baris said:

Don't give up so easily

Yes! Yes! Yes!

JavaScript is a prototypal language, which makes it a bit confusing, but very powerful. You can structure your programs in various ways. I'm only gonna go into how it's (mostly) done in nodeJS:

Although you define objects with members, it is uncommon to actually create instances of those objects. JS has a new keyword that allows you to do that, but there it gets messy with this and what it means. Sometimes this points to an object, sometimes a function - it all depends on where you use it and who called what. You should read up on 'function scope' and 'closure', in case you're interested. I myself still like to use it. But anyway, most code you will encounter treats objects as static. Also, function parameters are passed by value rather than by reference (you may know this from C as passing a variable vs. passing a pointer). Let's have a look:

var aNumberVariable = 50;

myMethod = function(functionsNumberVariable) {
    functionsNumberVariable = 100;
    console.log(functionsNumberVariable);
};

myMethod(aNumberVariable);
console.log(aNumberVariable);

When you execute this snippet, the output will read 100 and then 50.

Your browser's console is awesome for fiddling! I use chrome.

Here you can see, that aNumberVariable is assigned 50, myMethod defined and then called. The parameter is passed to the function, where only its value is received, and put into a newly created variable functionsNumberVariable. This then is set to 100 and written to the console. The function call (myMethod()) is done synchronously, so everything following 'waits' for the function to finish/return. It does so with no value (aka undefined), but the console ignores that. Then the last line is executed, showing that aNumberVariable's value is still 50. (The showing undefined in the picture is actually not from anywhere in my code. I think that is the result of a function, the console's VM wraps your statements in. Once again the scope thingy.)

But this also means, that once myMethod is through doing its thing, you have no way to access functionsNumberVariable anymore (before mentioned closure or function scope). Even worse, it's a candidate to be gobbled up by the garbage collector in one of its next runs. Now, you could, of course, easily just return its value, but this is where nodeJS's magic of asynchronicity gets in between.

Everything below would have to wait for your function to return, so a read from a database or from a network resource could easily block your process for a lengthy amount of time - not so good.
Fortunately, JavaScript is capable of passing functions as parameters. . This - we're almost there - is the callback (in our case here; you can do heaps of other neat things with it). You basically pass the next step in your chain of processing as this function, passing it the values which you otherwise would have returned. I fail to come up with a quick and easy code example to run, right now. So I'll draw it in a diagram:
Untitled (3).png
At this point you'd be perfectly right to say: "Well, nice story. But there's also an invisible return in queryDatabase which the program has to wait for.". Yes, it is. That's why I painted its declaration yellowish. Your JavaScript runs also in nodeJS on a single thread, but under the hood, when V8 (JS engine) passes such calls to the I/O layer it utilises multi-threading. This game of passing callbacks is played along the chain, until V8 'queries' some system I/O function. There it is split off into another thread, from where (on the return path, if you will) the callbacks are called one after another. The actual functions returns are done as soon V8 has branched off this new thread. I'm not 100% sure about this last part, though. I urge everyone who knows more about this to correct me!

By the way: Every function comes with several 'invisible' variables set. One of which is arguments. That's an array of all parameters passed to the function. Disregarding the declarations (function(var1, var2)) you did. So even when you define your function as myMethod = function(), arguments is available and filled with everything that has been passed in the call. Great for exploring!

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