Develop Reference

something better than base64 to encode data that doesn't take up too much processing power

I have a web site in which users can add multiple items and sometimes the URL can be long. I thought by using base64 encoding, I'd pass the URL along but it contains a slash which I use to separate items because my web server cannot handle path names (anything between 2 slashes) longer than 255 characters or I'd get a 403 error.
Is there another way I can encode data quickly in javascript so that theres a 0% chance that a slash will occur in the result?
I'm looking for something not too processor intensive and if possible, I want to go for something better than character swapping.
I will understand if I need to visit a library, but the only encoding built-in to javascript (to my knowledge) is base64 (via the atob function) and I want something different.
I also want to be able to make the solution work with older web browsers as well.

What you need is encodeURIComponent, which is part of the javascript spec and automatically included in all javascript environments
var url = 'example.com/someextenstion/' + encodeURIComponent(theString);

There are many ways to address this but one of the simplest is going to be to take an implementation of atob and btoa and modify it to use a - instead of a / when encoding. You'll have to rename the functions so they don't mask the standard function, but here's some JavaScript source code that does the trick: github. In that particular implementation just replace the / in _ALPHA with a - (or any character of your choosing).
It might be faster to just do as Amit suggests: use the standard functions and do a quick string replace of / on conversion: str.replace(/\//g,'-'); and perform the reverse on decoding, but it doesn't seem like performance will be critical in this application.

finding the meaning of the obfuscated javascript

i saw this piece of code in an obfuscated javascript :
if(1s Q.is.ep=='a')
do you have any idea what this might mean? Im quite confused about the space..
thanks :)

The code looks like generated by Dean Edwards' packer (or another similar one). You could unpack it with this tool.
It's indeed JavaScript, however replaced keywords, method, variables with meaningless strings. The bottom half of the file you provided is actually a mapper between obscured and original.
And this, it the power of eval (and don't use eval if by all means you could do without it).

Writing a Parser for javascript code

I want to extract javasscript code and find out if there are any dynamic tag creations like document.createElement('script'); I have tried to do this with Regular expressions but using regular expressions restricts me to get only some formats so i thought of writing a javascript parser which extracts all the keywords, strings and functions from the javascript code.

In general there is no way to know if a given line of code will ever run, you would need to solve the halting problem.
If you restrict your analysis to just finding occurances of a function call you don't make much progress. Naive methods will still be easy to trick, if you just regex match for document.createElement, you would not be able to match something as simple as document["create" + "Element"]. In general you would need to not only parse the code but evaluate it as well to get around this. And to be sure that you can evaluate the code you would again need to solve the halting problem.

Maybe you should try using Burrito

Well the first rule is never use regex for big things like this, or DOM, or ... . You have to parse it by tokens. The good news is that you don't have to write your own. There are a few JS to JS parsers.
UglifyJS
narcissus
Esprima
ZeParser
They may be a bit hard to work with it. But well better to work with them. There are other projects that are uses these such as burrito or code surgeon. So you can have a look at the source code and see how they uses them.
But there is bad news too, which people can still outsmart other people, let alone the parsers and the code they write. At least you need to evaluate the code with some execution time variables and see if it tries to access the DOM or not.

Knowing the language used in a form

I'm working on a site with forms that require a certain language to be used. (Something like a dictionary). I want to know if there is a way in JavaScript/HTML to check the language. For example, if I need the input in Japanese and the user gives me an English word, it is detected as an error.

You could take a look at the Google AJAX Language API and there's an example of language detection.

No, there is not a way to do this with straight java/html. The best you could do is hope that UTF-8 is the encoding, and then you check the character class. But that still doesn't really tell you which language is used.

No, you need to implement a function or use a specific library.

Best way to decompress javascript files

There are many tools to compress a Javascript file (Packer YUI for example).
But how can I decompress them back to a human readable format?
I have compressed a file using a tool like Packer YUI , but I couldn't reach the source back again.
Is there any good software or tricks you can suggest to decompress the JS ?

You can't. Javascript compression is usually a lossy one, and the information is lost forever.
What you can do, is use a source formatter and a good refactoring tool and -- painfully -- reconstruct the original source. Even if you are not familiar with the code it should be possible; Jeff and a few others reverse engineered the WMD javascript code from a minified version.
Finally, you should consider using a version control system and proper backups to keep your source code safe.

This website is really cool. You can paste a minified JS, then you get a human readable view.

Try JSMinNpp (now called JSToolNpp) plugin for notepad++ (to compress and decompress).
http://www.sunjw.us/jstoolnpp/

DECOMPRESS JAVASCRIPT
A typical JavaScript compressed with /packer/ starts with the following code:
`eval(function(p,a,c,k,e,r)`…
`eval` can simply be replaced by alert.
The eval function evaluates a string argument that contains JavaScript. In most packers, eval is used, followed by document.write.
To decompress JavaScript, replace these methods by one of the following:
1. Replace eval by alert (The alert will simply print the code in a popup-window)
2. If the JavaScript appears after the <body> element, you can add a <textarea> like so:
`<textarea id="code"></textarea>`
Then, replace eval(…); by document.getElementById("code").value=…;.

A linter like ESLint can be handy as well. It can format the code using the "fix all auto-fixable problems" to a point where you can at least start doing manual editing with greater ease.

I never used Packer YUI. But if you use this javascript packer, you can always get your code back using this javascript beautifier which also decompresses the code.
Some javascipt minifier shorten the variable names while compressing the js. In that case you could never get your original code back even if you beautify it.