Jay Phelps, former member of the RxJS core team, recently explained how to test code that leverages code using RxJS, the reactive programming library used by the Angular front-end framework for asynchronous programming. RxJS provides a testing API with a DSL to express timed sequences and lifecycle events.

RxJS provides an Observable type that encloses a mechanism to produce a potentially unbounded sequence of values. That sequence of values can originate from a source that is external to the program (e.g., button clicks), or result from transformations applied to existing sequences. RxJS provides additional APIs, types, and methods that let developers transform sequences (e.g., operators); and express the relationship between sources and sequences (e.g., Subject, schedulers).

Asynchronous code that leverages RxJS thus typically expresses a sequence of events as an Observable and applies transformations to it to express some business logic or processing. The resulting observable can then be further transformed. It can alternatively be subscribed to, with the program performing effects based on the values enclosed in the observable (e.g., API calls). In that context, testing asynchronous code is often testing that a transformation function takes an input observable into an output observable that contains an expected sequence of values (e.g., a sequence of purchases is taken into a sequence of updated basket content).

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