Build your own groupBy

Almost every list you render is grouped by something: orders by status, students by year, commits by author. Write the helper once and you stop hand-rolling it.

Implement groupBy(items, keyFn). It takes an array and a function that returns the grouping key for an item, and returns an object mapping each key to the array of items that produced it.

What it has to do

• Preserve the original order of items inside each group.
• Handle an empty array by returning an empty object.
• Accept keys that are numbers as well as strings.
• Do not mutate the input array.

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