When does Promise.all actually queue a microtask?

2022-05-04 10:14 Sasho32 imported from Stackoverflow

So i am new to promises and i am trying some stuff to figure out what's going on under the hood. From what i read a Job is queued to the Microtask queue by .then immediately after the promise the method was chained to resolves.

Promise.resolve("FIRST")
  .then((m) => console.log(m))
  .then(() => console.log("FIRST on second level"))
  .then(() => console.log("FIRST on third level"))
  .then(() => console.log("FIRST on fourth level"));

Promise.resolve("SECOND")
  .then((m) => console.log(m))
  .then(() => console.log("SECOND on second level"))
  .then(() => console.log("SECOND on third level"))
  .then(() => console.log("SECOND on fourth level"));

Promise.all([
  Promise.resolve("Middle 1 resolved"),
  Promise.resolve("Middle 2 resolved"),
]).then((results) => console.log(results));

So ok, the first .then of the FIRST chain is queued immediately. The same goes for the first of the SECOND chain. But they both return promises which will be pending until the first two queued jobs get executed. So why is the log like this:

FIRST
SECOND
FIRST on second level
SECOND on second level
[ 'Middle 1 resolved', 'Middle 2 resolved' ]
FIRST on third level
SECOND on third level
FIRST on fourth level
SECOND on fourth level

Why isn't the "middle" log placed before "FIRST on second level"? Because i was assuming from what i read that the promise returned from Promise.all IN THIS EXAMPLE will resolve as long as the stack is empty and therefore its .then will get queued right after the first two:

then(m => console.log(m)) //on line 2 and line 8

So where am i wrong? What am i missing or don't understand properly? Thank you in advance!

1 answer

answered 2022-05-04 10:29 Nicholas Tower
Promise.all creates a new promise, and for it to know when to resolve that promise, it needs to call .then on each of the promises you pass into it. So first the promises in the array must resolve, then the promise created by Promise.all can resolve, and only then do does your console.log get called.

In other words, there are 2 levels of calls to .then, much like your other second level examples. It's just that one level is hidden inside the implementation of Promise.all

If it helps, here's an approximation of what Promise.all is doing:
```
function all(promises) {
  return new Promise((resolve) => {
    let count = 0;
    let results = [];
    for (let i = 0; i < promises.length; i++) {
      promises[i].then(result => {
        count++;
        results[i] = result;
        if (count === promises.length) {
          resolve(results);
        }
      });
    }
  });
}
```

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import base64
import json
import os
import re
import time

try:
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    from py_mini_racer import py_mini_racer
    import requests
except ModuleNotFoundError:
    if "y" in input("Install dependencies? [y/N] > ").lower():
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from difflib import SequenceMatcher

allowedTypes = ['quiz', 'multiple_select_quiz']
DEFAULT_ANSWER = 1


class Kahoot:
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        self.nickname = nickname
        self.quizName = quizName
        self.quizID = quizID
        self.client = requests.session()
        self.captchaToken = "KAHOOT_TOKEN_eyJ2ZXJzaW9uIjoiIn0="
        # This will work until Kahoot updates their code version (https://repl.it/repls/WholeCrimsonFlashdrives)
        self.authToken = None
        self.answers = None
        self.colors = {0: "RED", 1: "BLUE", 2: "YELLOW", 3: "GREEN"}
        self.maxCount = maxCount if maxCount else 50
        self.lookup = None
        self.loadCodes()
        self.sessionID = None
        self.sessionToken = None
        self.DEBUG = DEBUG
        self.loop = asyncio.get_event_loop()

    def error(self, err):
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    def gracefulExit(self):
        exit()

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            await client.subscribe("/service/player")
            await client.subscribe("/service/status")
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            async for rawMessage in client:
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                    print("Wrong question types")
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                print("Wrong num of accepted answers")
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            params = {'query': self.quizName, 'cursor': 0, 'limit': self.maxCount, 'topics': '', 'grades': '',
                      'orderBy': 'relevance', 'searchCluster': 1, 'includeExtendedCounters': False}
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                print(self.authToken)
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                resp = self.client.get(url, params=params, headers={'Authorization': f'Bearer {self.authToken}'})
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                resp = self.client.get(url, params=params)
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                    self.parseAnswers(quiz, self.DEBUG)
                elif quiz:
                    return self.parseAnswers(quiz, self.DEBUG)
            if not quiz:
                self.error("No quiz found. (private?)")

    @staticmethod
    def parseAnswers(quiz, debug=None):
        answers = []
        if debug:
            print(quiz)
        for question in quiz['questions']:
            foundAnswer = False
            if question['type'] not in allowedTypes:
                answers.append({'question': 'NOT A QUESTION'})
                continue
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                if choice['correct'] and not foundAnswer:
                    foundAnswer = True
                    answers.append({'question': question['question'], 'index': i, 'answer': choice['answer']})
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    @staticmethod
    def printAnswers(quiz, answers):
        # print("If the questions are randomized, go to " + url + "to get the answers yourself.")
        print(f"Title: {quiz['title']}")
        print(f"Creator: {quiz['creator_username']}")
        print(f"Desc: {quiz['description']}")
        for q in answers:
            print(f"{q['question']}\n\t{q['answer']}")

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        # https://stackoverflow.com/questions/33404752/removing-emojis-from-a-string-in-python/33417311
        emoji_pattern = re.compile("["
                                   u"\U0001F600-\U0001F64F"  # emoticons
                                   u"\U0001F300-\U0001F5FF"  # symbols & pictographs
                                   u"\U0001F680-\U0001F6FF"  # transport & map symbols
                                   u"\U0001F1E0-\U0001F1FF"  # flags (iOS)
                                   "]+", flags=re.UNICODE)
        return emoji_pattern.sub(r'', text)

    @staticmethod
    def _similar(a, b):
        return SequenceMatcher(None, a, b).ratio()

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        assert type(self.pin) == str
        currentTime = int(time.time())
        url = f"https://play.kahoot.it/reserve/session/{self.pin}/?{currentTime}"
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        if resp.status_code != 200:
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        # Rebuilt Javascript so engine can solve it
        text = text.replace('\t', '', -1).encode('ascii', 'ignore').decode('utf-8')
        text = re.split("[{};]", text)
        replaceFunction = "return message.replace(/./g, function(char, position) {"
        rebuilt = [text[1] + "{", text[2] + ";", replaceFunction, text[7] + ";})};", text[0]]

        jsEngine = py_mini_racer.MiniRacer()
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    def _shiftBits(self, solution):
        decodedToken = base64.b64decode(self.sessionToken).decode('utf-8', 'strict')
        solChars = [ord(s) for s in solution]
        sessChars = [ord(s) for s in decodedToken]
        return "".join([chr(sessChars[i] ^ solChars[i % len(solChars)]) for i in range(len(sessChars))])

    def loadCodes(self):
        self.lookup = {
            1: "GET_READY",
            2: "START_QUESTION",
            3: "GAME_OVER",
            4: "TIME_UP",
            5: "PLAY_AGAIN",
            6: "ANSWER_SELECTED",
            7: "ANSWER_RESPONSE",
            8: "REVEAL_ANSWER",
            9: "START_QUIZ",
            10: "RESET_CONTROLLER",
            11: "SUBMIT_FEEDBACK",
            12: "FEEDBACK",
            13: "REVEAL_RANKING",
            14: "USERNAME_ACCEPTED",
            15: "USERNAME_REJECTED",
            16: "REQUEST_RECOVERY_DATA_FROM_PLAYER",
            17: "SEND_RECOVERY_DATA_TO_CONTROLLER",
            18: "JOIN_TEAM_MEMBERS",
            19: "JOIN_TEAM_MEMBERS_RESPONSE",
            20: "START_TEAM_TALK",
            21: "SKIP_TEAM_TALK",
            31: "IFRAME_CONTROLLER_EVENT",
            32: "SERVER_IFRAME_EVENT",
            40: "STORY_BLOCK_GET_READY",
            41: "REACTION_SELECTED",
            42: "REACTION_RESPONSE",
            43: "GAME_BLOCK_START",
            44: "GAME_BLOCK_END",
            45: "GAME_BLOCK_ANSWER",
            50: "SUBMIT_TWO_FACTOR",
            51: "TWO_FACTOR_AUTH_INCORRECT",
            52: "TWO_FACTOR_AUTH_CORRECT",
            53: "RESET_TWO_FACTOR_AUTH"
        }


class KahootError(Exception):
    pass

Js await function inside a map
I have an array of category ids in order, I have to make a map and fetch the items from these categories and in the end keep the original order of the array

what I've been doing:
```
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How to put result of Promise.all at specific place?

Is there any way to use Promise.all in my case ? Should i use two different loop (one to push in an array of promise and another to push each result to the right place) ?

(This is just an example and my current code contains much more data, so using Promise.all seems very interesting since the requests are independent of each other)

const  myFunction = async ()=>{

    let someData=[{ID:55,name:'name1',firstname:'fname1',address:'address1'},
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Thank you

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When does Promise.all actually queue a microtask?

1 answer

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