Async.eachSeries vs forEach

//I first wrote the migrate script something like this with a forEach.
//And thought that it, itself was not blocking:

    import mysql  from 'mysql';
    import mongo  from 'mongodb';
    import assert from 'assert';
    import argv from 'yargs';
    import async from 'async';
    import config from './config.json';


    const url = config.mongodb;

    const MongoClient = mongo.MongoClient;

    let databases = config.databases;
    let neededFields= config.neededFields;
    let settings = new Array();
    let posts = new Array();
    let mongo_posts = null
    let mongo_settings = null
    let mongodb = null

    function complete(){

      console.log( 'complete');
      mongo_settings.insert(settings, (err, result)=>{
        mongo_posts.insert(posts, (err, result)=>{
          mongodb.close();
        })
      });

    }

    function syncSettings(){

     databases.forEach( (database ) => {
        console.log( database );
        let connection = mysql.createConnection({
          host: config.host,
          user: config.user,
          password: config.password,
          database: database
        });

        connection.connect();

        connection.query("select * from wp_options ", (err, rows, fields) => {
          console.log( "select * from wp_options ");
          let domain_setting = {}
          if(!err){


             rows.forEach( (item,index) => {
               if( neededFields.find( ( x ) =>  x == item['option_name']   )){
                 if( item['option_name'] == 'siteurl')
                  domain_setting['domain'] = item['option_value'].replace(/.*?:\/\//g, "");
                 else
                  domain_setting[item['option_name']] = item['option_value'];
               }
             });

            settings.push(domain_setting);
          }


          connection.query("select * from wp_posts ", (err, rows, fields) => {
            console.log( "select * from wp_posts ");
            if(!err){

              rows.forEach( (item,index) => {
                try{
                  let insertObj = {};

                  insertObj['post_name']= item['post_name'];
                  insertObj['post_title']  = item['post_title'];
                  insertObj['post_content']= item['post_content'].replace(/\r\n/g, '<br />');
                  insertObj['post_date']= item['post_date'];
                  insertObj['post_modified'] = item['post_modified'];
                  insertObj['post_status'] = item['post_status'];
                  insertObj['post_status'] = item['post_status'];
                  insertObj['domain'] = domain_setting['domain']

                  posts.push(insertObj)
                }catch(e){
                  console.error( e );
                  process.exit(1);
                }

              });

            }



          });

        });


          console.log('end')
          connection.end();
      });

      complete();
    }

    MongoClient.connect(url, (err, db) => {

      assert.equal(null, err);
      mongodb = db;
      console.log("Connected correctly to server");
      mongo_posts = db.collection('posts')
      mongo_settings = db.collection('settings')
      syncSettings();

    });


//But then I figured out why this version here works correctly

    import mysql  from 'mysql';
    import mongo  from 'mongodb';
    import assert from 'assert';
    import argv from 'yargs';
    import async from 'async';
    import config from './config.json';
    // Connection URL



    const url = config.mongodb;

    const MongoClient = mongo.MongoClient;

    let databases = config.databases;
    let neededFields= config.neededFields;
    let settings = new Array();
    let posts = new Array();
    let mongo_posts = null
    let mongo_settings = null
    let mongodb = null

    function complete(){

      mongo_settings.insert(settings, (err, result)=>{
        mongo_posts.insert(posts, (err, result)=>{
          mongodb.close();
        })
      });

    }

    function syncSettings(){

      async.eachSeries( databases, ( database, next ) => {

        let connection = mysql.createConnection({
          host: config.host,
          user: config.user,
          password: config.password,
          database: database
        });

        connection.connect();

        connection.query("select * from wp_options ", (err, rows, fields) => {
          let domain_setting = {}
          if(!err){


             rows.forEach( (item,index) => {
               if( neededFields.find( ( x ) =>  x == item['option_name']   )){
                 if( item['option_name'] == 'siteurl')
                  domain_setting['domain'] = item['option_value'].replace(/.*?:\/\//g, "");
                 else
                  domain_setting[item['option_name']] = item['option_value'];
               }
             });

            settings.push(domain_setting);
          }


          connection.query("select * from wp_posts ", (err, rows, fields) => {
            if(!err){

              rows.forEach( (item,index) => {
                try{
                  let insertObj = {};

                  insertObj['post_name']= item['post_name'];
                  insertObj['post_title']  = item['post_title'];
                  insertObj['post_content']= item['post_content'].replace(/\r\n/g, '<br />');
                  insertObj['post_date']= item['post_date'];
                  insertObj['post_modified'] = item['post_modified'];
                  insertObj['post_status'] = item['post_status'];
                  insertObj['post_status'] = item['post_status'];
                  insertObj['domain'] = domain_setting['domain']

                  posts.push(insertObj)
                }catch(e){
                  console.error( e );
                  process.exit(1);
                }

              });

            }


          connection.end();
          next();

          });

        });


      }, complete );


    }

    MongoClient.connect(url, (err, db) => {

      assert.equal(null, err);
      mongodb = db;
      console.log("Connected correctly to server");
      mongo_posts = db.collection('posts')
      mongo_settings = db.collection('settings')
      syncSettings();

    });