How to return PostgreSQL database result by using PG and Node only?

Question

I am trying to give myself a practice to just use pure node as my server and PG to connect to PostgreSQL. What I want to do here is to return the result after querying from database and know it is problem of async. So how could I return the value from the callback function inside client.query.

function showAllMovies(){


pg.connect(connectionString, function (err, client, done) {
    if (err) {
      console.log('Can not log into database');
    } else {
      console.log('Connect to database...');
      client.query('SELECT * FROM movies', function (err, result) {
        done();  // client idles for 30 seconds before closing
        var result = JSON.stringify(result.rows[0].movie);
        console.log(result);
        return result;
      });
    }
  });
  pg.end();
}

Possible duplicate of How do I return the response from an asynchronous call? — Ben Fortune
– Ben Fortune, Commented Dec 18, 2015 at 8:41
Thanks for this information, @BenFortune , but I still don't know how to return the result value from this previous question. — TotoLai
– TotoLai, Commented Dec 20, 2015 at 7:19

jcaron · Accepted Answer · 2015-12-20 09:44:30Z

3

You can't return the value directly. You need to add a callback of your own to your function, which you will need to call, and provide the result to that callback.

For instance:

function showAllMovies(callback)
{
  pg.connect(connectionString, function (err, client, done)
  {
    if (err)
    {
      console.log('Can not log into database');
    }
    else
    {
      console.log('Connect to database...');
      client.query('SELECT * FROM movies', function (err, result)
      {
        callback(result.rows[0].movie);
        done();
      });
    }
  });
}

In this case, you can use it in your response handler like this:

showAllMovies(function(movie) { response.write('<li>'+movie+'</li>'); });

Note that this is just an example, you should actually do HTML escaping before output.

Alternatively, if you're responding to a request and using Express, you can simply pass down the response object and call one of its methods. For instance:

app.get('/movies', function(req,res)
{
  pg.connect(connectionString, function (err, client, done)
  {
    if (err)
    {
      console.log('Can not log into database');
    }
    else
    {
      console.log('Connect to database...');
      client.query('SELECT * FROM movies', function (err, result)
      {
        res.write('<li>'+result.rows[0].movie+'</li>');
        done();
      });
    }
  });
});

The same HTML escaping warning applies here.

edited Dec 20, 2015 at 9:44

answered Dec 18, 2015 at 10:34

jcaron

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4 Comments

TotoLai Over a year ago

Thanks for this simple instruction, but I still stuck somehow. Because I will use this showAllMovies function in my requestHandler. Something like this: response.write('<li>' + database.showAllMovies() + '</li>'); Could you give me some more guild line for this? thanks !

jcaron Over a year ago

Again, either you add a callback to your showAllMovies function, which will call it to do the output, or you pass down the response, and your function does the output.

jcaron Over a year ago

Edited the answer to illustrate how you can do it.

TotoLai Over a year ago

Thank you very very very much! It's what I exactly need! I always thought how to 'return' the value back from showAllMovie function, but now I realize how much is the power of callback !!

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