How to send mysql queries sequentially using express in Node.js

Question

I wrote the following code, where I have a post request that executes 2 queries and some other operations:

router.post('/', (req, res) => {
    dbController.query(
        "SELECT * FROM users WHERE username = 'myUserName'",
        (err, result) => {
            console.log('<---- 1 ---->')
        }
    )
    // do something
    console.log('<---- 2 ---->')
    // do something
    dbController.query(
        "SELECT * FROM users WHERE username = 'myUserName'",
        (err, result) => {
            console.log('<---- 3 ---->')
    })
    res.send('ok')
})

I want to execute all instructions inside the function sequentially, so instead of getting this output (which is the one I get after executing the code):

<---- 2 ---->
<---- 1 ---->
<---- 3 ---->

I wanna get this one:

<---- 1 ---->
<---- 2 ---->
<---- 3 ---->

Note that after adding the keyword async to the function and await to the queries, nothing changes

Heiko Theißen · Accepted Answer · 2022-06-17 08:55:53Z

1

Since you give dbController.query a callback function (err, result) => {...} as third parameter, it does not return a promise, but instead invokes the callback asynchronously. If you wrap dbController.query in util.promisify, you get a function that returns a promise which

either resolves to result, which you can await,
or rejects as err, which you can catch.

The two flavors are functionally equivalent, but with util.promisify, you can use async and await:

router.post('/', async (req, res) => {
  var queryPromise = util.promisify(dbController.query.bind(dbController));
  try {
    var result = await queryPromise("SELECT * FROM users WHERE username = 'myUserName'")
    console.log('<---- 1 ---->')
    console.log('<---- 2 ---->')
    result = await queryPromise("SELECT * FROM users WHERE username = 'myUserName'")
    console.log('<---- 3 ---->')
    res.send('ok')
  } catch(err) {
    res.status(400).send(err);
  }
})

(Many other database client libraries offer the promise flavor out of the box.)

edited Jun 17, 2022 at 8:55

answered Jun 15, 2022 at 14:56

Heiko Theißen

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1 Comment

Heiko Theißen Over a year ago

Sorry, mysql does not offer the promise flavor directly, but you can achieve it with util.promisify.

Mohammed Saber · Accepted Answer · 2022-06-15 14:41:28Z

0

You need to wrap your logic in a .then() Your code should look something like this:

router.post('/', (req, res) => {
    dbController.query(
        "SELECT * FROM users WHERE username = myUserName",
        (err, result) => {
            console.log('<---- 1 ---->')
        }
    ). then(async function(rows){
// do something
    console.log('<---- 2 ---->')
    // do something
})
dbController.query(
        "SELECT * FROM users WHERE username = myUserName",
        (err, result) => {
            console.log('<---- 3 ---->')
    })
    res.send('ok')
})

You might also need to wrap the second query in a .then()

answered Jun 15, 2022 at 14:41

Mohammed Saber

1713 silver badges13 bronze badges

1 Comment

arrmani88 Over a year ago

Hello, thanks for your answer, but this will not work because: TypeError: dbController.query(...).then is not a function

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