Is there a difference between promise.then.then vs promise.then; promise.then [duplicate]

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You have asked about “chaining” vs. “branching”.

Assuming that f1 and f2 represent asynchronous operations that return promises, yes there is a significant difference. For option 1:

  1. It serializes fn1 and fn2 so that fn2 is not called until after the promise returned by fn1 has been resolved.
  2. .catch() applies to an error in either fn1 or fn2 or if aPromiseObj rejects.
  3. fn2 will not be called if fn1 rejects.

For option 2:

  1. fn2 does not wait for fn1 to resolve. fn2 is called as soon as fn1 returns similar to fn1(); fn2();. This means the async operations started by fn1 and fn2 will both be in-flight at the same time (sometimes referred to running in parallel instead of running serially).
  2. The .catch() does not apply to either because it is not on the promise that is created by either of the .then() calls. The .catch() in option 2, only applies to if aPromiseObj rejects, not f1() or f2().
  3. Both fn1 and fn2 will be called regardless of an error in either.

Another related question/answer: Understanding javascript promises; stacks and chaining

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