Express JS 'this' undefined after routing with app.get (..)

Question

I have a basic Node JS server that is designed to be used as an API, I created a log module and a database module, and I started adding other modules to work with different types of queries.

I am using Express.js and node-mysql

When I am /v1/group , I get the following error:

 TypeError: Cannot read property 'database' of undefined at Group.getAll (C:\code\javascript\node\api\api\v1\groups.js:12:23) at callbacks (C:\code\javascript\node\api\node_modules\express\lib\router\index.js:161:37) ... 

So, I think that after receiving the request and calling group.getAll() that this is undefined, but I don’t understand why, is there a way to install this or have I structured my application incorrectly?

sever.js

 "use strict"; var Express = require('express'); var Log = require('./database/log'); var Database = require('./database/database'); var dbConfig = require('./dbconfig.json'); var Group = require('./api/v1/groups'); //Init express var app = new Express(); //Init log and database var log = new Log(); var database = new Database(dbConfig, log); var initCallback = function() { //Init routes var group = new Group(database, log); //Group routes app.get('/v1/group', group.getAll); app.get('/v1/group/:id', group.getByID); app.listen(3000); log.logMessage("INFO", "Listening on port 3000"); }; //Test database connection database.getConnection(function(err, connection) { if (err) { log.logMessage("FATAL", "Error connecting to database, check database is running and the dbconfig.json file is present and correct."); process.exit(1); } connection.end(); initCallback(); }); 

database.js

 "use strict"; var mysql = require('mysql'); var Database = function(dbConfig, log) { this.connected = false; this.log = log; this.log.logMessage("INFO", "Connecting to database with: Host - " + dbConfig.dbhost + ", Database port - " + dbConfig.dbport + ", Database name - " + dbConfig.dbname + ", User " + dbConfig.dbuser + ", Password length - " + dbConfig.dbpass.length); this.pool = mysql.createPool({ host : dbConfig.dbhost, user : dbConfig.dbuser, port: dbConfig.dbport, password : dbConfig.dbpass, database: dbConfig.dbname }); }; Database.prototype.getConnection = function() { var args = arguments; return this.pool.getConnection.apply(this.pool, arguments); }; module.exports = Database; 

groups.js

 "use strict"; var Group = function(database, log) { this.database = database; this.log = log; }; Group.prototype.getAll = function(req, res) { console.log(this); // --> undefined var query = 'SELECT * FROM invgroups WHERE published = 1'; this.database.getConnection(function(err, connection) { // --> error line if (err) { res.send(500, "Database error"); } connection.query(query, function(err, results) { if (err) { res.send(500, "Database error"); } res.send(results); }); connection.end(); }); }; Group.prototype.getByID = function(req, res) { console.log(this); res.send({name: "Group Item 1"}); }; module.exports = Group; 

Answer 1

You need to bind the function correctly.

 app.get('/v1/group', group.getAll); 

passes only the getAll function as a handler, but the function itself has no concept of this . this determined based on the associated context or based on how the function is called. This blog post is useful for understanding how a function context works.

 app.get('/v1/group', group.getAll.bind(group));