Develop Reference

Use of $ and _ in Javascript

I'm trying to interpret this piece of Javascript code. What is the use of the $ and _ signs here? In particular, is $ an alias for the JQuery library, and does this also apply for $set?
Template.postEdit.events({
'submit form': function(event) {
event.preventDefault();
var currentPostId = this._id;
var postProperties = {
url: $(event.target).find('[name=url]').val(),
title: $(event.target).find('[name=title]').val()
}
Posts.update(currentPostId, {$set: postProperties}, function(error) {
if (error) {
// display the error to the user
throwError(error.reason);
} else {
Router.go('postPage', {_id: currentPostId});
}
});
}
});

Explaination
$ and _ are mostly appended in Javascript variables to give more readability and to be more distinguishable ( visually most of the times ) from other variables. They are just conventions used by JS developers. Not necessary you've to use them. Major libraries/frameworks like Jquery, Angular like to follow this style in their frameworks.
Usage of $
Jquery have wrapped their features in $ . If you have included jQuery in your application, then $ used alone stands for jquery Object. Jquery being a really popular modern library, have somehow snatched the variable in JS. But its not like they have licensed the variable ( Just think, if variable name could be licensed, development would be more painful than it is now :p ), its more like they have dominated the use of $.
var selector = $('.someclass'); /* This is jquery object similar to
var selector = jQuery('.someclass') */
var $somestring = 'some string'; // Here $ character is appended to a variable.
//It doesn't adds any special behavior to the variable.
Some people have make good use of $, check it out here.
A naive developer who have used too much or the only library as JQuery in his lifespan is prone to this confusion. When he sees source code from framework like AngularJS, he tries to relate things with his former love jQuery. In Angular variables like $scope, $compile, etc, they seem confusing to him, as they have heavily appended $ to name their objects. Its just another name, you can write code with or without it. Angular uses this convention to distinguish variables from local to special objects. Big guns always try to dominate their conventions over the developer community. Can't blame them much, its for the betterment for all
Usage of _
Riding in similar vogue bandwagon, _ was nearly snatched by another useful ( really ? we can live without it ) library called Underscore Js . So they use the _ as their Underscore object, or mostly developers are to be blamed for this abuse, as they have paved its path to the vanity. But we can't blame developers for this, they were just using following good naming conventions.
var _myName = 'Who Cares'; // similar to $ no special behavior
var currentPostId = this._id; // In your case it seems
Well _ is mostly used by developers to distinguish the variables as private data members of their class, but only naming doesn't guarantee the access level. A good post briefly explaining for this is here
The best part is that, all the special characters that are allowed in Javascript to create distinguishable variable names are already invaded by biggies. So no more confusion. It is understandable why underscore invaded _ as their supremo, it stands for its meaning. But I am still curious why $ was chosen by jQuery. It doesn't even rhyme with it. No distant relation, it seems jQuery just took it as their property. I don't find any post explaining their invasion over it.
Sorry for being so dramatic, comic and sarcastic. Feel free to downnvote if it does not suit your appetite. Here to just help and make this space more interesting.
P.S : A list of valid characters for the naming convention used in JS
is explained here

Translating conditional statements in string format to actual logic

I have a good knowledge of real time graphics programming and web development, and I've started a project that requires me to take a user-created conditional string and actually use those conditions in code. This is an entirely new kind of programming problem for me.
I've tried a few experiments using loops and slicing up the conditional string...but I feel like I am missing some kind of technique that would make this more efficient and straightforward. I have a feeling regular expressions may be useful here, but perhaps not.
Here is an example string:
"IF#VAR#>=2AND$VAR2$==1OR#VAR3#<=3"
The values for those actual variables will come from an array of objects. Also, the different marker symbols around the variables denote different object arrays where the actual value can be found (variable name is an index).
I have complete control over how the conditional string is formatted (adding symbols around IF/ELSE/ELSEIF AND/OR
well as special symbols around the different operands) so my options are fairly open. How would you approach such a programming problem?

The problem you're facing is called parsing and there are numerous solutions to it. First, you can write your own "interpreter" for your mini-language, including lexer (which splits the string into tokens), parser (which builds a tree structure from a stream of tokens) and executor, which walks the tree and computes the final value. Or you can use a parser generator like PEG and have the whole thing built for you automatically - you just provide the rules of your language. Finally, you can utilize javascript built-in parser/evaluator eval. This is by far the simplest option, but eval only understands javascript syntax - so you'll have to translate your language to javascript before eval'ing it. And since eval can run arbitrary code, it's not for use in untrusted environments.
Here's an example on how to use eval with your sample input:
expr = "#VAR#>=2AND$VAR2$==1OR#VAR3#<=3"
vars = {
"#": {"VAR":5},
"$": {"VAR2":1},
"#": {"VAR3":7}
}
expr = expr.replace(/([##$])(\w+)(\1)/g, function($0, $1, $2) {
return "vars['" + $1 + "']." + $2;
}).replace(/OR/g, "||").replace(/AND/g, "&&")
result = eval(expr) // returns true

I need a Javascript literal syntax converter/deobfuscation tools

I have searched Google for a converter but I did not find anything. Is there any tools available or I must make one to decode my obfuscated JavaScript code ?
I presume there is such a tool but I'm not searching Google with the right keywords.
The code is 3 pages long, this is why I need a tools.
Here is an exemple of the code :
<script>([][(![]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]+(!![]+[])[+[]]][([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(![]+[])[+!+[]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]]()[(!![]+[])[!+[]+!+[]+!+[]]+(+(+[])+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+!+[]+[+[]]]+(![]+[])[+!+[]]+(![]+[])[!+[]+!+[]]])(([]+[])[([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+
Thank you

This code is fascinating because it seems to use only nine characters ("[]()!+,;" and empty space U+0020) yet has some sophisticated functionality. It appears to use JavaScript's implicit type conversion to coerce arrays into various primitive types and their string representations and then use the characters from those strings to compose other strings which type out the names of functions which are then called.
Consider the following snippet which evaluates to the array filter function:
([][
(![]+[])[+[]] // => "f"
+ ([![]]+[][[]])[+!+[]+[+[]]] // => "i"
+ (![]+[])[!+[]+!+[]] // => "l"
+ (!![]+[])[+[]] // => "t"
+ (!![]+[])[!+[]+!+[]+!+[]] // => "e"
+ (!![]+[])[+!+[]] // => "r"
]) // => function filter() { /* native code */ }
Reconstructing the code as such is time consuming and error prone, so an automated solution is obviously desirable. However, the behavior of this code is so tightly bound to the JavaScript runtime that de-obsfucating it seems to require a JS interpreter to evaluate the code.
I haven't been able to find any tools that will work generally with this sort of encoding. It seems as though you'll have to study the code further and determine any patterns of usage (e.g. reliance on array methods) and figure out how to capture their usage (e.g. by wrapping high-level functions [such as Function.prototype.call]) to trace the code execution for you.

This question has already an accepted answer, but I will still post to clear some things up.
When this idea come up, some guy made a generator to encode JavaScript in this way. It is based on doing []["sort"]["call"]()["eval"](/* big blob of code here */). Therefore, you can decode the results of this encoder easily by removing the sort-call-eval part (i.e. the first 1628 bytes). In this case it produces:
if (document.cookie=="6ffe613e2919f074e477a0a80f95d6a1"){ alert("bravo"); }
else{ document.location="http://www.youtube.com/watch?v=oHg5SJYRHA0"; }
(Funny enough the creator of this code was not even able to compress it properly and save a kilobyte)
There is also an explanation of why this code doesn't work in newer browser anymore: They changed Array.prototype.sort so it does not return a reference to window. As far as I remember, this was the only way to get a reference to window, so this code is kind of broken now.

Javascript refactoring in Vim

I don't need anything super fancy, but some scope aware refactoring would be nice.
Refactoring something in function scope is one of the most common scenarios for me:
var funyfun = function(arg1, arg2) {
arg1 = ...arg2;
arg2....();
}
Is there a vim plugin that would allow me to refactor arg1, for ex, in the scope of that function or do I have to invent my own "select block, find, replace" shortcut.
For extra kudos, something that would "refactor on the fly" as I type, so I can see where changes are being made. Netbeans does an excellent job of this.

This is not limited to a certain block, but this how I would do it with plain Vim:
Move cursor on top of arg1 and type *N
Type ciw and insert replacement.
Now you can use n and N to navigate the occurrences and by pressing . Vim will redo the replacement on the current match
Here's a shortcut for it:
" Simple word refactoring shortcut. Hit <Leader>r<new word> on a word to
" refactor it. Navigate to more matches with `n` and `N` and redo refactoring
" by hitting the dot key.
map <Leader>r *Nciw

Sound a bit like you only want renaming instead of refactoring. Full refactoring is tricky for Javascript, though some IDEs provide approximations. Since the question is about Vim specifically, and hacks aren't excluded, I'll just jump on the scope-aware aspect:
I've got a modified DoctorJS to generate scope-aware tags, with a Vim plugin for scope-aware navigation based on those tags (see blog post/screencast).
The hacky part comes in how navigation is implemented for Vim: I generate a search pattern that includes the scope of the variable and excludes all nested scopes for the same name. So, you could use that search pattern (function scoped_tags#FindTagInScope) to implement renaming (only if all uses are in the same file, and it doesn't exclude comments and the like..). Or you could use the scoped navigation to jump through variable occurrences manually and use '.' to rename them..

A few JavaScript-aware commands for Vim are provided by tern_for_vim, such as :TernRename for scope-aware renaming of variables, :TernDef for jumping to the definition of the thing under the cursor, and so on. You will need to have nodejs and npm installed, make sure to read the installation instructions in the repo.

As a partial solution you can use Eclim with JSDT, which allows you to use power of Eclipse refactoring/debugging/auto-completion/plugins with Vim.
In my experience, it may be a bit slow on older machines, but it's worth giving it a try.

In addition to doing actual scope-aware refactoring and the like, consider :%s/varName/newNav/gc. Per :help :s_c, the c flag passed to :s enters a quick confirmation mode for find/replace operations that prompts you (y/n) on whether each match should be replaced or not.

you can do:
:.,/^}/ s/\<arg1\>/new_name/g
the .,/^}/ is a range that many Ex commands accept: from cursor line to next line starting with a closing brace.

Benoit and Epeli has some good points, however, I find it a bit tedious to write .,/^}/ before my substitute statement, and since it only modifies code from the cursor position to the next line starting with a }, it depends on having the cursor position at the beginning of the function or block (and it will not work for an entire function with an if statement).
So instead I use visual mode in combination with textobjects. For instance, typing vi{ will select all the code inside the closest matching pair of {}, va{ will include the {} characters, and if you do this with visual line (vi{V), you'll get the entire function declaration as well. Then you can just do a :s/\<arg1\>/new_name/g to rename arg1 to new_name, including function parameters.

Good resources for extreme minified JavaScript (js1k-style)

As I'm sure most of the JavaScripters out there are aware, there's a new, Christmas-themed js1k. I'm planning on entering this time, but I have no experience producing such minified code. Does anyone know any good resources for this kind of thing?

Google Closure Compiler is a good javascript minifier.
There is a good online tool for quick use, or you can download the tool and run it as part of a web site build process.
Edit: Added a non-exhaustive list of tricks that you can use to minify JavaScript extremely, before using a minifier:
Shorten long variable names
Use shortened references to built in variables like d=document;w=window.
Set Interval
The setInterval function can take either a function or a string. Pass in a string to reduce the number of characters used: setInterval('a--;b++',10). Note that passing in a string forces an eval invokation so it will be slower than passing in a function.
Reduce Mathematical Calculations
Example a=b+b+b can be reduced to a=3*b.
Use Scientific Notation
10000 can be expressed in scientific notation as 1E4 saving 2 bytes.
Drop leading Zeroes
0.2 = .2 saves a byte
Ternery Operator
if (a > b) {
result = x;
}
else {
result = y;
}
can be expressed as result=a>b?x:y
Drop Braces
Braces are only required for blocks of more than one statement.
Operator Precedence
Rely on operator precedence rather than adding unneeded brackets which aid code readability.
Shorten Variable Assignment
Rather than function x(){a=1,b=2;...}() pass values into the function, function x(a,b){...}(1,2)
Think outside the box
Don't automatically reach for standard ways of doing things. Rather than using d.getElementById('p') to get a reference to a DOM element, could you use b.children[4] where d=document;b=body.
Original source for above list of tricks:
http://thingsinjars.com/post/293/the-quest-for-extreme-javascript-minification/

Spolto is right.
Any code minifier won't do the trick alone. You need to first optimize your code and then make some dirty manual tweaks.
In addition to Spolto's list of tricks I want to encourage the use of logical operators instead of the classical if else syntax. ex:
The following code
if(condition){
exp1;
}else{
exp2;
}
is somewhat equivalent to
condition&&exp1||exp2;
Another thing to consider might be multiple variable declaration :
var a = 1;var b = 2;var c = 1;
can be rewritten as :
var a=c=1,b=2;
Spolto is also right about the braces. You should drop them. But in addition, you should know that they can be dropped even for blocks of more expressions by writing the expressions delimited by a comma(with a leading ; of course) :
if(condition){
exp1;
exp2;
exp3;
}else{
exp4;
exp5;
}
Can be rewritten as :
if(condition)exp1,exp2,exp3;
else exp4,exp5;
Although it's not much (it saves you only 1 character/block for those who are counting), it might come in handy. (By the way, the latest Google Closure Compiler does this trick too).
Another trick worth mentioning is the controversial with functionality.
If you care more about the size, then you should use this because it might reduce code size.
For example, let's consider this object method:
object.method=function(){
this.a=this.b;
this.c++;
this.d(this.e);
}
This can be rewritten as :
object.method=function(){
with(this){
a=b;
c++;
d(e);
}
}
which is in most cases signifficantly smaller.
Something that most code packers & minifiers do not do is replacing large repeating tokens in the code with smaller ones. This is a nasty hack that also requires the use of eval, but since we're in it for the space, I don't think that should be a problem. Let's say you have this code :
a=function(){/*code here*/};
b=function(){/*code here*/};
c=function(){/*code here*/};
/*...*/
z=function(){/*code here*/};
This code has many "function" keywords repeating. What if you could replace them with a single(unused) character and then evaluate the code?
Here's how I would do it :
eval('a=F(){/*codehere*/};b=F(){/*codehere*/};c=F(){/*codehere*/};/*...*/z=F(){/*codehere*/};'.replace(/function/g,'F'));
Of course the replaced token(s) can be anything since our code is reduced to an evaluated string (ex: we could've replaced =function(){ with F, thus saving even more characters).
Note that this technique must be used with caution, because you can easily screw up your code with multiple text replacements; moreover, you should use it only in cases where it helps (ex: if you only have 4 function tokens, replacing them with a smaller token and then evaluating the code might actually increase the code length :
var a = "eval(''.replace(/function/g,'F'))".length,
b = ('function'.length-'F'.length)*4;
alert("you should" + (a<b?"":" NOT") + " use this technique!");

In the following link you'll find surprisingly good tricks to minify js code for this competition:
http://www.claudiocc.com/javascript-golfing/
One example: (extracted from section Short-circuit operators):
if (p) p=q; // before
p=p&&q; // after
if (!p) p=q; // before
p=p||q; // after
Or the more essoteric Canvas context hash trick:
// before
a.beginPath
a.fillRect
a.lineTo
a.stroke
a.transform
a.arc
// after
for(Z in a)a[Z[0]+(Z[6]||Z[2])]=a[Z];
a.ba
a.fc
a.ln
a.sr
a.to
a.ac
And here is another resource link with amazingly good tricks: https://github.com/jed/140bytes/wiki/Byte-saving-techniques

First off all, just throwing your code into a minifier won't help you that much. You need to have the extreme small file size in mind when you write the code. So in part, you need to learn all the tricks yourself.
Also, when it comes to minifiers, UglifyJS is the new shooting star here, its output is smaller than GCC's and it's way faster too. And since it's written in pure JavaScript it should be trivial for you to find out what all the tricks are that it applies.
But in the end it all comes down to whether you can find an intelligent, small solution for something that's awsome.

Also:
Dean Edwards Packer
http://dean.edwards.name/packer/
Uglify JS
http://marijnhaverbeke.nl/uglifyjs

A friend wrote jscrush packer for js1k.
Keep in mind to keep as much code self-similar as possible.
My workflow for extreme packing is: closure (pretty print) -> hand optimizations, function similarity, other code similarity -> closure (whitespace only) -> jscrush.
This packs away about 25% of the data.
There's also packify, but I haven't tested that myself.

This is the only online version of #cowboy's packer script:
http://iwantaneff.in/packer/
Very handy for packing / minifying JS