JavaScript - how to build string with escape character?

I am writing `JScript` to be run using `Windows Script Host`.

Say I have a simple string variable:

s1 = '\n'

I want to build `s2` from two separated chars: `\` & `n`.
**naively** I would like to do:

s2 = '';
s2 += '\\';
s2 += 'n';
But this of course lead to `s1 != s2`

Can I build `s2` in such way that it has the same **interpreted meaning** as `s1`?

### Example:

WScript.Echo("1\n2")
var s;
s += '1';
s += '\\';
s += 'n';
s += '2';
WScript.Echo(s)

I'd wish both `WScript.Echo()` to print exactly the same thing.

### Note
I'm well aware that this question seems **completely idiotic**. I would probably think the same had I read it without knowing all the details. I don't expect anyone to understand the purpose . just curious to see whether it is feasible or do I need to re-think the whole thing.

You can't, this is impossible. You cannot concatenate two or more characters together to yield **one character**, which is what `\n` is. It is a string containing one single character.

You can't build it directly by appending, but you can take a string with escape sequences in it and parse it to a string with the escaped characters. In this case, you'd probably use `JSON.parse`:

var s1 = '\n',
s2 = '' + '\\' + 'n'; // '\\n'
console.log(s1 == JSON.parse('"' + s2 + '"')) // true

It's still somewhat unclear to me what you're actually trying to accomplish. However, if you're saying that you have some arbitrary string like `var s1 = '\n\r\t';`, and you want to produce from that the _literal_ string `"\n\r\t"` so that when you parse the literal value you get a result that is equivalent to your original string, perhaps you can try something like this:

window.specialChars = {
8: "\\b", //backspace
9: "\\t", //tab
10: "\\n", //newline
12: "\\f", //form-feed
13: "\\r", //carriage return
39: "\\'", //single-quote
92: "\\\\" //literal backslash
};

function escapedString(source) {
if (! source) {
return undefined;
}

var result = "";
for (var index = 0; index < source.length; index++) {
var code = source.charCodeAt(index);
result += specialChars[code] ? specialChars[code] : String.fromCharCode(code);
}

return result;
};

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Of course there are a number of problems you may run into here. Particularly if your source string may contain non-ASCII characters or spurious escape sequences (for instance, `var s1 = '\q';` is valid JavaScript and will store a value of `"q"` in `s1`, and when processing that there's no way to tell that the original source string has a spurious `\` in it).

It may be that there's a better alternative to solving whatever underlying problem you're attempting to solve with this "unparse the string" approach.

I actually have the need to do this today. In particular, I need it for building up octal and hexadecimal escape sequences of the form `\x39` or `\123`.

The method I'm using is simply to use an `eval`:

var str = eval('"\\x' + '3' + '9' +'"') // str === "9"

That's a reasonable generic solution. For octal and hex codes, it would be better to parse the string into an int and then build a string from that

var str = String.fromCharCode(parseInt('39', 16))