Best way to “flatten” an array inside an RxJS Observable

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You can use concatAll() or mergeAll() without any parameter.

dataFromBackend.pipe(
  tap(items => console.log(items)),
  mergeAll(), // or concatAll()
)

This (including mergeMap) works only in RxJS 5 because it treats Observables, arrays, array-like objects, Promises, etc. the same way.

Eventually you could do also:

mergeMap(val => from(val).pipe(
  tap(item => console.log(item)),
  map(item => item.name),
)),
toArray(),

Jan 2019: Updated for RxJS 6

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