How to return the promise for further usage.

Hello!

I’m somewhat new to promises and stuff regarding this so… I did a small DB module which I’m importing wherever I need it.

var DB = function() {
    this.pool = require('mysql2').createPoolPromise({ host: '', user: '', password: '', database: '' });
}

DB.prototype.query = function( sql, format ) {
    this.pool.getConnection().then( conn => {
        var result = conn.query( sql , format );
        conn.release();
        return result;
    }).catch( err => {
        console.log( err );
    });
}

module.exports = new DB();

Now, in another module, I’m requiring the DB.js module as follows

var db = require( './controllers/DB.js' );

How I would like to use it is as follows:

db.query( 'SELECT * FROM users WHERE id = ? LIMIT 1', [ 5 ] ).then( result => {
     console.log( result );
});

Though everything “seems” to be working, I’m having an error as follows:

TypeError: Cannot read property 'then' of undefined

What am I doing wrong ? I guess that the result var is not a Promise type of object ? If so, what should I return in order to chain the result in other modules.

Thank you

[sidorares]

first of all I suspect that inner loop might be removed with some kind of join

if you want to call cb( merchant ); only after all inner loop promises are resolved you can use something like this

db.query( 'SELECT * FROM merchants WHERE id = 5' )
    .then( ([ rows, columns ]) => {
        var merchant = rows[0];
        merchant.shops = [];
        return merchant;
    })
    .then( merchant => {
         var products = [];
         var orders = [];
         db.query( 'SELECT * FROM shops WHERE merchantID = ?', [ merchant.id ] )
          .then([shops] => {
              products = shops.map( shopId => db.query( 'SELECT * FROM products WHERE shopID = ? ORDER BY name ASC', shopId );
              orders = shops.map( shopId => db.query( 'SELECT * FROM products WHERE shopID = ? ORDER BY name ASC', shopId );
             Promise.all(products)
                .then( products => /* do something with products */ )
                .then( () => {
                     Promise.all(orders).then(
                        // all ready, call callback?
                     );
                 }


          });

there might be better solution, I personally prefer traditional callbacks or async library and not very experienced in promise patterns

also instead of using loops to append array I’d use something like this

    var arr1 = [1, 2, 3];
    var arr2 = [3, 4, 5];
    console.log(arr1.concat(arr2)); // instead of for (var i=0; i < arr2l ++i) { arr1.push(arr2[i]); }

[sidorares]

it’s not wasted, you still learned something 😃