TypeScript: Varargs Overload Signatures
I thought I’d write up a short post to explain a TypeScript problem that took up far too much of my time.
The TL;DR is that if you have overload signatures for a function that takes a variable number of (initial) arguments, the ordering of the signatures matters in a way that is not obvious. And the ‘simplest’ signature should be placed first.
The types I was working with were in RxJS, but to keep things general let’s use an example that combines a bunch of arrays in some way, like this:
The combine function will take elements from each of the input arrays that are passed — the caller can pass as many as is necessary — and will return an array containing the combined elements. (How they are combined doesn’t matter; we’re only interested in the types.)
combine could be typed like this:
Without going into too much detail, Inputs is mapped type that maps a tuple of element types to a tuple of read-only arrays — for the input array parameters —…
