Reference error is not thrown from MongoDB callback

Introduction All people know that if we call `undefined.test` we will receive the following error (same for both: NodeJS and Javascript): ``` $ node > undefined.test TypeError: Cannot read property 'test' of undefined at repl:1:11 at REPLServer.self.eval (repl.js:110:21) at Interface.<anonymous> (repl.js:239:12) at Interface.EventEmitter.emit (events.js:95:17) at Interface._onLine (readline.js:202:10) at Interface._line (readline.js:531:8) at Interface._ttyWrite (readline.js:760:14) at ReadStream.onkeypress (readline.js:99:10) at ReadStream.EventEmitter.emit (events.js:98:17) at emitKey (readline.js:1095:12) ``` That's correct! How did I find the problem? Passed week I wasted about 30 minutes in debugging the following problem: A script was stopping accidentally and no error was thrown. I had the `urls` variable that was supposed to be an object: ``` var urls = settings.urls || {}; ``` Then in next lines I needed to get `shop` key of `urls` that was a string: ``` var shop = urls.shop || "/"; ``` I started adding `console.log` to find the values of variables: ``` console.log(urls); // undefined var shop = urls.shop || "/"; console.log("Passed"); // This isn't run. ``` The problem in my script was that I was redefining a new `urls` variable that was making the `urls` `undefined`, but the question is: why cannot read property "shop" of undefined didn't appear here? Because `urls` was really `undefined`. We know that the following is happening in both: NodeJS and Javascript: ``` var a = 10; foo(function () { console.log(a); // undefined var a = 10; }); function foo(callback) { callback(); } ``` The question After debugging the problem I found that this problem comes from Mongo: inside of Mongo callbacks if we call `undefined.something` we DON'T get the error. I've created a small script that demonstrates this: ``` var mongo = require("mongodb"); // Mongo server var server = mongo.Server("127.0.0.1", 27017); var db = new mongo.Db("test", server, { safe: true }); console.log("> START"); // Open database console.log("Opening database."); db.open(function(err, db) { if (err) { return console.log("Cannot open database."); } // get collection console.log("No error. Opening collection."); db.collection("col_one", function(err, collection) { if(err) { return console.log(err) } // do something with the collection console.log("No error. Finding all items from collection."); collection.find().toArray(function(err, items) { if(err) { return console.log(err) } console.log("No error. Items: ", items); console.log("The following line is: undefined.shop." + "It will stop the process."); console.log(undefined.test); // THE ERROR DOES NOT APPEAR! console.log("> STOP"); // this message doesn't appear. }); }); }); ``` My questions are: - Why the error doesn't appear? Which is the reason? (It would be great to debug together the MongoDB source code to find it.) - Why the process is stopped when calling `undefined.something`? - How can be this solved? I've created a Github repository where you can download my small application that demonstrates the issue. Interesting: If we add a `try {} catch (e) {}` statement we find the error and the process continue showing the `STOP` message. ``` try { console.log(undefined.test); } catch (e) { console.log(e); } ``` LOGS: ``` > START Opening database. No error. Opening collection. No error. Finding all items from collection. No error. Items: [] The following line is: undefined.shop. It will stop the process. [TypeError: Cannot read property 'test' of undefined] > STOP ```