var res = "";


function testComment(comment) {
    var words = comment.split(" ");
    words.forEach(function(word, i){
        (function(index){
            connection.query('select good_words from words where bad_words = ' + connection.escape(word), function(err, result){
                if(err){
                    console.log(err);
                    return;
                }
                if(result.length != 0){
                    this.res = comment.replace(word, result[0].good_words);
                    console.log("inside:" + this.res);
                }
            });
        })(i);
    });
    console.log("outside:" + this.res);
    return this.res;
}

function callback(){
   return res;
}

function testComment(comment, callback){
  ..all the working..(forEach loop which has a asyn function in it)
  return callback();
}

var wordsProcessed = 0;
(function(index){
    connection.query('select good_words from words where bad_words = ' + connection.escape(word), function(err, result){
        wordsProcessed++;
        if(err){
            console.log(err);
            return;
        }
        if(result.length != 0){
            this.res = comment.replace(word, result[0].good_words);
            console.log("inside:" + this.res);
        }
        if(wordsProcessed >= words.length){
            //THIS IS WHERE YOU RETURN YOUR RES
        }
    });
})(i);

var Promise = require('bluebird');
var promiseArray = words.map(function(word){
    return new Promise(function(resolve, reject){
        connection.query('select good_words from words where bad_words = ' + connection.escape(word), function(err, result){
            if(err) reject(err);
            else resolve(result);
        });
    });
});

Promise.all(promiseArray).then(function(result){
    return this.res;
});