupsum, etc. about d3-array

[mbostock]

These seem to work well:

function nest(values, ...keys) {
  return (function regroup(values, i) {
    if (i >= keys.length) return values;
    const map = group(values, keys[i]);
    return new Map(Array.from(map, ([k, v]) => [k, regroup(v, i + 1)]));
  })(values, 0);
}

function rollup(values, reduce, ...keys) {
  return (function regroup(values, i) {
    if (i >= keys.length) return reduce(values);
    const map = group(values, keys[i]);
    return new Map(Array.from(map, ([k, v]) => [k, regroup(v, i + 1)]));
  })(values, 0);
}

[mbostock]

I think the path forward here is to deprecate d3-collection entirely and adopt ES collections (Map, Set).

The proposed d3.group works well for single-level grouping, but it would be nice to have something analogous to d3.nest for multi-level grouping. Here’s an example two-level nested group:

new Map(Array.from(
  group(data, d => d.name),
  ([name, values]) => [name, group(values, d => d.date)]
))

And here’s an example with multi-part keys using the bleeding edge array.flatMap:

new Map(Array.from(group(data, d => d.name)).flatMap(([name, data]) => {
  return Array.from(group(data, d => d.date), ([date, data]) => {
    return [[name, date], data];
  });
}))

Perhaps there’s a d3.nest(data, ...keys) function?

[mbostock]

Some notes here: https://beta.observablehq.com/@mbostock/nested-groups