All at once: mergeMap in RxJS
RxJS interview exercise on Flattening, difficulty 2 of 3. Solve it in the editor beside a worked example; the tests run the moment you do.
Write uploadAll(files$, upload): start upload(file) for every file as soon as it arrives, all in parallel, and emit each result when it is ready.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Progress is saved in your browser, and in your account if you sign in.