node.js callback function needs null if no error?

Question

Just looking at sample code from mongodb driver: http://mongodb.github.io/node-mongodb-native/2.2/tutorials/projections/

var MongoClient = require('mongodb').MongoClient
  , assert = require('assert');

// Connection URL
var url = 'mongodb://localhost:27017/test';
// Use connect method to connect to the server
MongoClient.connect(url, function(err, db) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  findDocuments(db, function() {
    db.close();
  });  
});


var findDocuments = function(db, callback) {
  // Get the documents collection
  var collection = db.collection( 'restaurants' );
// Find some documents
  collection.find({ 'cuisine' : 'Brazilian' }, { 'name' : 1, 'cuisine' : 1 }).toArray(function(err, docs) {
    assert.equal(err, null);
    console.log("Found the following records");
    console.log(docs)
    callback(docs);
  });
}

Shouln't last line callback(docs) be callback(null, docs) ?

According node.js callback notation it should, but developer could use own style. In this style, callback doesn't accept error at all. — alexmac
– alexmac, Commented Aug 3, 2017 at 16:00

Jose Lopez Garcia · Accepted Answer · 2017-08-03 16:04:36Z

2

It depends on your callback.

There are error-first callbacks, which do take an error as the first argument, and the data as the second argument, like in: callback (err, data)

However, in Mongo's official example webpage (the one you pointed out), they pass a callback without an error argument. Error-first callbacks are everywhere inside Node's built-in modules, but Node does NOT enforce you, by any means, to use them. That's what, in this example, Mongo developers decided to do.

You can easily re-write the Mongo example to use an error-first callback, though.

answered Aug 3, 2017 at 16:04

Jose Lopez Garcia

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