A language interview tests more than syntax recall. You need to state what the function promises, choose a data structure, and handle edge cases while keeping the public types usable. These six questions combine collection behavior, asynchronous ordering, subscriber lifetimes, literal statuses, immutable updates, and exhaustive branching.
Work through one question at a time. Trace a small input before coding, then test the case that could invalidate your approach: an empty list, an explicit zero, a duplicate subscriber, or a failure branch. You may implement a different algorithm as long as it satisfies the same contract. The scenarios are original practice questions and don't claim to come from a particular employer. Runtime and compiler checks both contribute to a passing answer.