JavaScript backslash (\) in variables is causing an error

Solution 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.

Solution 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.

Solution 3:

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 "".