Count items by a computed key

Filter sidebars show counts: "Open (12)", "Closed (3)". Those numbers come from tallying a list by some derived property rather than storing them separately.

Implement countBy(items, keyFn). For each item, call keyFn(item) to get its bucket, then return a plain object mapping each bucket to the number of items in it. Because these are object properties, the returned keys are strings: a keyFn returning the number 1 produces the key "1", and one returning true produces "true". Buckets with no items never appear. An empty input array returns an empty object.

What it has to do

• Return a plain object mapping each key produced by keyFn to its count.
• Keys are the string form of whatever keyFn returns, so numbers and booleans become strings.
• Only keys that actually occur appear in the result.
• Return {} for an empty input array.

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