What’s a clean way to map several keys to the same value in JavaScript?

You can use an array or any other object as a Map key, but Map compares objects by identity, not by their contents. So a fresh [1, 2, 3] lookup will not match a different array with the same numbers. That works only if you keep and reuse the original array object.

If each individual item should find the same result, you don’t need a tuple key at all. Loop through each group and add every member as its own key in a Map. In the example, 1, 2, and 3 would each map to the same value.

Use for...of when iterating arrays. A for...in loop gives you property names, which is not what you want here.

If you need to look up a whole combination, a nested Map is often clearer than turning an array into a string. Store the first key in the outer Map, the next key in an inner Map, and continue for however many parts the combination has. That avoids object-identity surprises and makes each level explicit.

For a small, well-defined set of primitive values, serializing the tuple with JSON.stringify can be a practical key. Just serialize both when storing and when looking up. Order matters, so [1, 2] and [2, 1] remain different, and serialization is not a universal solution for arbitrary values or objects.

One thing to pin down first: are you mapping every member of a group to one value, or mapping the entire combination to a value? Those are different data structures. For the first, use one Map entry per member. For the second, use nested maps or a carefully defined composite key.