How to write reusable code in node.js

Question

I am trying to understand how to design node.js applications, but it seems there is something I can't grasp about asynchronous programming.

Let's say my application needs to access a database. In a synchronous environment I would implement a data access class with a read() method, returning an associative array.

In node.js, because code is executed asynchronously, this method can't return a value, so, after execution, it will have to "do" something as a side effect. It will then contain some code which does something else than just reading data.

Let's suppose I want to call this method multiple times, each time with a different success callback. Since the callback is included in the method itself, I can't find a clean way to do this without either duplicating the method or specifying all possible callbacks in a long switch statement.

What is the proper way to handle this problem? Am I approaching it the wrong way?

Answer 1

Since you can pass functions as parameters, it is common to pass a "callback" function to these sorts of functions. So, for example, you may have a "read" function that looks like this:

function getUser(userId, callback) {
  var mydbAPI = something;
  var myQuery = somethingElse;

  mydbAPI.getUser(myQuery, function(data) {
    //parse data a little here:
    myParsedData = ...;

    callback(myParsedData);
  });
}

You can then call the read method like this:

getUser(userId, function(parsedData) {
  //...do something with parsedData
});