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Encode and decode a string in JavaScript

Let's say I have a string, called str, which is equal to "Hello, world!"
Is there a way to choose some unicode characters like "azertyuiopqsdfghjklmwxcvbn1234567890-" and return an encoded string that contains only the choosen characters?
It should return something like "hvfebi iehfhe" (well, something encoded and not human readable, but which is decodable)?
Thanks

There's no such a thing as a function that you configure an arbitrary set of characters to enconde and decode, unless you find a library that does that or you implement it yourself.
But, you can use base64 encoding, which uses "A-Z", "a-z", "0-9", "+", "/" and "=" characters to encode the string.
The native browser functions are on window: btoa() to enconde and atob() to decode.
Edit after your comments:
Any function you make up to solve this will be decodable anyway analysing the code, so no point in hiding it. If you don't want to be simply base64, you can make a function that encodes it several times.

From php json_encode to jquery decode

Im using a well known "hack" (the json encode function) to prevent some characters to mess up my html, im receiving from an API a description field that can content single or double quotes (and other special chars). So:
<div class="someThing" data-fulldescription=<?=json_encode($textFromApi);?>>
...
</div>
Now I read that data field using jquery and then print it inside a div:
$('#brand-modal-content').html($(this).parents('.someThing').data('fulldescription'));
The problem is, the quotes are now coded by the PHP function, and some characters get replaced by "\u00e8" or "\u00f9", is there a way to reformat the text using jquery?

You are injecting content into HTML in a very wrong (and even unsafe) way. Do this instead:
data-fulldescription="<?=htmlspecialchars(json_encode($textFromApi));?>"
This way the JSON will be properly encoded and safely injected no matter what is inside; then, you can decode it like so:
var decoded = JSON.parse($(this).parents('.someThing').data('fulldescription'));
The combination of these steps will perfectly preserve the JSON no matter what it represents (you can take shortcuts if you assume it's a string, but why not be always 100% safe?). You can then do whatever you want with the decoded value.

How to encode and decode HTML special characters to get entity name

The character set is mentioned at Special Character Map. I need a Java-script or J-Query encoding code to get entity name.
for e.g. if I pass £ then I should get "&pound ;" or for ¥ it should return "&yen ;".
Even I copy the symbols instead of typing in then also it should work.
I am trying to use following J-Query code but it doesn't seem to work when I copy-paste strings.
function krEncodeEntities() {
var s = $('#input').val();
return $('#lblEncode').text($("<div/>").text(s).html());
}
function krDencodeEntities() {
var s = $('#lblEncode').text();
return $('#lblDecode').text($("<div/>").html(s).text());
}
Can anyone please help me?

JavaScript has no concept of HTML identities. To JS, everything is UCS16 (a forerunner of UTF16).
You have a couple of options.
Option 1
Make a big translation object of characters and their identities.
Option 2
See if some other form of encoding will work for you.
When are you supposed to use escape instead of encodeURI / encodeURIComponent?

Escaping quotation marks in PHP for JavaScript function argument

I'm having trouble escaping a quotation mark in PHP.
I have a table of products and each row has an onclick function, with the name of the product as the argument.
The name contains the length which is measured in inches, so the name contains a quotation mark. I wrapped an addslashes() around the string. This adds a backslash before the quotation mark but for some reason it doesn't seem to escape the character!
Here's a snippet of my code:
<?$desc1 = addslashes($row['Desc1']);?>
<tr class='tableRow' onclick='afterProductSelection("<?=$desc1?>")'>
<td><?=$row['Desc1']?></td>
When I inspect element in Google Chrome, the colour of the syntax indicates that this has not been escaped, clicking on it gives me a syntax error.
Probably something simple that I'm missing. Hope you can help!

There are a lot of different cases where you need to escape a string. addslashes() is the wrong answer to pretty much all of them.
The addslashes() function is an obsolete hang-over from PHP's early days; it is not suitable for any escaping. Don't use it. Ever. For anything.
In your particular case, since you're creating Javascript data from PHP, use json_encode().
json_encode() will take a PHP variable (whether it's a string, array, object or whatever) and convert it into a JSON string. A JSON string is basically fully escaped Javascript variable, including the quotes around your strings, etc. This is what you need to do.

The addslashes() function is an obsolete hang-over from PHP's early days; it is not suitable for any escaping. Don't use it. Ever. For anything. -Spudley
I think the function you're looking for is htmlentities()
<?=htmlentities($desc1, ENT_QUOTES)?>
http://ca1.php.net/htmlentities

You are generating a JavaScript string encoded as HTML so you need to encode twice:
Use json_encode() to generate the string
Use htmlspecialchars() to encode as HTML

Use json_encode to output variables from the backend in JavaScript:
<tr onclick='afterProductSelection(<? print json_encode($desc1); ?>)'>
N.B.: For string output there is no need for extra quotes.

what kind of encoding is this?

I've got some data from dbpedia using jena and since jena's output is based on xml so there are some circumstances that xml characters need to be treated differently like following :
Guns n ' Roses
I just want to know what kind of econding is this?
I want decode/encode my input based on above encode(r) with the help of javascript and send it back to a servlet.
(edited post if you remove the space between & and amp you will get the correct character since in stackoverflow I couldn't find a way to do that I decided to put like that!)

Seems to be XML entity encoding, and a numeric character reference (decimal).
A numeric character reference refers to a character by its Universal
Character Set/Unicode code point, and uses the format
You can get some info here: List of XML and HTML character entity references on Wikipedia.
Your character is number 39, being the apostrophe: ', which can also be referenced with a character entity reference: '.
To decode this using Javascript, you could use for example php.js, which has an html_entity_decode() function (note that it depends on get_html_translation_table()).
UPDATE: in reply to your edit: Basically that is the same, the only difference is that it was encoded twice (possibly by mistake). & is the ampersand: &.

This is an SGML/HTML/XML numeric character entity reference.
In this case for an apostrophe '.