One after another: concatMap in RxJS
RxJS interview exercise on Flattening, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write saveInOrder(changes$, save): call save(change) for each change, but never two at a time: the next save starts when the previous one has completed. Emit each save's result.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.