solve callback hell on class initializer for node.js

Question

solve callback hell on class initializer for node.js

I am using npm package "mssql" In order to open connection. I have to do:

var getOneToken = function (callback) {
    var token = "";

    var connection = new sql.Connection(dbConfig, function(err){   //<-----line 3
        if(err) console.log(err);

        var sqlrequest = new sql.Request(connection);

        sqlrequest.query('select top 1 [accessToken] from AccessToken', function(err, recordset){
            if(err) console.log(err);

            token = recordset[0]['accessToken'];

            connection.close();

            callback(token);
        })
    });
    //how to await the whole block at the top and prevent early execution of code afterwards.
};

The code in this block need to be delayed until database return the result, which is the token. Then further operation can be carried out.

I don't know how to await the code.

Since I tried:

connection = await { new sql.Connection(dbConfig, defer (var err)) }

It fails. Because tamejs don't allow you to put keyword 'await' over there.

I also use bluebird, asyncawait library, however, it is very hard to flatten the whole block and 'await' on class initialize especially for line 3.

How to deal with this problem ?

javascript
node.js
jquery-callback

Answer 1

function sqlConnect(dbConfig, cb) {
  var conn = new sql.Connection(dbConfig);
  conn.connect(function (err) {
    cb(err, conn);
  });
}

Usage:

sqlConnect(dbConfig, function (err, conn) {
  // do your stuff here
});

Or using tamejs:

var err, conn;
await { sqlConnect(dbConfig, defer(err, conn); }
// do your stuff here

To layout callbacks, you may also want to take a look at async

Update: If what you want is only avoid callback chain, you don't need to extract a new function for establishing connection. Instead, you can rewrite your function like this:

var getOneToken = function (dbConfig, cb) {
    var err, recordset;
    var token = "";

    var conn = new sql.Connection(dbConfig);

    await { conn.connect(defer(err)); }
    if (err) {
        console.log(err);
        cb(token);
        return;
    }

    var request = new sql.Request(conn);
    await { request.query('select top 1 [accessToken] from AccessToken', defer(err, recordset)); }
    if (err) {
        console.log(err);
    } else {
        token = recordset[0]['accessToken'];
    }

    conn.close();
    cb(token);
};