scriptcraft/docs/YoungPersonsGuideToProgrammingMinecraft.md

The Young Person's Guide to Programming in Minecraft

Table of Contents

• Introduction
• Installation
• Configuring your Server (optional)
• Learning Javascript
• First Steps
• Variables
• Functions
• Building stuff in Minecraft
• Common Block Materials
• Dimensions
• More shapes
• The Drone Object
  • Movement
  • Chaining - combining building and movement.
  • Exercise - Build a simple dwelling
  • Remembering where you started.
• Saving your work
• Your First Minecraft Mod!
• Parameters
• true or false
• More fun with true or false
• ...and Again, and Again, and Again,...
  • Counting to 100
  • Saying "Hi!" to every player
• While Loops
• utils.foreach() - Yet another way to process Arrays
  • Exercise
• Putting for loops to use - Building a Skyscraper
• Making Decisions
• Event-Driven programming
  • Stop listening to events.
• Keeping Score - Lookup tables in Javascript
• Counting block break events for each player
• Next Steps

Introduction

Minecraft is an open-ended 3D game where you can build and craft anything you like. Minecraft can be extended and enhanced using 'Mods' (short for 'modifications') - additional bits of code that are added to the Game. ScriptCraft is one such Mod - it lets you program in Javacript right within the game, making it possible to ...

• Build using simple javascript statements.
• Extend the game in other interesting ways - add new Items, change the game behaviour and create mini-games.

Minecraft can be played in single-player or multi-player mode (with friends). Most people play Minecraft in Multi-player mode where players connect to a Minecraft Server on the internet or locally (running on the player's computer).

Installation

CanaryMod is a version of the Minecraft server software which allows easy addition of 'Mods' and extensions to Minecraft. ScriptCraft is a 'Mod' for use with CanaryMod. Adding Mods to Minecraft can be difficult but CanaryMod makes it easy. Follow these steps to Install ScriptCraft on your computer...

1. Download and install CanaryMod then follow the CanaryMod Installation Instructions.

2. Start the CanaryMod server, then once it has started up, stop it by typing 'stop'. If you go to the CanaryMod folder (see step 1) you should see some new files and subfolders.

3. Download the latest version of the ScriptCraft Mod. Then copy the ScriptCraft.jar file to the plugins folder (This folder won't be created until you run CanaryMod for the first time (see previous step).

4. Start up the CanaryMod server again (see instructions for starting the server).

5. In the CanaryMod command window type op {your_username} and hit enter, replacing {your_username} with your own minecraft username. This will give you operator access meaning you can perform more commands than are normally available in Minecraft. You should make yourself a server operator (Server operators have full privileges for the server) permanently by editing the ops.txt file and adding your username (one username per line).

6. In the CanaryMod command window type js 1 + 1 and hit enter. You should see > 2 .

... Congratulations! You just installed your own Minecraft Server with the ScriptCraft Mod and are now ready to begin programming in Minecraft.

Normally, Minecraft Mods are written in Java. This makes writing your own extension or game rules difficult because you must first learn Java. Java is different enough from Javascript. With the ScriptCraft plug-in installed, you don't have to learn Java, you can extend and customize Minecraft your way using Javascript. Javascript is easier to learn than Java but it's also more flexible and powerful and is used for creating interactive web sites and many other applications.

Configuring your Server (optional)

Once you've installed CanaryMod, depending on your specific needs, you might want to consider setting the following properties in the server.properties or world.properties files...

# completely flat worlds are best for building from scratch
level-type=FLAT
generate-structures=false

# creative mode
gamemode=1
pvp=false

# turns off authentication (for classroom environments)
online-mode=false
spawn-npcs=false
spawn-monsters=false

Learning Javascript

To begin creating cool stuff in Minecraft using ScriptCraft, you don't have to know much JavaScript. ScriptCraft comes with lots of functions to help you create buildings of any size, and lets you experiment while you play. However, as you learn Javascript you will be able to create cooler stuff in Minecraft - not just buildings, you'll be able to add new rules and items to the game - even create mini-games for you and your friends. If you want to get started learning JavaScript, check out this fun Javascript Tutorial. If you want to dive right in to ScriptCraft, read on...

First Steps

If you don't already know Javascript, don't worry, you'll learn a little about Programming and Javascript along the way. You've set up a Minecraft server and are ready to connect ...

1. Launch Minecraft.

2. Click 'Multi-Player'

3. Click 'Add Server'

4. Type any name you like in the name field then type localhost in the address field. localhost is a special internet address that points to your own computer.

5. Click 'Join Server' to join the server. If the version of Minecraft is incompatible with the version of the server you will not be able to connect to the server. To fix this, you can create a Minecraft profile in your client. Profiles let you decide which version of Minecraft client you want to run so that your client and server are compatible.

6. Once you've joined the game, press the / key located at the bottom right of your keyboard. A prompt will appear. Type the following then press enter: js 1 + 1 The number 2 should be displayed.

... Well Done! You've just confirmed you can run Javascript code from within the Minecraft Console.

Variables

A variable is how you name something for the computer (and you the programmer) to remember. You create a new variable in Javascript using the var keyword...

/js var location = 'Blackrock Castle'

... creates a new variable called location and stores the text Blackrock Castle in it. Now the computer has a new item in its memory called location. We can use that name like this...

/js echo( location )

... and the following is displayed...

Blackrock Castle

...You might be wondering why there's no enclosing ' single quotes. When telling the computer to store some text, you have to put ' (that's the single-quote character) at the start and end of the text. The computer doesn't store these quote characters, only the text between them. The computer will store the variables while the Minecraft Server is running. Repeat the last command you entered by pressing the / key then the UP arrow key on your keyboard, then pressing enter. You can repeat that statement as many times as you like and the computer will always display the same value. You can change the value like this...

/js location = 'Mahon Point'

...notice this time I didn't use the var keyword. I didn't need to. The var keyword is only needed when you first create the variable. Now execute this command...

/js echo( self, location ) 

...and it displays...

Mahon Point

Variables can be created and changed easily in Javascript. Along with the variables you'll create in your in-game commands and scripts, there are handy free variables created for you by ScriptCraft. One such variable is self, it contains information about the current player (that's you)...

/js echo ( self, self.name )

... displays something like the following...

walterh

... for me but the message displayed will be different for every player.

Functions

ScriptCraft comes with a couple of extra functions not normally found in Javascript. These functions will help you build new structures and buildings which would otherwise take hours to build by hand. Before looking at the building functions let's look at the echo() function.

echo() - as its name implies - will echo back at you whatever you tell it. For example, type ...

/js echo( self, 'Hello')

... and the game will display...

Hello

... type ...

/js echo( self, 5 + 7 )

... and the game will display...

12    

... While you can now use Minecraft to help with Maths homework - I don't recommend it. Homework and Minecraft don't mix! The echo() function will display anything you tell it to - Text, Numbers and other types...

/js echo( self, new Date() )

... prints today's date. If the statement above looks confusing - don't worry - new Date() creates a new date object - I'll talk about objects later ...

Tue Jan 08 2013 20:53:37 GMT-0000 (GMT)

echo() is a very useful function but it is not part of the Javascript Language. You can't use it outside of Minecraft. There are many other functions in Javascript all of which you can also use in Minecraft. For example...

/js Math.max( 6, 11 )

... returns the larger of the 2 numbers you give it (max is short for maximum). While...

/js Math.min( 6, 11 )

... returns the smaller of the 2 numbers. That's another thing - functions can return stuff. You can store the result of a function (what it returns) in a variable like this...

/js var biggest = Math.max( 6, 11 )

... Now type...

/js biggest

... Not all Javascript functions return data but most do. As well as the functions provided to you by the Javascript Language and ScriptCraft, you can write your own functions like this...

/js function whatTimeIsIt () { return new Date() }

... Here you've created a new function called whatTimeIsIt and told the function it should return a new Date object every time it's called. You'll notice the above statement didn't actually do anything

• it certainly didn't display the current time. That's because all you've done is is say what the function should do when it's called, you haven't called it yet. To call the function...

  /js whatTimeIsIt()

... The current time is displayed. Congrats! You've just written your first Javascript function - you're well on your way to becoming a Minecraft Modder. There are many functions for working with Text, numbers and dates in Javascript...

/js Math.random()

... prints out a random number every time you call it. Try it! Then press the / key then the UP Arrow key to repeat the last statement in your in-game console. You'll see the number displayed is different each time. Think of Math.random() as a Dice with many many sides. You can rely on it to never return the same value twice.

Building stuff in Minecraft

Now we get to the fun stuff - creating structures and buildings in Minecraft. Building by hand is fun but tedious when you want to build big - Towers, Castles and Fortresses. That's where ScriptCraft comes in. ScriptCraft comes with a couple of javascript functions that can be combined to build interesting things. Let's start small though to get a feel for how ScriptCraft's building functions work. The function you'll probably use most for building is called box() and - as its name implies - it is used to create cubes and cuboids of any size. A cube is a 3D shape whose sides are all the same length. A cuboid is a 3D shape whose width, height and length can differ.

You can create a Cube or a Cuboid in ScriptCraft using the box() function. You must tell the function what material you want the shape to be made of. For example, in the game, point the cross hairs at the ground, then type the following and hit enter...

/js box( blocks.oak )

... This will change the targeted block to wood. What's happened here is the box() function has created a single new wooden block. blocks is another one of those free variables you get in ScriptCraft, you can see a list of block materials by typing ...

/js blocks.

... then pressing the TAB key. Repeatedly pressing the TAB key will cycle through all of the block materials. Alternatively, you can see many more current materials and the numbers Minecraft uses for them by visiting the Minecraft Data Values site.

Common Block Materials

In Minecraft Programming, Materials aren't known by their name, instead numbers (sometimes 2 numbers) are used to indicate which material should be used. For example the number 2 is grass, 1 is cobblestone etc, while 5 is wood (oak). There are different types of wood so the text '5:1' means Spruce, '5:2' means Birch and '5:3' means Jungle wood. There are many different materials in the Minecraft world, the most commonly used materials for building are:

• '4' - Cobblestone or blocks.cobblestone
• '5' - Wooden Planks or blocks.oak
• '5:2' - Birch wood Planks (light wood) or blocks.birch
• '98' - Stone bricks or blocks.brick.stone
• '45' - Red bricks or blocks.brick.red
• '68' - Sign or blocks.sign
• '102' - Glass panes (for windows) or blocks.glass_pane

You can create a single wooden block using the numeric values or the blocks variable. For example...

/js box( '5' )

... and ...

/js box( blocks.oak ) 

... both do exactly the same thing but I personally prefer /js box( blocks.oak ) because it's easier to remember. For reference, here is a chart of all of the blocks (not items) in the Minecraft world...

Dimensions

box() can do more than just create single blocks - it can create cubes and cuboids of any size. Take a look at the following picture which shows how shapes are measured in 3D space. There are 3 dimensions (or sizes) to consider.

1. Width
2. Height
3. Depth (or length) - not to be confused with how deep underground a mine-shaft can go. Think of Depth (or length if you prefer) as how far away you want something to extend.

More shapes

• box0( block, width, height, depth ) - creates an empty box (with the insides hollowed out - perfect for dwellings. box0 will remove both the floor and ceiling too.
• cylinder( block, radius, height ) - creates cylinders, perfect for Chimneys.
• cylinder0( block, radius, height ) - creates empty cylinders - perfect for Towers. cylinder0 will remove both the floor and ceiling too.
• prism( block, width, depth ) - creates a Prism - good for roofs.
• prism0( block, width, depth ) - creates an empty prism.

The Drone Object

ScriptCraft is a Minecraft Mod that lets you execute Javascript code in the game. It also lets you write your own Mod in Javacript. One such mod that comes bundled with ScriptCraft is called the Drone mod. The Drone is an (invsible) object you create every time you execute any of the building or movement functions. When you execute...

/js box(5,3,2,4)

... a new Drone object is created and does the work of building on your behalf. Think of a Drone as something like a remote control plane that can move about freely and build things for you. Moving the Drone is easy...

Movement

• up( numberOfBlocks ) - moves the Drone Up. For example: up() will move the Drone 1 block up. You can tell it how many blocks to move if you want it to move more than one block.
• down( numberOfBlocks ) - moves the Drone Down.
• left( numberOfBlocks ) - moves the Drone Left.
• right( numberOfBlocs ) - moves the Drone Right.
• fwd( numberOfBlocs ) - moves the Drone Forward (away from the player).
• back( numberOfBlocs ) - moves the Drone Back (towards the player)
• turn( numberOfTurns ) - Turns the Drone Clock-wise (right). For example: turn() will make the Drone turn right 90 degrees. turn(2) will make the Drone turn twice so that it is facing in the opposite direction.

Chaining - combining building and movement.

You can make a Drone move around before and after building by daisy-chaining the building and movement functions together. In the game, point at the ground then type the following...

/js up(1).box( blocks.oak ).fwd(3).box( blocks.oak )

A series of 2 boxes is created 3 blocks apart.

Exercise - Build a simple dwelling

OK. You know enough now about the Drone functions to be able to build a simple dwelling. The dwelling should be a hollow building with a sloped roof. Don't worry about doors or windows for now. The walls should be made of Cobblestone ('4') and the roof made of wood ('5'). You can use the following Drone functions to create a dwelling 7 blocks wide by 3 blocks high by 6 blocks long with a wooden sloped roof. It's up to you to figure out how.

• up()
• box0()
• prism0()

Your dwelling should end up looking something like this...

Remembering where you started.

Sometimes when you're building something big that requires lots of manoeuvering by your Drone, you need to leave breadcrumbs as you go so your Drone can return to where it started. Every new Drone has a 'start' checkpoint that it can return to by executing move('start') ...

/js box('5').up(3).left(4).box('1').turn(3).fwd(5).right().box('1').move('start')

... A genius would have trouble figuring out how to get back to where they started. Fortunately, they don't have to - the move('start') function will take the Drone back to its starting point.

• chkpt( breadCrumb ) - Leaves a mark at your Drone's current location so it can return there later. Think of it as giving a name to the place where your Drone is located. chkpt is short for Check-Point - a place in a game where you usually save your progress.

• move( breadCrumb ) - Moves your Drone to a location you named using chkpt() . It brings your Drone back to the place where you saved it.

Both chkpt() and mark() are useful for when you want to build complex things that require your Drone to move about a lot ( for example, Castles, mansions, palaces, etc).

Saving your work

You can build cool things using the in-game command-prompt and the /js command but sooner or later you'll probably want to build something more complex and save your commands so you can run them again when you quit the game and start it up again.

Notepad++ Is a special text editor (like Notepad which comes installed on every Windows machine) that is well suited for writing code. If you don't already have it on your machine, you can install Notepad++ here. I recommend using NotePad++ rather than plain old Notepad because it understands Javascript. If you prefer coding on a Macintosh, then TextWrangler is a good programming editor which also understands Javascript code.

Your First Minecraft Mod!

So, You've learnt a little bit about Javascript and what the Drone() object can do, let's use that knowledge to create a Minecraft Mod!

Once you've installed Notepad++, Launch it, create a new file and type the following...

exports.greet = function( player ) {
    echo( player, 'Hi ' + player.name);
}

... then save the file in a new directory scriptcraft/plugins/{your_name} (replace {your_name} with your own name) and call the file greet.js (be sure to change the file-type option to '. All Files' when saving or NotePad++ will add a '.txt' extension to the filename. Now switch back to the Minecraft game and type...

/js refresh()

... to reload all of the server plugins. Your mod has just been loaded. Try it out by typing this command...

/js greet(self)

... it should display ...

Hi {your-username-here}

... where {your-username-here} will be replaced with your own minecraft username. Congratulations - You've just written your very first Minecraft Mod! With ScriptCraft installed, writing Minecraft Mods is as simple as writing a new javascript function and saving it in a file in the scriptcraft/plugins directory. This function will now be avaible every time you launch minecraft. This is a deliberately trivial minecraft mod but the principles are the same when creating more complex mods.

The exports variable is a special variable you can use in your mod to provide functions, objects and variables for others to use. If you want to provide something for other programmers to use, you should export it using the special exports variable. The syntax is straightforward and you can use the same exports variable to export one or more functions, objects or variables. For example...

thrower.js

exports.boo = function(player){
  echo( player, 'Boo!');
}
exports.yo = function(player){
  echo( player, 'Yo!');
}

... is a plugin which provides 2 javascript functions called boo() and yo() which can be invoked from the in-game prompt like this /js boo(self) or /js yo(self).

Parameters

If you want to change the greet() function so that it displays a greeting other than 'Hi ' you can change the code in the greet() function, or better still, you can use Parameters. Parameters are values you provide to a function so that the function behaves differently each time it is called.

Change the greet() function so that it looks like this...

exports.greet = function ( greeting , player) {
    echo( player, greeting + player.name );
}

... Save your greet.js file and issue the /js refresh() command in minecraft. Now enter the following command in Minecraft...

greet('Hello ',self);

... Now try ...

greet('Dia Dhuit ',self);

... you should see the following messages in your chat window...

Hello {your name}
Dia Dhuit {your name}

... Parameters let you provide different values to functions each time they're called. As you'll see later, Parameters are very useful when changing the behaviour of MineCraft.

true or false

Try entering each of the following statements and make a note of the answers given by minecraft...

/js 1 < 2

/js 1 > 2

... the answer given by the first statement ( 1 < 2 ) should be true since 1 is less than 2. The < symbol - usually found near the bottom right of your keyboard - means test to see if something is less than another so 1 < 2 is a way of asking the computer "is 1 less than 2 ?". This is a silly example of course since we know 1 is less than 2 but when dealing with variables we might not know in advance what its value is or whether it's greater than (bigger) or less than (smaller) another number or value. The result of the 2nd statement (1 > 2) should be false since 1 is not greater than 2. Now try this...

/js 1 = 2

... The result won't be what you expected. You'll see an Error message

• that's OK. What's happened here is I've tried to test to see if 1 is equal to 2 but I've made one of the most common mistakes even experienced programmers make. If you want to test to see if two things are the same, you use == that's two equals signs right next to each other. Let's try again...

  /js 1 == 2

... this time you should get an answer false since 1 obviously isn't equal to 2. These are the different operators used when comparing things...

• < Is less than ?
• > Is greater than ?
• == Is equal to ?
• <= Is less than or equal to ?
• >= Is greather than or equal to ?
• != Is not equal to ?

... try comparing some more numbers yourself - say for example, compare the ages of your friends or siblings to your own age.

More fun with true or false

TODO: This needs to be changed for CanaryMod

You can find out if you can Fly in minecraft by typing the following statement...

/js self.allowFlight 

... the result will be true or false depending on whether you can fly or not. You can turn on and off your ability to fly by setting your allowFlight property to true or false. Try it...

/js self.allowFlight = true

... Now you can fly! To turn off flight...

/js self.allowFlight = false

... and you come crashing down to earth. This is just one example of how true and false are used throughout ScriptCraft - these are called boolean values - named after George Boole, a 19th Century Maths Professor at University College Cork. There are plenty more examples of boolean values in Minecraft. You can find out if monsters are allowed in your minecraft world by typing the following statement...

/js self.location.world.allowMonsters

... The result of this statement will be either false (Phew!) or true (Yikes!) depending on how your server has been configured. However, typing the following statement doesn't work as expected...

/js self.location.world.allowMonsters = true

... This statement won't work as expected - it will give an Error message. This is because sometimes we can read variables but we can't change them the same way we read them (this is because of how Javascript, Java and the CanaryMod API work together). To turn on or off the spawning of monsters, type the following...

/js self.location.world.setSpawnFlags(false, true)

... the setSpawnFlags() method takes 2 parameters, the first parameter says whether or not monsters can spawn, and the 2nd says whether or not Animals can spawn. (SIDENOTE: You may be wondering how to change other aspects of the Minecraft game - pretty much all aspects of the game can be changed. Changes are made using what are called API calls - these are calls to functions and methods in Minecraft - you can read more about these on the CanaryMod API Reference.)

...and Again, and Again, and Again,...

One of the things Computers are really good at is repetition. Computers don't get tired or bored of doing the same thing over and over again. Loops are handy, if you want to run the same code over and over again, each time with a different value.

Counting to 100

At the in-game command prompt (hint: press 't') type the following then hit Enter...

/js for (var i = 1 ; i <= 100 ; i = i + 1) { echo( i ); }

... The above code will count from 1 to 100. The first thing you'll notice if you run the above code is how quickly the count happened. You're probably curious how long it would take to count to 1000. Try it out for yourself. Change the above line of code so that it counts to 1000 instead of 100. If you're feeling adventurous, see how long it takes to count to ten thousand, one hundred thousand or even one million.

The for statement is useful when you want to repeat something over and over. It has 4 parts...

1. The initialiser: var i = 1 - this happens once at the start of the loop.
2. The test: i <= 100 - this happens at the start of each run around the loop. If the test fails, then the loop ends.
3. The increment: i = i + 1 - this happens at the end of each run around the loop. If you didn't have a statement here, the loop might never finish. i = i + 1 is often written as i++ - it's shorter and does basically the same thing.
4. The body - everything that appears between the { and } (opening and closing curly braces).

for loops becomes very useful when you combine it with Arrays - remember, an Array is just a list of things, for example - the players connnected to a server, the worlds of a server and so on.

Saying "Hi!" to every player

At the in-game command prompt type the following then hit Enter...

/js var utils = require('utils');
/js var players = utils.players();
/js for (var i = 0;i < players.length; i++){ echo(players[i], 'Hi!'); }

... Lets look at these statements in more detail. We had to enter the statements on a single line at the in-game command prompt but the statements could be written like this...

var utils = require('utils');
var players = utils.players();
for (var i = 0;i < players.length; i++) { 
  echo(players[i], 'Hi!'); 
}

... On the 2nd line, a new variable players is created and assigned a value by calling utils.players(). On the next line, the for loop is declared, a counter variable i is set to 0 (zero - arrays in javascript start at 0 not 1) and each time around the loop is tested to see if it's less than the number of players online. At the end of each run around the loop the i variable is incremented (increased by 1) so that the next player can be messaged. Inside the body of the for loop (everything between the opening { and closing } curly braces) the players[i] expression refers to the player in the players array at position[i]. Imagine there are 4 players online on a minecraft server, the players array might look like this...

• players[0] = 'CrafterJohn'
• players[1] = 'MinerPaul'
• players[2] = 'ExplorerRingo'
• players[3] = 'TraderGeorge'

... in this case players.length will be 4 (since there are 4 online players), the for-loop will go around 4 times starting from position 0 and going all the way up to position 3, sending a message to each of the players in the array. It's time for a new scriptcraft function. Open the hi.js file you created earlier (using NotePad++ , TextWrangler or your editor of choice) and add the following code at the bottom of the file...

var utils = require('utils');
exports.hiAll = function () {
  var players = utils.players();
    player,
    i;
  for ( i = 0; i < players.length; i++) {
    player = players[i];
    echo( player, 'Hi!' );
  }
}

... save the file, at the in-game command prompt type /js refresh() and then type /js hiAll(). This will send the message Hi! to all of the players connected to your server. You've done this using a for loop and arrays. Arrays and for loops are used heavily in all types of software, in fact there probably isn't any software that doesn't use for loops and Arrays to get things done.

While Loops

Another way to repeat things over and over is to use a while loop. The following while loop counts to 100...

var i = 1;
while ( i <= 100 ) {
    console.log( i );
    i = i + 1;
} 

A while loop will repeat until its condition is false - the condition in the above example is i <= 100 so while i is less than or equal to 100 the code within the while block (everything between the starting { and ending } curly braces) will run. It's important that you change the variable being tested in a while loop, otherwise the while loop will never it - it will run forever. Try running the following code...

/js var i = 1; while (i <= 100){ echo( i ); }

The code above will contine printing out the number 1 until the end of time (or until you unplug your computer). That's because the i variable is never incremented (remember - incrementing just means adding 1 to it) so i will always be 1 and never changes meaning the loop goes on forever. Again - this is a mistake even experienced programmers sometimes make.

Just like for loops, while loops can be also be used to loop through arrays. The following loop prints out all of the players on the server...

var utils = require('utils');
var players = utils.players();
var i = 0;
while ( i < players.length ) {
    console.log( players[i] );
    i = i + 1;
}

... whether you chose to use a for loop or a while loop is largely a matter of personal taste, for loops are more commonly used with Arrays but as you see from the example above, while loops can also loop over Arrays.

utils.foreach() - Yet another way to process Arrays

Both the for statement and while statement are standard commonly used javascript statements used for looping. ScriptCraft also comes with a special function for looping called utils.foreach(). utils.foreach() is a convenience function, you don't have to use it if you prefer the syntax of javascript's for and while loops. utils.foreach() takes two parameters...

1. An array
2. A function which will be called for each item in the array.

...that's right, you can pass functions as parameters in javascript! Let's see it in action, the following code will console.log() (print) the name of each online player in the server console window...

var utils = require('utils');
var players = utils.players;
utils.foreach( players, console.log );

... in the above example, the list of online players is processed one at a time and each item (player) is passed to the console.log function. Note here that I used console.log not console.log(). The round braces () are used to call the function. If I want to pass the function as a parameter, I just use the function name without the round braces. The above example uses a named function which already exists ( console.log ), you can also create new functions on-the-fly and pass them to the utils.foreach() function...

/*
  give every player the ability to fly.
*/
var utils = require('utils');
var players = utils.players();
utils.foreach( players, function( player ) { 
  player.capabilities.setMayFly(true);
} );

... Another example, this time each player will hear a Cat's Meow...

/*
  Play a Cat's Meow sound for each player.
*/
var utils = require('utils');
var players = utils.players();
var sounds = require('sounds');
utils.foreach( players, function( player ) { 
  sounds.catMeow( player );	       			  	     
} );

Exercise

Try changing the above function so that different sounds are played instead of a Cat's Meow. To see all of the possible sounds that can be played, load the sounds module at the in-game prompt using the following statement:

/js var sounds = require('sounds');

... then type /js sounds. and press the TAB key to see a list of all possible sounds.

Loops are a key part of programming in any language. Javascript provides for and while statements for looping and many javascript libraries also provide their own custom looping functions. You should use what you feel most comfortable with.

Putting for loops to use - Building a Skyscraper

For loops can be used to build enormous structures. In this next exercise I'm going to use a for loop to build a skyscraper. This skyscraper will be made of Glass and Steel (just like most skyscrapers in real-life). The first thing to do is see what a single floor of the skyscraper will look like. Place a block (of any type) where you want to eventually build the skyscraper, then while your cursor is pointing at the block, type the following into the in-game prompt...

/js var drone = box(blocks.iron,20,1,20).up().box0(blocks.glass_pane,20,3,20).up(3)

... you should a large (20x20) iron floor with 3 block high glass all around.

... A skyscraper with just a single floor isn't much of a skyscraper so the next step is to repeat this over and over. This is where for loops come in. Open your favorite text editor and create a new file in your scriptcraft/plugins/{your-name} directory, name the file myskyscraper.js, then type the following code and save:

function myskyscraper( floors ) {
  var i ;
  if ( typeof floors == 'undefined' ) {
    floors = 10;
  }
  // bookmark the drone's position so it can return there later
  this.chkpt('myskyscraper'); 
  for ( i = 0; i < floors; i++ ) {
    this
      .box(blocks.iron,20,1,20)
      .up()
      .box0(blocks.glass_pane,20,3,20)
      .up(3);
  }
  // return the drone to where it started
  this.move('myskyscraper'); 
};
var Drone = require('drone'); 
Drone.extend( myskyscraper );

So this takes a little explaining. First I create a new function called myskyscraper that will take a single parameter floors so that when you eventually call the myskyscraper() function you can tell it how many floors you want built. The first statement in the function if (typeof floors == 'undefined'){ floors = 10; } sets floors to 10 if no parameter is supplied. The next statement this.chkpt('myskyscraper') saves the position of the Drone so it can eventually return to where it started when finished building (I don't want the drone stranded atop the skyscraper when it's finished). Then comes the for loop. I loop from 0 to floors and each time through the loop I build a single floor. When the loop is done I return the drone to where it started. The last 2 lines load the drone module (it must be loaded before I can add new features to it) and the last line extends the 'Drone' object so that now it can build skyscrapers among other things. Once you've typed in the above code and saved the file, type /js refresh() in your in-game prompt, then type:

 /js myskyscraper(2);

A two-story skyscraper should appear. If you're feeling adventurous, try a 10 story skyscraper! Or a 20 story skyscraper! Minecraft has a height limit (256 blocks from bedrock) beyond which you can't build. If you try to build higher than this then building will stop at that height.

I'll leave it as an exercise to the reader to create a city block of skyscrapers, 5 blocks apart using a for loop. Once you've figured that out, creating an entire city of blocks of skyscrapers is the next logical step. Of course, Minecraft doesn't have the same constraints as real-world densely populated areas so let your imagination go wild.

Making Decisions

All the programs we have seen so far have been fairly predictable - they went straight through the statements, and then went back to the beginning again. This is not very useful. In practice the computer would be expected to make decisions and act accordingly. The javascript statement used for making decisions is if. While standing on the ground in-game, type the following at the command prompt:

/js if ( self.onGround ) { echo('You are not flying!'); }

the following message should have appeared on your screen:

You are not flying!

Now double-tap the space bar to start flying in-game (tap the space bar twice in rapid succession), then press and hold space to rise above the ground. Now enter the same statement again (If you don't want to type the same statement again, just press / then press the UP cursor key on your keyboard, the statement you entered previously should reappear.

/js if ( self.onGround ) { echo('You are not flying!'); }

This time no message should appear on your screen.

The if statement tests to see if something is true or false and if true then the block of code between the curly braces ( { and } ) is executed - but only if the condition is true. The condition in the above example is !self.onGround (self is not on ground) which will be true if you are currently flying or false if you aren't.

What if you wanted to display a message only if a condition is not true ? For example to only display a message if the player is not on the ground:

/js if ( !self.onGround ) { echo ('You are flying!'); }

This code differs in that now there's a ! (the exclamation mark) before self.onGround. The ! symbol negates (returns the opposite of) whatever follows it.

What if you want to display a message in both cases - whether you're flying or not? This is where the if - else construct comes in handy. Open your favorite editor and type the following code into a new file in your scriptcraft/plugins directory...

exports.flightStatus = function( player ) {
  if ( player.onGround ) { 
    echo(player, 'You are not flying!' );
  } else {
    echo(player, 'Hey, You are flying!' );
  }
}

... now type /js refresh() at the in-game prompt then type /js flightStatus(self) and an appropriate message will appear based on whether or not you're currently flying. Type the /js flightStatus() command while on the ground and while flying. The message displayed in each case should be different.

Event-Driven programming

So far we've written code which executes when you invoke the /js command. What if - for example - you want to have some special behaviour which occurs when a player joins the game? What if you wanted to display a custom welcome message (in addition to the MotD - message-of-the-day which is configurable in your server.properties file) ? This is where Event-Driven Programming comes in. Event-Driven Programming is just a fancy way of saying 'Do this when that happens' where 'this' is a function you define, and 'that' is some event which occurs. There are hundreds of events in the minecraft game...

• Every time someone joins the server - that's an event!
• Every time someone breaks a block - that's an event!
• Every time someone shoots an arrow - that's an event! and so on...

You can write a function which will be called whenever a specific type of event occurs, it's probably best to illustrate this by example. The following code sends a message to any player who breaks a block in the game...

function myBlockDestroyHook( event ){
  var breaker = event.player;
  echo( breaker, 'You broke a block');
}
events.blockDestroy( myBlockDestroyHook );

The events.blockDestroy() function is just one of the many events functions which can be used to register a function to be called whenever a particular type of event occurs. In the above code the blockDestroy function takes as a parameter a function I want to be called when that event occurs. The function I want called in turn takes 1 parameter. The event object has all the information about the event which just occurred. I can tell who broke the block and send a message to the player. The important thing to note is that the myBlockDestroyHook function defined above will not be called until a player breaks a block. Try it - save the above code in a new file in the scriptcraft/plugins directory then type /js refresh() to reload scriptcraft. Then break a block in the game and you should see the message 'You broke a block'.

There are many types of events you can listen for in Minecraft. You can browse all possible event registration functions in the API Reference.

For custom events (events which aren't in the net.canarymod.hook tree) just specify the fully qualified class name instead. E.g. ...

events.on ( net.yourdomain.events.YourEvent, function( event ) {
    ...
});

Stop listening to events.

If you want an event handler to only execute once, you can remove the handler like this...

function myBlockDestroyHook( evt ) { 
  var breaker = evt.player;
  echo( breaker, 'You broke a block');
  this.unregister();
} 
events.blockDestroy( myBlockDestroyHook );

The this.unregister(); statement will remove this function from the list of listeners for the event. The this keyword when used inside an event handling function refers to a Listener object provided by ScriptCraft, it has a single method unregister() which can be used to stop listening for events.

To unregister a listener outside of the listener function...

function myBlockDestroyHook( evt ){
  var breaker = evt.player;
  echo( breaker, 'You broke a block');
}
var myBlockBreakListener = events.blockDestroy( myBlockDestroyHook );
...
myBlockBreakListener.unregister();

Keeping Score - Lookup tables in Javascript

In the Event-Driven Programming section, I defined a function which displayed a message to players every time they broke a block. Imagine if I wanted to keep a count of how many blocks each player has broken? This is where Javascript's Objecct literals come in handy. An object literal in javascript is simply a way of creating a new Object on-the-fly in your code. This is an example...

var myNewObject = { name: 'walter', country: 'Ireland' };

... I created a new object with two properties 'name' and 'country'. The notation used to create this object is called JSON which is short for JavaScript Object Notation. If I want to find out the 'country' property of the myNewObject variable there are a few ways I can do it...

var playerCountry = myNewObject.country;

... or ...

var playerCountry = myNewObject['country']

... JavaScript lets you access any object property using either dot-notation ( object.property ) or by index ( object['property'] ). The result in both cases is the same - playerCountry will be 'Ireland'. When accessing the object by indexing, the property doesn't even have to be a string literal - it can be a variable like this...

var propertyName = 'country';
var propertyValue = myNewObject[propertyName];

... in the above example, the propertyName variable is used when indexing. What this means is that every object in JavaScript can act like a lookup table. What's a lookup table? A table you 'look up' of course. This is a table of names and scores...

Name            Score
--------        -----
walter          5
tom             6
jane            8
bart            7

... If I want to find Jane's score, I look down the list of names in the name column until I find 'jane' then look across to get her score. In Javascript, an object which stored such a table would look like this...

var scoreboard = {
  walter: 5,
  tom:    6,
  jane:   8,
  bart:   7
};

... and if I wanted to write a function which took a player name as a parameter and returned their score, I'd do it like this...

function getScore(player){
  return scoreboard[ player ];
}

... I might call such a function like this...

var janesScore = getScore('jane'); // returns 8

... putting it all together, a hypothetical scoreboard.js mdoule might look something like this...

var utils = require('utils');
var scores = {};

exports.initialise = function(names){
  scores = {};
  utils.foreach(names, function(name){ 
    scores[name] = 0;
  });
};

/* 
  changes score by diff e.g. to add 6 to the player's current score
  updateScore('walter',6); // walter's new score = 5 + 6 = 11.
*/
exports.updateScore = function(name, diff){
  scores[name] += diff; 
};

exports.getScore = function(name){
  return scores[name];
};

Counting block break events for each player

I can use a Javascript lookup table (a plain old Javascript object) to keep a count of how many blocks each player has broken ...

block-break-counter.js

var breaks = {};

/*
  every time a player joins the game reset their block-break-count to 0
*/
function initializeBreakCount( event ){
  breaks[event.player.name] = 0;	 
}
events.connection( initializeBreakCount );

/* 
  every time a player breaks a block increase their block-break-count
*/
function incrementBreakCount( event ){
  breaks[event.player.name] += 1; // add 1
  var breakCount = breaks[event.player.name];
  echo( event.player, 'You broke ' + breakCount + ' blocks');
}
events.blockDestroy( incrementBreakCount );

With a little more work, you could turn this into a game where players compete against each other to break as many blocks as possible within a given time period.

Next Steps

This guide is meant as a gentle introduction to programming and modding Minecraft using the Javascript Programming Language. Javascript is a very powerful and widely-used programming language and there are many more aspects and features of the language which are not covered here. If you want to dive deeper into programming and modding minecraft, I recommend reading the accompanying ScriptCraft API reference which covers all of the ScriptCraft functions, objects and methods. I also recommend reading the source code to some of the existing scriptcraft plugins, followed by Anatomy of a ScriptCraft Plug-in. The online CanaryMod API Reference provides lots of valuable information about the different objects and methods available for use by ScriptCraft.