loop synchronously through array while calling asynchronous function

Question

My problem: i have an array with users and an user_hash and need to find the matching user. I came up with the following solution:

//results is the array user objects
//safety_first.getUserHash is a function that calculates a user_hash, it retrieves the user-record based on the id and then combines values of some fields and a key and turns this into a hash.
if (results.length > 0)
    {
      var i = 0;
      function checkUserhash(user_id, user_hash, callback) {
        safety_first.getUserHash(user_id, function(check_user_hash) {
          if (user_hash == check_user_hash)
          {
            callback(user_id);
          }
          else
          {
            if ((i+1) < results.length)
            {
              i++;
              checkUserhash(results[i].id, user_hash, callback);
            }
            else
            {
              callback(false);
            }
          }
        });
      }

      checkUserhash(results[i].id, user_hash, function(user_id) {
        if (user_id)
        {
          console.log("MATCH: "+user_id);
        }
        else
        {
          console.log("NO MATCH");
        }
      });


    }

I first tried to do this in a for-loop but cause it calls the checkUserhash asychronously i could not break the loop when the match was found.

I'm looking for other possible solutions, please share your thoughts.

regards, Pieter

Is safety_first a service you could change? E.g. return a promise instead of using a callback? — Yoshi
– Yoshi, Commented Jan 24, 2019 at 12:53

Andy · Accepted Answer · 2019-01-24 13:44:18Z

2

You can map over your user array to create an array of promises. Use Promise.all to wait for those promises to resolve and then iterate over the responses to check whether the hash matches the id.

In this example I've used async/await. I've also mocked up a hashing function routine so you can see it in action. Simply change the resolve from 'id' to 'id + 1' and rerun the demo to see the Match become No match.

Hope this is some use.

const safety_first = {
  getUserHash(id) {
    return new Promise(resolve => {
      setTimeout(() => resolve(id), 1000);
    });
  }
}

async function checkUserHashes(userList, promises) {
  try {

    // await the promises to all resolve
    // `Promise.all` preserves the order...
    const res = await Promise.all(promises);

    // ...which makes it useful if you need to check the returned
    // results against the promise-producing array
    userList.forEach(({ id }, i) => {
      if (id === res[i]) console.log(`${id}|${res[i]} - Match`);
      if (id !== res[i]) console.log(`${id}|${res[i]} - No match`);
    });

  } catch (e) {
    console.log(e);
  }
};

const userList = [{ id: 1, userHash: 1 }, { id: 2, userHash: 2 }];

// `map` over the fields and return a
// validation promise for each
const promises = userList.map(({ id }) => {
  return safety_first.getUserHash(id);
});

// Pass in the original userList, and the generated promises
checkUserHashes(userList, promises);

Update: if you want to break out of the loop when a match has been found that's a little easier:

const safety_first = {
  getUserHash(id) {
    return new Promise(resolve => {

      // This will match the second object (index 1)
      setTimeout(() => resolve(id === 1 ? id: id + 1), 1000);
    });
  }
}

async function checkUserHashes(userList) {

  // iterate over the array of objects
  for (let [index, obj] of userList.entries()) {

    // await for that one hash check
    const res = await safety_first.getUserHash(obj.id);

    // if it matches return the id
    if (obj.id === res) return obj;
  }

  // otherwise return null if there are no matches
  return null;
}

const userList = [{ id: 0, userHash: 1 }, { id: 1, userHash: 2 }];

(async () => {
  const id = await checkUserHashes(userList);
  console.log(id);
})();

edited Jan 24, 2019 at 13:44

answered Jan 24, 2019 at 12:27

Andy

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2 Comments

Overbeeke Over a year ago

With this solution hashes are generated for all users in my result list, right? I'm looking for a solution where i can generate one user hash, check it, not good then the next one. So that when i have 4000 users and i find the right one after 5 loops, the program wil break out the loop.

Andy Over a year ago

Updated the answer @Overbeeke.

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