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Parser: How to Turn a JavaScript String into One Line of JavaScript?

I'm trying to make a parser for formatting JavaScript in a contextual format. First I want to be able to convert the input JavaScript into one line of JavaScript and then format the code based on my requirements. This does not remove all of the enters or white space:
txt = $.trim(txt);
txt = txt.replace("\n", "");
How can I convert the text into one line?

Use a regular expression with the "global" flag set:
txt.replace(/\n/g, "");
However, you should be careful about removing linebreaks in Javascript. You might break code that was depending on semicolon insertion. Why don't you use an off-the shelf parser like Esprima?

Use :
\s character that represents any space character (Carriage return, Line Feed, Tabs, Spaces, ...)
the "greedy" g flag.
var text = txt.replace(/\s+/g, ' ');
Hope it helps

If the text comes from some operating systems, it may have the \r\n line ending, so it is worth removing both...
You should also use /\r/g this replaces ALL \rs not just the first one.
var noNewLines = txt.replace(/\r/g, "").replace(/\n/g, "");
You have to be pretty sure there are no single-line comments and that there are no missing semi-colons.

You can try to minify your code, using something like https://javascript-minifier.com/
however this will also change your variable names

Proper Regex to find and replace escaped UTF-8 strings

(edited) I am reading a JSON file that includes some UTF-8 characters that are encoded like this: "\uf36b". I am trying to write a RegExp to convert this to an HTML entity that looks like "🍫". This displays the character correctly in my html page.
I haven't been able to correctly display the character that should be associated with "\uf36b", especially when in a longer sentence that also includes other text.
How can I write a regexp that replaces strings like "\uf4d6" and "\uf36b" but leaves other text alone?
Example:
var str = "I need \uf36b #chocolate";
This should be converted to:
I need 🍫 #chocolate;

The \uf36b here is a Unicode code point that represents a character, it should be possible to have your page support characters like this without needing to escape them by encoding to UTF-8.
That being said, the printable ASCII range is from \u0020 (space character) to \u007e (tilde), so you should be able to use something like the following to only escape the characters you need to:
var escaped = "I need \uf36b #chocolate".replace(/[^\x20-\x7e]+|%/g, escape);
This will call escape() only on the non-ASCII or non-printable ASCII characters in your string, as well as any % characters.
You can then use var str = escaped.replace(/%(..)/g,"&#x1f") + ";"; to do your conversion, although this looks pretty strange and I can't really see how it would do anything too useful. You probably actually want something like the following:
var str = escaped.replace(/%(?:u([0-9a-f]{4})|([0-9a-f]{2}))/gi, "&#x$1$2;");

Printing all ASCII characters in Javascript

I need to do something like this:
Have a variable of some type.
Run in a loop and assign all the possible ASCII characters to this variable and print them, one by one.
Is something similar possible for UNICODE also?

I'm not sure how exactly you want to print, but this will console.log printable ascii
for(var i=32;i<127;++i) console.log(String.fromCharCode(i));
You can document.write then if that's your intention. And if the environment is unicode, it should work for unicode as well, I believe.

Others have shown how to print the printable Ascii characters. It is possible to print all other Ascii characters, too, though they are control characters with system-dependent effect (often no effect). To create a string containing all Ascii characters into a string, you could do this:
var s = '';
for (var i = 0; i <= 127; i++) s += String.fromCharCode(i);
Unicode is much more tricky, because the Unicode coding space, from 0 to 0x10FFFF, contains a large number of unassigned code points as well as code points designated as noncharacters. There are also Private Use code points, which may be used to denote characters by “private agreement” but have no generally assigned meaning. Moreover, many Unicode characters are nonspacing, i.e. meant to combine with the preceding character (e.g., turning “a” to “â”), so you can’t visually print them in a row. There is no simple way in JavaScript to determine, from a integer, the class of the corresponding code point – you might need to read the UnicodeData.txt file, parse it, and use the information there to classify code points.
Finally, there is the programming issue that the JavaScript concept of character corresponds to a 16-bit code unit (not code point), and any Unicode code point larger than 0xFFFF needs to be represented using two code units (so-called surrogates). If you are using JavaScript in the context of an HTML document and you want to print characters in th HTML content, then the simplest way is to use character references like 𐐀 (which denotes the Unicode character at code point 10400 hexadecimal) and assign the string to the innerHTML property of an element.
If you need to write ranges of Unicode characters, you might take a look at the Full Unicode Input utility that I recently wrote. Its source code illustrates some ways of dealing with Unicode characters in JavaScript.

There are some of the ASCII characters that are non-printable, but for example getting the characters from 32 (space) to 126 (~), you would use:
var s = '';
for (var i = 32; i <= 127; i++) s += String.fromCharCode(i);
The unicode character set has more than 110,000 different characters (see Unicode), but a normal font doesn't contain all of them, so you can't display them anyway. You would have to specify what parts of the character space you are interested in.

JavaScript backslash (\) in variables is causing an error

In Javascript, when I put a backslash in some variables like:
var ttt = "aa ///\\\";
var ttt = "aa ///\";
Javascript shows an error.
If I try to restrict user in entering this character, I also get an error:
(("aaa ///\\\").indexOf('"') != -1)
Restricting backslashes from user input is not a good strategy, because you have to show an annoying message to the user.
Why am I getting an error with backslash?

The backslash (\) is an escape character in Javascript (along with a lot of other C-like languages). This means that when Javascript encounters a backslash, it tries to escape the following character. For instance, \n is a newline character (rather than a backslash followed by the letter n).
In order to output a literal backslash, you need to escape it. That means \\ will output a single backslash (and \\\\ will output two, and so on). The reason "aa ///\" doesn't work is because the backslash escapes the " (which will print a literal quote), and thus your string is not properly terminated. Similarly, "aa ///\\\" won't work, because the last backslash again escapes the quote.
Just remember, for each backslash you want to output, you need to give Javascript two.

You may want to try the following, which is more or less the standard way to escape user input:
function stringEscape(s) {
return s ? s.replace(/\\/g,'\\\\').replace(/\n/g,'\\n').replace(/\t/g,'\\t').replace(/\v/g,'\\v').replace(/'/g,"\\'").replace(/"/g,'\\"').replace(/[\x00-\x1F\x80-\x9F]/g,hex) : s;
function hex(c) { var v = '0'+c.charCodeAt(0).toString(16); return '\\x'+v.substr(v.length-2); }
}
This replaces all backslashes with an escaped backslash, and then proceeds to escape other non-printable characters to their escaped form. It also escapes single and double quotes, so you can use the output as a string constructor even in eval (which is a bad idea by itself, considering that you are using user input). But in any case, it should do the job you want.

You have to escape each \ to be \\:
var ttt = "aa ///\\\\\\";
Updated: I think this question is not about the escape character in string at all. The asker doesn't seem to explain the problem correctly.
because you had to show a message to user that user can't give a name which has (\) character.
I think the scenario is like:
var user_input_name = document.getElementById('the_name').value;
Then the asker wants to check if user_input_name contains any [\]. If so, then alert the user.
If user enters [aa ///\] in HTML input box, then if you alert(user_input_name), you will see [aaa ///\]. You don't need to escape, i.e. replace [\] to be [\\] in JavaScript code. When you do escaping, that is because you are trying to make of a string which contain special characters in JavaScript source code. If you don't do it, it won't be parsed correct. Since you already get a string, you don't need to pass it into an escaping function. If you do so, I am guessing you are generating another JavaScript code from a JavaScript code, but it's not the case here.
I am guessing asker wants to simulate the input, so we can understand the problem. Unfortunately, asker doesn't understand JavaScript well. Therefore, a syntax error code being supplied to us:
var ttt = "aa ///\";
Hence, we assume the asker having problem with escaping.
If you want to simulate, you code must be valid at first place.
var ttt = "aa ///\\"; // <- This is correct
// var ttt = "aa ///\"; // <- This is not.
alert(ttt); // You will see [aa ///\] in dialog, which is what you expect, right?
Now, you only need to do is
var user_input_name = document.getElementById('the_name').value;
if (user_input_name.indexOf("\\") >= 0) { // There is a [\] in the string
alert("\\ is not allowed to be used!"); // User reads [\ is not allowed to be used]
do_something_else();
}
Edit: I used [] to quote text to be shown, so it would be less confused than using "".

The backslash \ is reserved for use as an escape character in Javascript.
To use a backslash literally you need to use two backslashes
\\

If you want to use special character in javascript variable value, Escape Character (\) is required.
Backslash in your example is special character, too.
So you should do something like this,
var ttt = "aa ///\\\\\\"; // --> ///\\\
or
var ttt = "aa ///\\"; // --> ///\
But Escape Character not require for user input.
When you press / in prompt box or input field then submit, that means single /.

Regex wordwrap with UTF8 characters in JS

i've already read all tha articles in here wich touch a similar problem but still don't get any solution working. In my case i wanna wrap each word of a string with a span. The words contain special characters like 'äüö...'
What i am doing at the moment is:
var textWrap = text.replace(/\b([a-zA-Z0-9ßÄÖÜäöüÑñÉéÈèÁáÀàÂâŶĈĉĜĝŷÊêÔôÛûŴŵ-]+)\b/g, "<span>$1</span>");
But what happens is that if the äüñ or whatever NON-Ascii character is at the end or at the beginning it also acts like a boundary. Being within a word these characters do't act as a boundary.
'Ärmelkanal' becomes Ä<span>rmelkanal</span> but should be <span>Ärmelkanal</span>
'Käse'works fine... becomes <span>Käse</span>
'diré' becomes <span>dir</span>é but should be <span>diré</span>
Any advice would be very appreciated. I need to do that on clientside :-( BTW did i mention that i hate regular expressions ;-)
Thank You very much!

The problem is that JavaScript recognizes word boundaries only before/after ASCII letters (and numbers/underscore). Just drop the \b anchors and it should work.
result = subject.replace(/[a-zA-Z0-9ßÄÖÜäöüÑñÉéÈèÁáÀàÂâŶĈĉĜĝŷÊêÔôÛûŴŵ-]+/g, "<span>$&</span>");