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why does '\\\[' equals '\\[' ? How does backslash work in string?

As the title
console.log('\\\[' === '\\[');
returns true.
Can anyone explain in detail what's the difference?

A backslash before most characters will only be parsed as an unnecessary escape character - the backslash will be ignored. This is what's happening in the second part of the first string. Before a certain few characters though, such as another backslash in \\, or \n, it will be parsed as a escape sequence. \\ is the escape sequence for a single literal backslash:
console.log('\\');
and is only one character.
A backslash before a [ will resolve to just the [, though:
console.log('\[');
So:
'\\\[' - A literal backslash, followed by an (unnecessarily escaped) [
'\\[' - A literal backslash, followed by a plain [
See MDN for a list of escape sequences.

In strings, the backslash (\) is a special character used to encode other special characters, including the backslash.
'\\[' is a JavaScript string literal that contains a backslash (\\) and an open square bracket ([). In the compiled program the string is \[.
'\\\[' is a JavaScript string literal that contains a correctly encoded backslash (\\) followed by the combination of characters \[ that looks like an escape sequence but doesn't mean anything. Because this combination is not defined and \ by itself does not mean anything, the JavaScript interpreter ignores the backslash and corrects the string; it becomes identical to the first one (\[).
The behaviour is documented:
For characters not listed in the table, a preceding backslash is ignored, but this usage is deprecated and should be avoided.

Backslash is a special character. Literally, JS talk to browser to interpret the symbol after \ as is. Sometimes it calls screening or shielding.
That is why we can write smth like that: console.log("Double \"quotes\" inside another one."); with the result of Double "quotes" inside another one. without any error. Although that is not the way we need to use anywhere.
"\\\[" separates into 2 parts: \\ and \[. First returns \ and the second returns [. Finally it is \[.
"\\[" separates into 2 parts: \\ and [. First returns \ and the second returns [. Finally it is \[.

Regex match with '\' slash and replace with '\\'?

I was converting normal string in to latex format.So i was created the latex code match and replace the \ single slash into \\ double slash.why the i need it Refer this link.I tried Below code :
function test(){
var tex="$$\left[ x=\left({{11}\over{2}}+{{\sqrt{3271}}\over{2\,3^{{{3}\over{2} $$";
var tex_form = tex.replace("/[\\\/\\\\\.\\\\]/g", "\\");
document.getElementById('demo').innerHTML=tex_form;//nothing get
}
test();
<p id="demo"></p>
Not getting any output data.But the match in this link
i wish to need replace the \ into \\

There are these issues:
The string literal has no backslashes;
The regular expression is not a regular expression;
The class in the intended regular expression cannot match sequences, only single characters;
The replacement would not add backslashes, only replace with them.
Here you find the details on each point:
1. How to Encode Backslashes in String Literals
Your tex variable has no backslashes. This is because a backslash in a string literal is not taken as a literal backslash, but as an escape for interpreting the character that follows it.
When you have "$$\left...", then the \l means "literal l", and so the content of your variable will be:
$$left...
As an l does not need to be escaped, the backslash is completely unnecessary, and these two assignments result in the same string value:
var tex="$$\left[ x=\left({{11}\over{2}}+{{\sqrt{3271}}\over{2\,3^{{{3}\over{2} $$";
var tex="$$left[ x=left({{11}over{2}}+{{sqrt{3271}}over{2,3^{{{3}over{2} $$";
To bring the point home, this will also represent the same value:
var tex="\$\$\l\e\f\t\[\ \x\=\l\e\f\t\(\{\{\1\1\}\o\v\e\r\{\2\}\}\+\{\{\s\q\r\t\{\3\2\7\1\}\}\o\v\e\r\{\2\,\3\^\{\{\{\3\}\o\v\e\r\{\2\}\ \$\$";
If you really want to have literal backslashes in your content (which I understand you do, as this is about LaTeX), then you need to escape each of those backslashes... with a backslash:
var tex="$$\\left[ x=\\left({{11}\\over{2}}+{{\\sqrt{3271}}\\over{2\\,3^{{{3}\\over{2} $$";
Now the content of your tex variable will be this string:
$$\left[ x=\left({{11}\over{2}}+{{\sqrt{3271}}\over{2\,3^{{{3}\over{2} $$
2. How to Code Regular Expression Literals
You are passing a string literal to the first argument of replace, while you really intend to pass a regular expression literal. You should leave out the quotes for that to happen. The / are the delimiters of a regular expression literal, not quotes:
/[\\\/\\\\\.\\\\]/g
This should not be wrapped in quotes. JavaScript understands the / delimiters as denoting a regular expression literal, including the optional modifiers at the end (like g here).
3. Classes are sets of single characters
This regular expression has unnecessary characters. The class [...] should list all individual characters you want to match. Currently you have these characters (after resolving the escapes):
\
/
\
\
.
\
\
It is overkill to have the backslash represented 5 times. Also, in JavaScript the forward slash and dot do not need to be escaped when occurring in a class. So the above regular expression is equivalent to this one:
/[\\/.]/g
Maybe this is, or is not, what you intended to match. To match several sequences of characters, you could use the | operator. This is just an example:
/\\\\|\\\/|\\\./g
... but I don't think you need this.
4. How to actually prefix with backslashes
It seems strange to me that you would want to replace a point or forward slash with a backslash. Probably you want to prefix those with a backslash. In that case make a capture group (with parentheses) and refer to it with $1 in this replace:
tex.replace(/([\\/.])/g, "\\$1");
Note again, that in the replacement string there is only one literal backslash, as the first one is an escape (see point 1 above).

why the i need it
As the question you link to says, the \ character has special meaning inside a JavaScript string literal. It represents an escape sequence.
Not getting any output data.But the match in this link
The escape sequence is processed when the string literal is parsed by the JavaScript compiler.
By the time you apply your regular expression to them, they have been consumed. The slash characters only exist in your source code, not in your data.
If you want to put a slash character in your string, then you need to write the escape sequence for it (the \\) in the source code. You can't add them back in with JavaScript afterwards.

Not sure if I understood the problem, but try this code:
var tex_form = tex.replace("/(\\)/g","\\\\");.
You need to use '(' ')' instead of '['']' to get a match for output.

Javascript strings with symbol '\'

I am having difficulties handling symbol \ in javascript. Strings seems to ignore it, for example alert('a\b') would only alert a.
My goal is to write following function which would give me latex image:
function getLatexImage(tex)
{
return 'http://chart.apis.google.com/chart?cht=tx&chl=' + tex;
}
However calling getLatexImage('\frac{a}{b}') gives me: "http://chart.apis.google.com/chart?cht=tx&chl=rac{a}{b}"
\f is being ignored.
Any suggestions?

\ is an escape character. It starts an escape sequence.
\n is a new line. \t is a tab. An escape sequence that has no special meaning usually gets turned into the character on the RHS (so \b is b).
To have a backslash as data in a string literal you have to escape it:
alert('a\\b');

Use \\. Single slashes are used for special signs (like \n, \t..)

The backslash \ is a escape character in JavaScript and many other programming languages. If you want to output it, you'll need to escape it by itself.
\\a
for example would output \a.
That being said, if you want to use it in an url you should encode it for safety.
%5c
translates to \

You can simply escape it by adding another backslash
'\\b'

JSON.parse with newline [duplicate]

This question already has answers here:
How do I handle newlines in JSON?
(10 answers)
Closed 7 years ago.
Why can't you parse a json with a \n character in javascript
JSON.parse('{"x": "\n"}')
However when you do JSON.parse(JSON.stringify({"x" : "\n"})), it is valid.
http://www.jslint.com/ says that {"x": "\n"} is a valid JSON. I wonder what does spec says about this?
Update:
For those who marked this duplicate, this is not the same question as "How to handle newlines in JSON". This question is more about why can't an unescaped newline character allowed in JSON.

JSON.parse('{"x": "\n"}') fails because '{"x": "\n"}' is not a valid JSON string due to the unescaped slash symbol.
JSON.parse() requires a valid JSON string to work.
'{"x": "\\n"}' is a valid JSON string as the slash is now escaped, so JSON.parse('{"x": "\\n"}') will work.
JSON.parse(JSON.stringify({"x" : "\n"})) works because JSON.stringify internally escapes the slash character.
The result of JSON.stringify({"x" : "\n"}) is {"x":"\n"} but if you try to parse this using JSON.parse('{"x":"\n"})' it will FAIL, as it is not escaped. As JSON.stringify returns an escaped character, JSON.parse(JSON.stringify()) will work.

It need to be:
JSON.parse('{"x": "\\n"}')
You must use \\ to escape the character.
Why it's invalid?
It' from rfc4627 specs
All Unicode characters may be placed within the quotation marks except for the characters that must be escaped: quotation mark, reverse solidus, and the control characters (U+0000 through U+001F)." Since a newline is a control character, it must be escaped.

In JavaScript "\n" is not the literal string \n. It is a string containing a newline character. That makes your JSON invalid.
To represent the literal string \n, you need to escape the initial backslash character with another backslash character: "\\n".

You need to escape the "\" in your string (turning it into a double-"\\"), otherwise it will become a newline in the JSON source, not the JSON data.
Try to replace \n with some other characters while parsing and again replace those characters with \n before assigning this value to some control.

Javascript replace unescaped slash

I want to replace an unescaped slash in a string with backslash. But something weird happened:
"\content\hs\gj\home.css".replace(/\\/gi,"/")
which returns "contenthsgjhome.css". I understand that if change it to
"\\content\\hs\\gj\\home.css".replace(/\\/gi,"/")`
Then it will work as expected, but I can't change the string because it's just the output by nodejs path.join("conetnt", "hs", "gj", "home.css").
What I should do?

The reason it is returning "contenthsgjhome.css" is that your string doesn't really have any backslashes in it at all because single backslashes in a string literal will be ignored unless they make sense to escape the following character (e.g., "\\" or "\n"). "\c" has no special meaning as an escape so it is interpreted as "c".
"\content\hs\gj\home.css"
Ends up the same as:
"contenthsgjhome.css"
So there are no backslashes for .replace() to find.
(Note that if you do have escaped backslashes in a string literal like "\\" that is part of the literal syntax only and once interpreted the resulting string has only one backslash "\".)
Perhaps if you could explain what you mean by "it's just the output by FS" somebody could offer more advice. This is a common problem when JSP/ASP/PHP/etc outputs JS code - the escaping needs to happen in the JSP/ASP/PHP/etc code before the JS interpreter sees it.

yourstring.split(String.fromCharCode(92)).join('/')