Node.js - Retrieving data from a database (mysql) - delay

Question

I am loading data from the database using mysql drivers for node.js. https://github.com/felixge/node-mysql

I want to load data into the array. When I am loading data into the array - there is a delay and the array is empty.

I want to retrieve data and then write the lines - console.log("END QUERY"); console.log(users);

var users = [];

client.query(
  'SELECT * FROM users',
  function selectCb(err, results, fields) {
    if (err) {
      throw err;
    }

if (results.length > 0) {
          var reader = results[0];
          console.log("ID: " + reader['id']); //ADD string - 'ID: 1'
          users = [reader['id']]
      } 
     }
);

console.log("END QUERY");

console.log(users);

OUTPUT:

WRITE LINE:

END QUERY
[]
ID: 1

Rohan Singh · Accepted Answer · 2012-02-05 20:12:18Z

2

This is how Node works — using an asynchronous model. First all your code as written executes, including console.log(users). At this point, users is an empty array.

Only once all your imperative instructions have executed do callbacks get fired — such as your selectCb function. It's only once this callback runs that you can have access to users.

This is the entire point and purpose of Node, actually, and it is very important to understand the fundamentals here. You can read more about it at: http://shinetech.com/thoughts/articles/139-asynchronous-code-design-with-nodejs-

Others have also recommended this video introducing Node from Ryan Dahl: http://www.youtube.com/watch?v=jo_B4LTHi3I

In your specific case, I would move the console.log("END QUERY") and console.log(users) lines into the body of the selectCb function.

answered Feb 5, 2012 at 20:12

Rohan Singh

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