PCS JS Immersion

Node, NPM, Modules, and Async callbacks (Sudoku part 1)


Creating Modules

Modules are created by placing code in a separate file. You can put whatever code you want to in that file, but you can expose values and objects that you want to through the object module.exports. In Node.js, all modules start with an empty module.exports object, so you can add properties directly to that.

 * @file output-functions.js

module.exports.say = function(message) {

Alternatively, you can directly assign the module.exports object.

 * @file output-functions.js

module.exports = {
  say: function(message) {

These two ways of exposing the say function are identical.

Quick Status Check

  • Create a new module that exports the say function in one of the ways above.
  • Expose a new function from this module called sayHello that simply logs "hello world".
  • Expose a new property from this module called version that indicates the version of this module's code (call it 0.1.0 for now).
  • Try switching to the other method of populating module.exports.

Using Modules

In order to use a module, you use require to access the values that it has exposed. You pass require one argument, a string that allows it to find the module. For now we're going to be using a relative path, but we'll later see other uses.

 * @file app.js

var outputFns = require('./output-functions');
var fruits = ['kiwi', 'strawberry', 'banana'];



npm is Node's package manager. It's a command line tool that will allow us to easily install and manage different modules. Its website is also useful for searching for modules.

NPM demo

Fork this repo to your group's organization. Then have each person clone your fork (not the original).

Once you've cloned the repo, take a look at the file package.json and notice that two modules are listed in the "dependencies" section.

Lodash and Underscore

Lodash is a general-purpose Javascript library which has includes several utilities which may be useful for our Sudoku project. Lodash is a cousin of another library Underscore and can be used interchangeably with it for some frameworks (including Backbone). Here's a discussion of the differences between them.

Take a look at the Lodash documentation to find some function which sound useful. You might begin with any of these: 'intersection', union, difference, xor, unique, chunk.



I/O is short for Input/output. This has meanings in both computer hardware and software. I/O in software generally refers to communication with hardware devices, mice, keyboards, monitors, hard drives, etc.


I/O is slow. It may not feel that way to you, but to a computer it's orders of magnitude slower than the code you've written so far. The difference is about the same as the difference between the speed at which a baby crawls and the speed of a fighter jet.

What does this mean to you?

It means we shouldn't wait for the I/O. We should continue doing whatever we can while I/O happens. We use callbacks!

Reading Files

Here's how you read a file:

var fs = require('fs');

fs.readFile('./path/to/file', { encoding: 'utf8' }, function(err, contents) {

And now, a tangent!

UTF-8 is a string encoding. That certainly made it a lot clearer. Computers work with zeros and ones. You've certainly heard that before. These are called bits. Eight bits make up a byte. So one byte could be something like 01100001. Strings are actually just sequences of characters, right? Well each character is backed by these zeros and ones somehow. Sometimes each character is represented by a single byte, and there's a one-to-one mapping between bytes and characters. The standardized mapping used to do this is called ASCII. The letter a, for instance, is represented by the byte 01100001, and b by the byte 01100010 (if you've never counted in binary, that's just 01100001 + 00000001). It works okay, but has limitations—in particular, it can only encode English text.

One byte is only able to represent 256 (2^8) different options. If you start mapping out all of the characters you need to represent all of the languages of the world (plus the emoji), you'll realize you run out pretty quickly. In fact ASCII only uses 7 bits per character. So it can only represent 128 characters. That's not a lot. The Unicode standard was introduced to address this limitation.

There are a few common encodings used with Unicode. They are UTF-8, UTF-16, and UTF-32. Each of these specifies the number of bits that it uses by default. UTF-8 is 8 bits (or 1 byte). UTF-16 is 16 bits (or 2 bytes). UTF-32 is 32 bits (or 4 bytes). You can read up on them independently, but the main thing to know is that UTF-8 is the dominant encoding that supports all of the world's languages.

  • UTF-8 is a variable-length encoding that's backwards-compatible with ASCII. What this means is that you can read ASCII-encoded files by pretending they're UTF-8. "Variable-length encoding" means that the number of bytes per character varies depending on the character being encoded. Variable-length encodings are somewhat slower to work with in certain circumstances.
  • UTF-16 is variable-width as well (either 2 or 4 bytes), but incompatible with ASCII. Don't use it unless you have to.
  • UTF-32 is fixed-width. That means every character is represented by 4 bytes. This results certain files being four times larger than UTF-8, but has the advantage that you know the 2,000th character starts at the 8,000th byte (2000*4). Unlike UTF-8 and UTF-16, you don't have to read the first 2,000 characters to know if any of them are represented by multiple bytes. This random access to characters is only needed in specific applications.

Unicode characters are often represented in hexadecimal, so a in UTF-8 is 0x96, z is 0x7a, and 💩 is 0xf09f92a9.

Why does all of this matter? When you write code that receives user input, you either need to specify that the input must be a certain encoding or will need to consider the possibility that the input could be encoded with any encoding. These are the common encodings, but since the history of computers goes back pretty far, there are some older encodings, some failed encodings, and some platform-specific encodings that exist. If you've been using the Internet for a while, you've probably seen garbled text in emails or comments. This usually happens when someone sends text in one encoding, but your computer displays it in a different one, for one reason or another. This problem occurs much less often now that browsers have gotten much better about indicating the encoding they've sent.

Some other time, you can watch this YouTube video that explains UTF-8.


Write a program that takes two arguments, the paths to two different text files. The program should output all of the words that are used in both files.

Benchmarking I/O

We talked about speed earlier, and we know that reading files can be slow. Let's analyze the performance of the code that we're writing now, though. How can we do this?

var time = function(name) {
  var start = Date.now();
  return function() {
    console.log('[time] %s: %dms', name, Date.now() - start);

var done = time('reading file');
fs.readFile('path/to/file', function(err, buffer) {

Analyze the performance of both reading the files as well as the code that figures out which words are shared. See which takes longer. Don't include the actual logging of shared words in how long it takes. Just the part that figures out which words are shared.

How long does your program take to compare the words here? It will depend on the speed of your computer, but it should be less than 100ms.

If you've got some extra time, add an option to write the files to a file instead of to the terminal.

Project Of The Week: Sudoku Solver

For the first week's project, you'll be building an application that solves Sudoku puzzles. More details will follow soon, but here's a broad outline.

This project will have 4 modules, each implementing a constructor:

  • DigitSet: each instance of a DigitSet will represent a subset of 9 possible characters '1'-'9'. Each set of digits should be unordered and have no duplicates; that is, each each possible digit is either present or absent from the set. You'll need this to represent the partial knowledge available in each Sudoku square.

  • Grid: each Grid instance will represent a 9x9 grid of squares, each holding a known digit or a set of possible digits. Each square belongs to three different groups: a row, a column, and a 3x3 block.

    This object will have methods for retrieving certain squares and groups, for getting and setting the possible digits in each square, and for importing and exporting the entire grid into other formats.

  • Viewer: a viewer instance will be responsible for the display of a board, with various methods to represent its board as a string or, eventually, a browser graphic.

  • Solver: a solver instance will be able to deduce the contents of uncertain squares and eventually solve the entire grid.

You will be responsible for writing the first three modules, DigitSet, Grid, and Viewer, according to a specification which will be compatible with a Solver. On Tuesday, we'll explore precisely what a "module" is and how they work together in projects.

As part of your development process, you'll need to write tests which validate the behavior of your modules. On Wednesday, we'll talk about writing test with Mocha and Chai, frameworks for testing and assertions.

On Thursday, we'll provide at least an initial version of a Solver which should be able to solve the Sudoku puzzles encoded in your Grid and use your Viewer to display it.


  • square or cell: one of the 81 spaces holding a single digit

  • block: one of the nine 3x3 grids of adjacent squares

  • row: one of the nine 1x9 rows of squares

  • column or col: one of the nine 9x1 columns of squares

  • group: a set of nine squares, either a row, column, or block. Each square on the board will belong to three groups, one of each type.

  • token: any Javascript value which is proprietary to your module but held temporarily by another module (e.g. the Solver). Your token will be generated by one of your own methods and given back as an argument to other methods; only your module knows its format and interpretation. A token may be a primitive (e.g. string or number) or any kind of object, including Arrays.

Module APIs

DigitSet constructor

  • new DigitSet() --> digitSet instance
  • new DigitSet(digitArray) --> digitSet instance
  • new DigitSet(singleDigit) --> digitSet instance

DigitSet instance methods

  • digitSet.size() --> integer 0-9; how many digits are possible here
  • digitSet.set(arrayOfDigits)
  • digitSet.add(digit) --> undefined, modify original
  • digitSet.add(digitSet) --> undefined, modify original
  • digitSet.eliminate(digit) --> modify
  • digitSet.eliminate(digitSet)
  • digitSet.toString() --> string of digits in set
  • digitSet.toArray() --> array of digits

  • digitSet.isUncertain() --> boolean

  • digitSet.contains(digit) --> boolean

Grid constructor

  • new Grid(initString) --> grid instance
  • new Grid() --> grid instance

Grid instance methods

  • grid.cells() --> Array of all cell tokens
  • grid.cells(groupToken) --> Array of cell tokens associated with groupToken
  • grid.groups() --> Array of all group tokens
  • grid.groups(cellToken) --> Array of all group tokens associated with cellToken

  • grid.getRow() --> array of groupTokens (all rows)

  • grid.getCol() --> array of groupTokens (all cols)

  • grid.getBlock() --> array of groupTokens (all blocks)

  • grid.getRow(cellToken) --> groupToken (row)

  • grid.getCol(cellToken) --> groupToken (col)

  • grid.getBlock(cellToken) --> groupToken (block)

  • grid.getPossible(cellToken) --> digitSet

  • grid.setPossible(cellToken, digitSet) --> ?

  • grid.groupHas(groupToken) --> digitset

  • grid.groupNeeds(groupToken) --> digitset

  • grid.neighborhood(cellToken) --> digitSet of all known digits in same row, col, or block OR --> array of digitSets for all neighbors

  • grid.fromString(initString) --> set up grid with known digits

  • grid.toString() --> initString

  • grid.save() --> savedState

  • grid.restore(savedState)

  • grid.isInvalid() --> return true if notices any problems, else false?

  • grid.remaining() --> number (0-81) of uncertain cells

Viewer constructor

  • new Viewer(grid) --> viewer instance

Viewer instance methods

  • viewer.showCertain() --> string depicting 9x9 grid of digits known with certainty /* use own game with game.toString(), then squrify that string */

  • viewer.updateView() --> decorate finished group?

  • viewer.showPossible() --> richer display including partial info

  • viewer.showDebug()

  • viewer.showHint(cellToken) --> show number of possibilities at some cell

  • viewer.snapshot() --> store snapshot /* call grid.toString(), store that */

  • viewer.playback() --> replay all shapshots /* call series of viewer.showCertain() for each stored snapshot? */

  • viewer.startTimer() viewer.checkTime()

solver methods

  • solver.deduce(cellToken) --> new DigitSet ??

  • solver.logProgress()