How to make async await work for a function that calls a database

Question

I have an async function processWaitingList() that needs to wait for the result of another function findOpenVillage() that gets it's data from my database.

I'm getting the following error and do not understand why:

(node:2620) UnhandledPromiseRejectionWarning: TypeError: Cannot read property '0' of undefined at processWaitingList (xx\server.js:151:36) at process._tickCallback (internal/process/next_tick.js:68:7) (node:2620) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)

I've read everything I can find on promises, callbacks, async and await, but I just cannot figure out how these work.

function slowServerLoop() {
    processWaitingList();
}
setInterval(slowServerLoop, 500);

async function processWaitingList() {
    let openVillage = await findOpenVillage();
    villageId = openVillage[0];
    openSpots = openVillage[1];
    addPlayerToVillage(waitingList[0], villageId, openSpots);
}

function findOpenVillage() {
    con.query(`SELECT id, maxVillagers FROM villages WHERE status='0'`, function (err, result, fields) {
        if (err) throw err;
        if (result.length === 1) {
            let villageId = result[0].id;
            let maxVillagers = result[0].maxVillagers;
            con.query(`SELECT COUNT(*) as villagerCount FROM villagers WHERE villageId='${villageId}'`, function (err, result, fields) {
                if (err) throw err;
                let villagerCount = result[0].villagerCount;
                let openSpots = maxVillagers - villagerCount;
                return [villageId, openSpots];
            });
        }
    });
}

Possible duplicate of How do I return the response from an asynchronous call? — nicholaswmin
– nicholaswmin, Commented Feb 8, 2019 at 14:14
I was just reading that one, trying to use the suggested option: ES2017+: Promises with async/await - but can't figure out how to get it to work — Pascal Claes
– Pascal Claes, Commented Feb 8, 2019 at 14:16
You need to convert con.query into a Promise so you can await it. — nicholaswmin
– nicholaswmin, Commented Feb 8, 2019 at 14:17

libik · Accepted Answer · 2022-06-22 06:52:30Z

2

there is important thing about await. If you put it behind some promise, it will await the promise to be resolved and then return the value to your code.

In your case, you are not returning promise, so there is nothing to be awaited.

When you need to combine promises with callbacks (async/await is just extension over promise chains), one of way is to wrap callback inside new promise.

In your case

return new Promise((resolve, reject) =>
{
con.query(`SELECT id, maxVillagers FROM villages WHERE status='0'`, function (err, result, fields) {
        if (err) { return reject(err); }
        if (result.length === 1) {
            let villageId = result[0].id;
            let maxVillagers = result[0].maxVillagers;
            con.query(`SELECT COUNT(*) as villagerCount FROM villagers WHERE villageId='${villageId}'`, function (err, result, fields) {
                if (err) { return reject(err); }
                let villagerCount = result[0].villagerCount;
                let openSpots = maxVillagers - villagerCount;
                resolve([villageId, openSpots]);
            });
        }
    });
}

Also remember that you need to return it (its already in code ^^ )

edited Jun 22, 2022 at 6:52

answered Feb 8, 2019 at 14:16

libik

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6 Comments

Pascal Claes Over a year ago

@ExplosionPills the function is not vulnerable to injection because it does not take any user input

Pascal Claes Over a year ago

@libik Thanks a lot! I could've never figured this out myself! This seems to be working!

Explosion Pills Over a year ago

Imagine a bad actor who is able to create a villageId with an injection

libik Over a year ago

@ExplosionPills - I just took the OP code and wrap it with Promise, which I also described in my answer... So I dont think it make sense to evaluate the code quality itself.

Explosion Pills Over a year ago

This is not a question of quality, it's a question of security. I think it's reasonable to propagate good security practices in SO answers.

|

Explosion Pills · Accepted Answer · 2019-02-08 14:28:05Z

2

findOpenVillage does not return a Promise. You need it to return a Promise in order to use async/await.

If possible, you should see if con.query has the ability to return a Promise. There is the mysql-promise library that you could use, among others. You could also use the Promise constructor yourself or the built in util.promisify. Let's use that last option as an example:

const util = require('util');

...
async function findOpenVillage() {
  const query = util.promisify(con.query);
  const maxResult = await query(`SELECT id, maxVillagers FROM villages WHERE status = '0'`);
  if (maxResult.length === 1) {
    const villageId = result[0].id;
    const maxVillagers = result[0].maxVillagers;
    const countResult = await query(
      `SELECT COUNT(*) as villagerCount FROM villagers WHERE villageId = ?`,
      villageId
    );
    const villagerCount = result[0].villagerCount;
    const openSpots = maxVillagers - villagerCount;
    return [villageId, openSpots];
  }
}

I've made this an async function so you could use await, but using promises with .then would also be reasonable. If you did that, you would have to return the promise.

Your original code was using string interpolation instead of parameterized queries. This makes the query vulnerable to injection. I have corrected this in my answer as well.

edited Feb 8, 2019 at 14:28

answered Feb 8, 2019 at 14:20

Explosion Pills

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3 Comments

Pascal Claes Over a year ago

Thanks for adding the parameterized queries, I'll definetely use those!

Pascal Claes Over a year ago

What is const util = require('util'); - what makes this method better than returning a Promise?

Explosion Pills Over a year ago

@PascalClaes nodejs.org/api/util.html#util_util_promisify_original — it’s hard to say if an approach is better than another. You’re already using an async function, so consistentcy is nice. Async functions implicitly return Promises.

Shahzeb Khan · Accepted Answer · 2019-02-08 14:17:05Z

-1

You need to access the code after the promise is fullfilled. Your function findOpenVillage() must return a promise object. You need to access the values in then. It means that you will be accessing the values after the promise is fullfilled.

openVillage.then(function(response){
villageId = openVillage[0];
    openSpots = openVillage[1];
    addPlayerToVillage(waitingList[0], villageId, openSpots);
});

answered Feb 8, 2019 at 14:17

Shahzeb Khan

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1 Comment

libik Over a year ago

Hi, sorry you are not correct about then, it works same as the await. You are correct about returning promise though.

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