I'm probably on the mild side of abusing it, I've never seen another developer on my team use it, but I also include self-describing variables for the values. If the reducer function itself is non-trivial, I've give it a name so the final product may look like:

    const idNameMap = itemList.reduce(toIdNameMap)

It should be intuitive that itemList is a list of objects familiar to the context of the code, and the result will be like:

    [{"id1":"Billy"}, etc.