Passing variables into callback function

Question

I have a piece of code like this:

var guid = 'unique_guid';
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, function(err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);
        // do more things which require guid
    }
}

In order to avoid callback hell, I give the call back function a name and refactor it as the following:

var checkExistence = function(err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    }
}
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence);

con is a connection created from node-mysql

Now my problem is that I can't reference guid in checkExistence(), and I don't want to make guid as a global variable.

Is it possible to get guid in checkExistence()?

Callback will be function(){ checkExistence(guid); } and accept guid as argument.. — Rayon
– Rayon, Commented Feb 18, 2016 at 10:56

jcubic · Accepted Answer · 2016-02-18 10:59:44Z

6

You can add guid as parameter and return a function:

var checkExistence = function(guid) {
    return function(err, rows) {
        if(err) throw err;
        if(rows.length == 0) {
            console.log('new guid: ' + guid);       // guid can't be referenced here
            // do more things which require guid
        } else {
            console.log('old guid: ' + guid);       // guid can't be referenced here
            // do more things which require guid
        }
    };
};
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence(guid));

answered Feb 18, 2016 at 10:59

jcubic

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1 Comment

Brian Over a year ago

Thank you :-). I'm new to javascript.

Ильяс Гарифуллин · Accepted Answer · 2016-02-18 10:59:36Z

0

You can use Function.bind function for thet, like this:

var checkExistence = function(guid, err, rows) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    }
}
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence.bind(null, guid));

answered Feb 18, 2016 at 10:59

Ильяс Гарифуллин

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Comments

Olivier Boissé · Accepted Answer · 2016-02-18 11:01:33Z

0

Maybe you could use the bind function,

var checkExistence = function(guid, err, rows) { ...

and call the method query like this

con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence.bind(null, guid);

answered Feb 18, 2016 at 11:01

Olivier Boissé

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Comments

Sanjeev Kumar · Accepted Answer · 2016-02-18 11:05:11Z

0

    var checkExistence = function(err, rows, guid) {
    if(err) throw err;
    if(rows.length == 0) {
        console.log('new guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    } else {
        console.log('old guid: ' + guid);       // guid can't be referenced here
        // do more things which require guid
    }
}
con.query('SELECT guid FROM myDB.myTable WHERE guid = ?', guid, checkExistence(err, rows, guid));

answered Feb 18, 2016 at 11:05

Sanjeev Kumar

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