Blog

RxJS Creation Functions and Cold vs Hot Observables: of, from, defer, timer, fromEvent and iif

A cold Observable restarts its producer for each subscriber, while a hot one attaches subscribers to a source that flows on its own; to wrap Promise-based work in an Observable, use defer, not from. We compare of, from, range, defer, iif, EMPTY, NEVER, throwError, timer, interval, fromEvent and fromFetch with a real server log, and show why retry with from(promise) sends no new request.