First of a two part slideshow that starts you off on the road towards building your own software extension blocks for the Scratch programming environment. Notes: . - Contains 39 slides. - Aimed at experienced Scratch users/teachers with the motivation to learn more. - Scratch is developed by the Lifelong Kindergarten Group at the MIT Media Lab. See http://scratch.mit.edu - Please check back for occasional updates/errata.

1. Understanding ScratchX
Extensions with JavaScript
© Darren Adkinson - 21 Jan 2016
Top-Down Learning
Approach Using A
Real World Example
Part 1 of 2
39 slides
V 1.4

2. 2
T h e a u t h o r r e t a i n s
copyright of these slides.
You are free to use them so
long as the author’s name
i s q u o t e d o n a l l
presentations and excerpts.
ISS Tracker Scratch extension
code is provided with kind
permission of Kreg Hanning.
NOTE: occasional updates may be
p u b l i s h e d ( o n S l i d e S h a r e o r
elsewhere) from time to time.
Please check back for new versions.
Scratch is developed by the
Lifelong Kindergarten Group
at the MIT Media Lab.
See http://scratch.mit.edu

3. Scene Setting
Assumed you’re familiar with Scratch: https://scratch.mit.edu/about/
Not a “stand up & talk” presentation; meant to be worked through at
home.
You should have some JavaScript experience, or do some learning on
your own to get the most from this (see next slide).
Aimed at: keen Scratch students who want to learn more; motivated
teachers and parents.
These slides won’t get you all the way by any means, and they’re not
“follow-along-with-me”. They should be used alongside the ScratchX
wiki.
Who am I?
A systems engineer who makes complex stuff a little more understandable.
3

4. Where these slides can help
4
Your JavaScript knowledge
Understanding
ISS Tracker
Making your own
toy blocks
What these slides will help you with
What you should have already
Absolute
Beginner
Master
Entry Point
JS Entry Point is roughly:
- be familiar with most of the syntax
- understand if/else and for loops, basic constructs
- get a feeling for what functions and methods are
- write your own small programs
- read some simple code and understand (most) of it
It varies by person but a couple of hours a day
spread over 3 - 4 days would be a good start
Making your own
useful extension

5. What are Extensions?
They enable you to
create blocks that don’t
currently exist in
Scratch.
ScratchX.org hosts a
bunch of experimental
extensions that you can
play around with.
Divided into hardware
and software
extensions.
5

6. How will these slides work?
We’ll take a (gradual) deep dive into one
particular extension: ISS Tracker, by
Kreg Hanning.
The hard work is all Kreg’s. I just
happened to choose this extension to
teach myself and hopefully make the
topic more accessible to other people.
Just like Scratch, ScratchX makes code
open, though you’ll have to dig around a
little in your browser or on GitHub* to
find it.
6
*see next slide where I’ll discuss jump off points.

7. Complexity
Required
Future usefulness
0 25 50 75 100
git/
GitHub
“Jump Off Points (JoPs)”
The visual aspect of slides is great, but they’re linear and we can’t cover
everything.
I’ve highlighted “jump off points” throughout where it would be good for
you to go off and do some research/study on your own (or in your team!).
7

8. 8
Complexity
Required
Future usefulness
0 25 50 75 100
The thing to go research.
There will usually be a web
link but don’t restrict your
self to that.
Totally
subjective
scale.
As it states. How hard is this
for a beginner? The higher the
number the less you should
worry if you don’t get it first or
even second time around.
How important is this
topic to learning how
ISS Tracker works (and
roughly, how to make
your own extension).
Moving away from ISS
Tracker and Scratch,
how useful is this topic
for general software
development, JavaScript
etc.
So this JoP means:
git/GitHub
- it has a learning curve - you’ll need to do a tutorial or two.
- you have to know some of it to make an extension.
- it’s *really* useful to know. Your time will not be wasted!
git/
GitHub
Recommend
you find and
take a
tutorial.
Don’t sweat
it. Just get a
rough feeling.
Example JoP

9. Let’s dive in
Go to http://scratchx.org/
#extensions. Find the ISS
Tracker extension and
open it by clicking on
“Sample Project”.
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Have a play around and
see what it does. Most
of the blocks are regular
Scratch blocks, so you
should be able to get a
rough idea quite easily.
Complexity
Required
Future usefulness
0 50 100
ISS
Tracker

10. New Blocks
The “ISS” sprite uses two
custom-made blocks:
10
Note that both blocks have
some type of input facility for
the user.

11. Sprite1 only uses one
new block type: an
event block that is -
presumably - triggered
when the ISS passes
over a certain place.
11
New Blocks

12. So this “ISS Tracker” Scratch extension is a piece of code that creates three new
block types, and that’s it.
The Sample Project uses these three new blocks to track the ISS as it orbits
Earth. It makes sense to create a sample project every time you create new
blocks (there must have been some reason for creating them in the first place).
All of the code is contained in a single JavaScript file.
The ScratchX wiki is here: https://github.com/LLK/scratchx/wiki. I recommend
that you look at the FAQs first (scroll down) and then dig into the business end
later, as you go through these slides.
12
New Blocks
Complexity
Required
Future usefulness
(100%)
0 50 100
ScratchX
FAQ

13. ISS Extension Info
The extension developer will usually put
a web page on GitHub explaining
something about his/her extension.
Note that this is aimed primarily at end
users of the blocks, probably not so
much at people developing new
extensions.
This extension’s info page is here:
http://khanning.github.io/scratch-
isstracker-extension/
The quick way to get to this page (that
all extensions should provide) is by
clicking on the drop down arrow and
selecting “About XYZ extension…”
13
Complexity
Required
Future usefulness
(100% compare)
0 50 100
ISS About
page

14. 14
Extension Relationships
For any extension we write there are a bunch of relationships to
think about. It looks complex but it’ll make more sense as you work
through the slides and Part Two.
ScratchXUser
Extension writer
ScratchX
wiki
reads
creates with
creates creates - via
extension .js file
Extension
info page
work
with
provides
support
for
reads / adds to / corrects
GitHub
loads extension .js file onto
reads
defines
hosts
writes
A
A
hosts
New blocks
.js file

15. The Extension Code
The ScratchX website itself doesn’t host the extension code. Instead the developer
puts it in a GitHub repository (an online store for software, mainly) and she/he tells
the ScratchX website where to get it from.
For now we can skip those details and just grab the extension code from our browser.
In your browser window that has ISS Tracker open up the Developer Tools or whatever
yours is called. Here I’ve used Chrome browser’s View -> Developer Tools option:
15
But, before we copy
the code into an
editor let’s take a
little diversion…

16. Dev tools
The developer tools can be intimidating as
they’re so powerful. This is also what makes
them great.
The Console tab is the most useful for us in
two ways: (1) we can play around with some
JavaScript at any time, and, more usefully
(2) it will show output from ISS Tracker if
the developer programmed his/her
extension that way.
16
Complexity
Required
Future usefulness
0 50 100
Browser
dev tools
(1)

17. 17
Dev tools
Now try this so we can see the more useful stuff. Run the ISS Tracker
program and check that it’s working ok.
Bring up the dev tools and select “Console”.
Now turn off your wifi or pull your ethernet cable. Just make sure your
computer can’t connect to the web.
Wait a 30s or so and you should start seeing error messages appear like
these:
(2)
Some kind of error
message, clearly
console.log output

18. 18
You don’t need to search for this
now but I’ve highlighted here
the part of Kreg’s code where he
specified this console.log output.
The function might look
inpenetrable right now but you
can get a rough idea that it’s
getting data about the ISS
from somewhere.
When you ‘pull the plug’ on your
internet connection it fails, and
throws up an error message.
Complexity
Required
Future usefulness
0 50 100
Get an
editor
A good code-aware text editor or Integrated
Development Environment will serve you well.
I’ve used Atom here; there are loads of free ones
out there.

19. Select the Sources tab and then navigate to the iss_extension.js file.
You can see the code right there in your dev tools window and it should be syntax color-
highlighted.
You can also find out where the code came from on a server if you mouseover the folder.
This is where your browser pulls it from. In our case it is at: https://
khanning.github.io/scratch-isstracker-extension/iss_extension.js. Put this in a new
browser tab and you can read it from there as well (plain black text, probably).
19
The Extension Code
Most of the
ScratchX files
are in here
We need this one
You can ignore the rest

20. 20
iss_extension.js - skeleton
So the breakdown is:
(1) a comment block
(2) a bracketed ‘main’ function
(3) a bunch of var declarations
(4) three private functions
(5a) three block methods
(5b) two ‘internal’ methods
(6) an object called “descriptor”
(7) a call to ScratchExtensions.register
(8) a call to one of the private functions
(9) a var called poller that does something
(10) slightly weird looking closing of (2),
above
1
2
3
4
5a
6
7
8
9
10
5b

21. Many environments give you some code to start with,
“boilerplate”, “framework”, etc.
They often contain code you must use as well as some helpful code
that completes standard foundational tasks for you (‘automate
the boring stuff’).
The ScratchX wiki gives you some boilerplate that we’ll look at
next.
One way to start off when looking at someone else’s extension is
to see what they added on top of the boilerplate.
21
Boilerplate code

22. 22
added code to clear the poller
same
defined the blocks + url for info page
almost the same; changed name

23. System methods
The boilerplate provides two methods that are meant to be ‘private’ and called by ScratchX
(hence they start with an underscore).*
“ext” is the name of our extension object so the notation ext._shutdown = etc essentially
means “add a function to our “ext” object and make it visible to other code” (this is roughly what
a method is: a function that you tell the world about).
ext._shutdown will be called by scratch_ext.js when it unregisters your code, for instance, if
you were to click on “Remove extension blocks”. This is where to put tidy up code.
ext._getStatus will be called by scratch_ext.js if it needs to get the status of your plugin
(more relevant to hardware than software plugins).
23
*There is a subtle distinction here since all the methods will only be called within
ScratchX. The difference is that the block methods will be indirectly called by the
ScratchX application programmer when he/she uses the new blocks, whereas these
two underscore methods will only potentially be called by the ScratchX environment.

24. The descriptor object
24
url for info page
any drop down menus
that our blocks will use
Op Code ie.
block type
formatted
block label method name
optional default
argument values
%s - replace with string
%m.<menu> - replace with choice from the ‘menus’ list
The ScratchX wiki
covers this quite well.

25. Some words about IIFEs
25
The extension code is all contained within a bracketed function expression.
This pattern is called an Immediately Invoked Function Expression (IIFE).
The brackets (parentheses) are to trick the JS engine into treating the
function as an expression so that it will be immediately invoked (ie. run).
It’s not a regular function statement that sits around until called upon.
The main reason for doing this is to shield all of our variables from being in
the global space - so they don’t clash with someone else’s variables.
code goes here
There are two common ways to lay this out:
A. ( function (<optional params>) { code } ) (<optional arguments>);
B. ( function (<optional params>) { code } (<optional arguments>) );

26. 26
A. ( function (<optional params>) { code } ) (<optional arguments>);
B. ( function (<optional params>) { code } (<optional arguments>) );
Some words about IIFEs
code goes here
ScratchX uses the type A, but A and B are equivalent.
The green parens are the ‘wrappers’ that make the whole function act like an
expression.
The function expects a parameter, called “ext” (this is the extension object)
We supply the function with a starting argument, which is an empty object.
The ‘ {} ‘ stands for an empty object in JS.

27. Block types
27
ISS Tracker uses three different types of block: a Hat block (h), an Asynchronous
reporter (R), and a Synchronous reporter (r).
Anything that runs synchronously means it could delay your whole function if you
don’t write it correctly or use it in the wrong place.
You should NOT use a synchronous block for anything that will take more than a
few milliseconds or will need to wait on some external source.
When we look at the specific methods in Part Two this will make more sense.

28. This code shows how we register our extension with ScratchX.
This is quite straightforward: just make sure your extension name is unique.
Here’s an example from a different extension (Xi Client, by Alan Yorinks):
28
Register our extension
Our extension name The descriptor object Our extension object
You can see that it’s highly unlikely anyone else would write an extension with the
same name. This makes it possible to have two or more extensions loaded within
ScratchX at the same time.

29. The .register call
29
lib.register = function (name, descriptor, handler, deviceSpec) {
if (name in handlers) {
console.log('Scratch extension "' + name + '" already exists!');
return false;
}
… more code here
};
You really don’t need to know this but when you call ScratchExtensions.register() the
method you’re calling is listed at http://scratchx.org/scratch_extensions/scratch_ext.js
Only hardware extensions
provide a deviceSpec
As we said earlier, ScratchX won’t let you load an extension if it has the same name
as an extension that you’re already using. Here I changed my test extension to be
called “ISS Tracker” and tried to load it when I had the real ISS Tracker running:

30. 30
Checkpoint
We’ve more or less covered the greyed out
areas.
What is left is the meat of this code.
We’ll handle the easy stuff first and do a
quick overview of the more complex stuff.
Part Two will cover (4) and (5a) in more
detail.
1
2
3
4
5a
6
7
8
9
10
5b

31. Variables
31
3
This is quite straightforward: two constants (or at least variables that are not
meant to be changed in any way - the convention is to capitalize them).
An empty object called “locations”. (Note that objects can contain nested objects,
arrays etc, so we simply don’t know yet how complex this object may end up being).
A variable called issData that has no value right now (we don’t know what type it
is).
These are available within our code but can’t be accessed by, and won’t clash with,
anyone outside - since our code is all wrapped inside a special type of function (see
earlier slides on IIFE).

32. Entry point
32
When our function loads it runs straight away, but what exactly runs?
First: it registers itself with ScratchX, as we’ve seen earlier.
Next it makes a call to its own function, updateISSLocation().
Then it declares a variable called poller and sets it off on its repeating task.
So where does “setInterval” come from? The three main choices are: (1) Kreg wrote it,
(2) it’s built in to the language, (3) it’s part of ScratchX or a library that ScratchX
uses.

33. setInterval()
33
If you’re not sure the simplest thing to do is google it and see what comes up. This one
is a built in method.
Normally I’d go to the MDN (Mozilla Developer Network) website, but in this case I
think W3Schools has a better explanation.
We get access to this method because we’re running in an open window.
setInterval calls the function updateISSLocation every (approx.) 2000 milliseconds
- which is every 2 seconds.
We don’t have to explicitly say window.setInterval(). Just setInterval() will work.

34. 34
setInterval()
setup a timer to call this function
roughly every 2s
until someone clears the interval
Remember the _shutdown method of
our ext object? That’s where the timer is
cancelled.
Complexity
Required
Future usefulness
0 50 100
MDN &
W3Schools
Get used to going online to
look up syntax, methods,
examples, etc.
Mozilla Developer Network
(MDN) is recommended.

35. 35
Checkpoint
A quick blast through the internal
functions (4) and the methods (5a) and
we’re pretty much done for Part One.
1
2
3
4
5a
6
7
8
9
10
5b

36. 36
Internal functions
Roughly - what it does
- Queries a server to get lat / long etc for a
specific place (initially Boston, Ma).
- Build up a locations object for each place
the user inputs.
How complex
(relatively)
- Queries a server to find out where the
International Space Station is.
- Helper function for the distanceFrom
method. This is where the Haversine
formula (see next slide) is actually used.

37. 37
Methods
taken from http://khanning.github.io/scratch-isstracker-extension/

38. 38
Checkpoint
That’s it for Part One. Part Two will cover: deeper dive into the methods and internal
functions; tips on how to set up your own test extension - sections 2 and 4 from the
ScratchX wiki; thoughts about what (software) extensions to make.
We covered quite a lot of 3.i and 3.ii from the wiki. (Part 3.iii won’t be covered as these
slides are about software extensions.)
Complexity
Required
Future usefulness
(100%)
0 50 100
ScratchX
wiki

39. Hope this was helpful.
Part 2 will be out sometime in Feb 2016.
Constructive comments, ideas, suggestions, corrections, etc are welcome.
GitHub username: darren-a
Email: dadkinson@gmail.com
39
Wrap Up
What extensions would you like to write?
How does the reverse-engineered, top-down learning approach suit you?
Would you be interested in learning JavaScript by taking real world code and deconstructing it like this?
Questions for you