Why is onRejected not called following Promise.all() where Promise.reject() included in array passed to Promise.all()?

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You need to understand that handling a rejection results in putting the promise back on the success path. One approach to this is to re-throw in the failure handler, like this:

var promises = [Promise.resolve("a"), Promise.reject("b")];

Promise.all(promises.map(function(p, index) {
  return p.then(function(data) {
    console.log("inside .map()", data, "index", index)
    return data
  }, function(err) {
    console.log(err);

    // RE-THROW!!
    throw err;                  

  })
}))
.then(...

If the purpose of the rejection handler is merely to log, then you could move it out of the chain:

Promise.all(promises.map(function(p, index) {

  // MOVE ERROR HANDLER OUTSIDE OF CHAIN
  p.catch(function(e) { console.log(e); });

  return p.then(function(data) {
    console.log("inside .map()", data, "index", index)
    return data
  })
}))
.then(...

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