How can I keep a reference to individual items when summarizing in a groupBy?

[pbeshai]

Sometimes it's useful to keep track of individual items after summarizing.

For example, if we want to go from this input:

[
  { "group": "a", "name": "n0", "value": 5 },
  { "group": "a", "name": "n1", "value": 10 },
  { "group": "b", "name": "n2", "value": 12 },
  { "group": "c", "name": "n3", "value": 35 },
  { "group": "c", "name": "n4", "value": 55 }
]

To this output:

[
  {
    "group": "a",
    "total": 15,
    "items": [
      {"group": "a", "name": "n0", "value": 5},
      {"group": "a", "name": "n1", "value": 10}
    ]
  },
  {
    "group": "b",
    "total": 12,
    "items": [{"group": "b", "name": "n2", "value": 12}]
  },
  {
    "group": "c",
    "total": 90,
    "items": [
      {"group": "c", "name": "n3", "value": 35},
      {"group": "c", "name": "n4", "value": 55}
    ]
  }
]

Is it possible to do this with summarize?

[pbeshai]

This can be accomplished by adding an identity summarizer: items => items, which adds a key with the array of grouped items as its value.

For example:

tidy(
  data,
  groupBy(
    ['group'],
    [summarize({ total: sum('value'), items: (items) => items })]
  )
)

Try it out here on Observable