A custom operator in RxJS
RxJS interview exercise on Subjects & sharing, difficulty 3 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 tapOnce(fn): an operator that calls fn with the FIRST value of each subscription and passes every value through untouched. Each subscription counts on its own: a second subscriber gets its own first-value call.
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.