JavaScript backslash (\) in variables is causing an error

Question

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?

Answer 1

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.

Answer 2

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.