I would like to clarify this point, as the documentation is not too clear about it;
Q1: Is Promise.all(iterable)
processing all promises sequentially or in parallel? Or, more specifically, is it the equivalent of running chained promises like
p1.then(p2).then(p3).then(p4).then(p5)....
or is it some other kind of algorithm where all p1
, p2
, p3
, p4
, p5
, etc. are being called at the same time (in parallel) and results are returned as soon as all resolve (or one rejects)?
Q2: If Promise.all
runs in parallel, is there a convenient way to run an iterable sequencially?
Note: I don't want to use Q, or Bluebird, but all native ES6 specs.
No, promises cannot "be executed". They start their task when they are being created - they represent the results only - and you are executing everything in parallel even before passing them to
Promise.all
.Promise.all
does only await multiple promises. It doesn't care in what order they resolve, or whether the computations are running in parallel.If you already have your promises, you can't do much but
Promise.all([p1, p2, p3, …])
(which does not have a notion of sequence). But if you do have an iterable of asynchronous functions, you can indeed run them sequentially. Basically you need to get fromto
and the solution to do that is using
Array::reduce
: