Probably I'm missing something as I don't know much this part, but I don't understand this. In this condition, you're checking that rootNode.n is an array and that each is element has an id property that I guess is a proxy for checking that each element is a node. So this part:

// in case when all the results are nodes we can ignore wrapper node
// and display only results for better UI/UX.

is clear. The part that I don't understand is:

// for values (strings, ints, bool) we have to keep the wrapping node
// otherwise it's not a tree and uast-viewer wouldn't be able to display it

Shouldn't you return rootNode.n if this rootNode.n.every(c => c.id) it's not true?

if (Array.isArray(rootNode.n)) {
    if (rootNode.n.every(c => c.id)) {
        return rootNode.n.map(c => c.id);
    } else {
        return rootNode.n;
    }
}

this function returns root ids, not nodes.
If all the results are nodes it would return ids of them.
Otherwise, it would return null and default root id (of the wrapper) would be used.

oh ok! Thanks for the explanation!

Is it possible that c.id is 0, making it false? Would it be safer something like every(c => c.id !== undefined)?

no, it's not possible. When we transform uast to flat json indexing starts with 1.

applied the suggestion in case the transformer logic would change in the future. 👍