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introduction to node

Note: this rough draft has turned into https://github.com/maxogden/art-of-node#art-of-node

go to https://github.com/maxogden/art-of-node#art-of-node to view the newer versions of this document

Introduction to Node.js

Table of contents

Understanding node

Node.js is an open source project designed to let you write JavaScript programs that talk to networks, file systems or other I/O (Input/Output) sources. Usually this kind of task is either:

  • difficult to code but yields super fast results (like writing your web servers from scratch in C)
  • easy to code but not very speedy (like when someone tries to upload a 5GB file and your server crashes)

Node's goal is to strike a balance between these two: relatively easy to understand and use and fast enough for most use cases.

Node isn't either of the following:

  • A web framework (like Rails or Django, though it can be used to make such things)
  • A programming language (it uses JavaScript but node isn't its own language)

Instead, node is somewhere in the middle. It is:

  • Relatively easy to understand and use
  • Useful for I/O based programs that need to be fast and/or handle lots of connections

At a lower level, node can be described as a tool for writing two major types of programs:

  • Network programs using the protocols of the web: HTTP, TCP, UDP, DNS and SSL
  • Programs that read and write data to the filesystem or local processes/memory

What is an "I/O based program"? Here are some common I/O sources:

  • Databases (e.g. MySQL, PostgreSQL, MongoDB, Redis, CouchDB)
  • APIs (e.g. Twitter, Facebook, Apple Push Notifications)
  • HTTP/WebSocket connections (from users of a web app)
  • Files (image resizer, video editor, internet radio)

Node does I/O in a way that is asynchronous which lets it handle lots of different things simultaneously. For example, if you go down to a fast food joint and order a cheeseburger they will immediately take your order and then make you wait around until the cheeseburger is ready. In the meantime they can take other orders and start cooking cheeseburgers for other people. Imagine if you had to wait at the register for your cheeseburger, blocking all other people in line from ordering while they cooked your burger! This is called blocking I/O because all I/O (cooking cheeseburgers) happens one at a time. Node, on the other hand, is non-blocking, which means it can cook many cheeseburgers at once.

Here are some fun things made easy with node thanks to its non-blocking nature:

Like any good tool, node is best suited for a certain set of use cases. For example: Rails, the popular web framework, is great for modeling complex business logic and for the rapid prototyping of database-driven web applications. While it is technically possible to do the same type of thing using node, there would be definite drawbacks since node is designed for solving I/O problems and it doesn't know much about 'business logic'. Hopefully this guide will help you gain an intuitive understanding of the strengths of node so that you know when it can be useful to you.

Core modules

Firstly I would recommend that you get node installed on your computer. The easiest way is to visit nodejs.org and click Install.

Node has a relatively small core group of modules (commonly referred to as 'node core') that are presented as the public API that you are intended to write programs with. Fundamentally node is a tool used for managing I/O across file systems and networks, and it leaves other more fancy functionality up to third party modules. For working with file systems there is the fs module and for networks there are modules like net (TCP), http, dgram (UDP).

The whole point of node is to make it easy to deal with file systems and networks so it made sense to have one pattern that was used everywhere. The good news is that most of the patterns like these (there are only a few anyway) have been figured out at this point and it is very unlikely that node will change that much in the future.

In addition to fs and network modules there are a number of other base modules in node core. There is a module for asynchronously resolving DNS queries called dns, a module for getting OS specific information like the tmpdir location called os, a module for allocating binary chunks of memory called buffer, some modules for parsing urls and paths (url,querystring,path`), etc. Most if not all of the modules in node core are there to support nodes main use case: writing fast programs that talk to file systems or networks.

The aforementioned patterns that are used in node core are: callbacks, events, streams and modules.

Callbacks

Callbacks are just functions that are executed asynchronously, or at a later time. Instead of the code reading top to bottom procedurally, async programs may execute different functions at different times based on the order and speed that earlier functions like http requests or file system reads happen.

The difference can be confusing since determining if a function is asynchronous or not depends a lot on context. Here is a simple synchronous example:

var myNumber = 1
function addOne() { myNumber++ } // define the function
addOne() // run the function
console.log(myNumber) // logs out 2

The code here defines a function and then on the next line calls that function, without waiting for anything. When the function is called it immediately adds 1 to the number, so we can expect that after we call the function the number should be 2.

Let's suppose that we want to instead store our number in a file called number.txt:

var fs = require('fs')
var myNumber = undefined // we dont know what the number is yet since it is stored in a file

function addOne() {
  fs.readFile('./number.txt', function doneReading(err, fileContents) {
    myNumber = parseInt(fileContents)
    myNumber++
  }
}

addOne()

console.log(myNumber) // logs out undefined

Why do we get undefined when we log out the number this time? In this code we use the fs.readFile method, which happens to be an asynchronous method. Usually things that have to talk to hard drives or networks will be asynchronous. If they just have to access things in memory or do some work on the CPU they will be synchronous. The reason for this is that I/O is reallyyy reallyyy sloowwww. A ballpark figure would be that talking to a hard drive is about 100,000 times slower than talking to memory (RAM).

When we run this program all of the functions are immediately defined, but they don't all execute immediately. This is a fundamental thing to understand about async programming. When addOne is called it kicks off a readFile and then moves on to the next thing that is ready to execute. If there is nothing to execute node will either wait for pending fs/network operations to finish or it will stop running and exit to the command line.

When readFile is done reading the file (this may take anywhere from milliseconds to seconds to minutes depending on how fast the hard drive is) it will run the doneReading function and give it an error (if there was an error) and the file contents.

The reason we got undefined above is that nowhere in our code exists logic that tells the console.log statement to wait until the readFile statement finishes before it prints out the number.

If you have some code that you want to be able to execute over and over again or at a later time the first step is to put that code inside a function. Then you can call the function whenever you want to run your code. It helps to give your functions descriptive names.

Callbacks are just functions that get executed at some later time. Let's put our console.log statement into a function and pass it in as a callback.

var fs = require('fs')
var myNumber = undefined

function addOne(callback) {
  fs.readFile('./number.txt', function doneReading(err, fileContents) {
    myNumber = parseInt(fileContents)
    myNumber++
    callback()
  }
}

function logMyNumber() {
  console.log(myNumber)
}

addOne(logMyNumber)

Now the addOne function can get passed in an argument that will become the callback variable inside the addOne function. After readFile is done the callback variable will be invoked (callback()). Only functions can be invoked, so if you pass in anything other than a function it will cause an error.

When a function get invoked in javascript the code inside that function will immediately get executed. In this case our log statement will execute since callback is actually logMyNumber. Remember, just because you define a function it doesn't mean it will execute. You have to invoke a function for that to happen.

To break down this example even more, here is a timeline of events that happen when we run this program:

  • 1: the code is parsed, which means if there are any syntax errors they would make the program break.
  • 2: addOne gets invoked, getting passed in the logMyNumber function as callback, which is what we want to be called when addOne is complete. This immediately causes the asynchronous fs.readFile function to kick off. This part of the program takes a while to finish.
  • 3: with nothing to do, node idles for a bit as it waits for readFile to finish
  • 4: readFile finishes and calls its callback, doneReading, which then in turn increments the number and then immediately invokes the function that addOne passed in (its callback), logMyNumber.

Perhaps the most confusing part of programming with callbacks is how functions are just objects that be stored in variables and passed around with different names. Giving simple and descriptive names to your variables is important in making your code readable to others. Generally speaking in node programs when you see a variable like callback or cb you can assume it is a function.

You may have heard the terms 'evented programming' or 'event loop'. They refer to the way that readFile is implemented. Node first dispatches the readFile operation and then waits for readFile to send it an event that it has completed. While it is waiting node can go check on other things. Inside node there is a list of things that are dispatched but haven't reported back yet, so node loops over the list again and again checking to see if they are finished. After they finished they get 'processed', e.g. any callbacks that depended on them finishing will get invoked.

Imagine you had 3 async functions a, b and c. Each one takes 1 minute to run and after it finishes it calls a callback (that gets passed in the first argument). If you wanted to tell node 'start running a, then run b after a finishes, and then run c after b finishes' it would look like this:

a(function() {
  b(function() {
    c()
  })
})

When this code gets executed, a will immediately start running, then a minute later it will finish and call b, then a minute later it will finish and call c and finally 3 minutes later node will stop running since there would be nothing more to do. There are definitely more elegant ways to write the above example, but the point is that if you have code that has to wait for some other async code to finish then you express that dependency by putting your code in functions that get passed around as callbacks.

Events

TODO

Streams

Early on in the project the file system and network APIs had their own separate patterns for dealing with streaming I/O. For example, files in a file system have things called 'file descriptors' so the fs module had to have extra logic to keep track of these things whereas the network modules didn't have such a concept. Despite minor differences in semantics like these, at a fundamental level both groups of code were duplicating a lot of functionality when it came to reading data in and out. The team working on node realized that it would be confusing to have to learn two sets of semantics to essentially do the same thing so they made a new API called the Stream and made all the network and file system code use it.

THE REST IS TODO

Modules

TODO

Real-time apps

TODO

@jamesmatthewgarner

In the fourth paragraph in "Core Modules," you wrote "node care," instead of "node core."
Looking forward to reading the rest once it's completed!

@thefoxis

In the first section, I'd maybe elaborate more on networking, file systems and other I/O. This sounds pretty scary for someone not that familiar with programming. Maybe some more examples of what that kind of programs can actually do? I know you can have countless options but it's good to have some, at least broad picture in mind to know if this is useful for me at all or not.

You could add few lines about how to load modules in the core modules section. I know it's kinda mr. obvious thing, since it's used later on in code snippet but it would be good to have it.

I think the callback section is good, explaines the idea of async really well!

PS. WHERE ARE THE NODE CATS?!??!!1

@diorahman

Streams! More on streams please! ;-) just like what you've wrote here http://maxogden.com/node-streams.html

@vjk2005

For callbacks, don't go with the standard API example because it's confusing as hell — it shows only one half of the picture and people will then scratch their heads trying to figure out what goes on behind the scenes in the other half (speaking from personal experience). Since Callbacks are simply functions that are run after the calling function has completed all of its operations, we can illustrate this with a simple example:

function readFile( file, callback ) {
    // commands to do fileIO on "file" in a separate thread or whatever
    // store results of fileIO in a variable called "fileContents"

    callback( fileContents )
}

readFile( 'list_of_countries.txt', function( data ) {
    console.log( data )             // value stored in "fileContents" is logged to console
})

While this is essentially the same as the fs.readFile example used in the Callback section, by showing what happens to the Callback inside the readFile function, people can see the whole picture instead of just the Callback declaration half of the picture.

This allows people to "see" the mechanism that allows the Callback pattern to exist — the line-by-line execution style of Javascript means that while we don't know WHEN the file IO will complete, we do know WHERE the file IO will complete — the last line of readFile function! So to run something after the file IO completes, simply put it as the last line of the readFile function, and boom!, we have "Callbacks".

We don't know "when", but we know "where" — that's the secret to intuitively understanding Callbacks. So when people read this:

Callbacks are just functions that get executed at some later time.

...instead of wondering,

"At some later time?" How the hell does my code figure out what that "later time" is and execute the callback?"

they'll just go,

Oh wait, my code doesn't need to know WHEN to execute the callback, because it knows WHERE Javascript will go to execute the next instruction after async operations are complete —- the line right after the async operations are complete, i.e., the last line of the function, A-ha!

I'm a designer, not a programmer, so it took me a fair amount of digging around some very gnarly source code before I finally "got" what Callbacks are. Now that I've understood them, hopefully sharing the fruits of that experience will save other noobs like me lots of time and frustration.

@mattlowe

That's an excellent introduction to some of the new concepts that Node provides for developers! Thanks a lot!

Here are some minor suggestions that I would offer to an already excellent piece of work:

Original:

Most if not all of the modules in node care are there

Change to: (Changed care to core)

Most if not all of the modules in node core are there

Original:

Instead of the code reading top to bottom procedurally async programs may execute different functions at different times based on the order and speed that earlier functions like http requests or file systems reads happen.

Change to: (Added a comma and fixed plurality with systems)

Instead of the code reading top to bottom procedurally, async programs may execute different functions at different times based on the order and speed that earlier functions like http requests or file system reads happen.

Original:

Now the addOne function can get passed in an argument that will become the callback variable inside the addOne function. After readFIle is done the callback variable will be invoked (callback().

Change to: (Changed case of readFile, becuase JavaScript is case sensetive, and closed the ending parenthesis)

Now the addOne function can get passed in an argument that will become the callback variable inside the addOne function. After readFile is done the callback variable will be invoked (callback()).

Initially I thought there was a code error because fs.readFile executes callback instead of logMyNumber. Because this was a little confusing for me, I am suggesting a little more explanation on the program flow. (If I wasn't going to reuse addOne, I might have just replaced "callback()" with "logMyNumber()", though that is a lot less flexible in the long run.)

Original:

2: addOne gets invoked which immediately causes the asynchronous fs.readFile function to kick off. This program takes a while to finish
3: with nothing to do, node idles for a bit as it waits for readFile to finish
4: readFile finishes and calls its callback, doneReading, which then in turn increments the number and then immediately invokes addOne's callback, logMyNumber.

Change to: (Changed to explain the callback more clearly)

2: addOne gets invoked passing in the function that we want called once the command is complete. This immediately causes the asynchronous fs.readFile function to kick off. This program takes a while to finish.
3: with nothing to do, node idles for a bit as it waits for readFile to finish
4: readFile finishes and calls its callback, doneReading, which then in turn increments the number and then immediately invokes the function that addOne passed in (its callback), logMyNumber.

Thanks for such an excellent intro. I look forward to reading the rest!

@maxogden
Owner

thanks everyone for the edits/suggestions, i'm incorporating them now

@ikari-pl

I'd only suggest moving "Perhaps the most confusing part of programming with callbacks is how functions are just objects " paragraph before the "When a function get invoked in javascript". It actually can be very confusing for people coming from other backgrounds indeed, and while reading, people may have a "WTF" moment and stop and think instead of getting to the (already good) explanation :)

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