Async in JavaScript: interview exercises
Async in JavaScript: 4 test-checked interview exercises (Fetch in parallel, Retry a failing call), each beside a worked example, with tests as you go. Free.
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.