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What i'm not able to understand is how logger.info is able to execute

Neither resolve not reject break the program's flow. Defers work like a normal Promises. They just fire an async event with the result of the Promise. The evaluation and calling of the resolve-/reject-handler will happen later, and the execution is queued by the JS VM.

If you need to break the flow at that point, you might want to use a return; statement:

const update = (selectionCriteria, updateCriteria, updateOptions, options) => {
  let q = Q.defer()
    , db = options.db
    , logger = options.logger

  db.collection(collectionName).update(selectionCriteria, updateCriteria, updateOptions, (err, doc) => {
    if (err) { 
      logger.error(err)
      q.reject(new Error("SOMETHING_WENT_WRONG",options))
    }
    if(doc.result.n > 0)
      q.resolve(true)
    else {
      q.resolve(false)
      return
    }
    logger.info("Updating")
  })
  return q.promise
}