How do you safely wrap a JS string variable in double quote chars?

Obviously when you're creating an actual string literal yourself, you backslash escape the double quote characters yourself.

var foo = "baz\"bat";

Just as you would with the handful of other control characters, like linebreaks and backslashes.

var bar = "baz\\bat\nmynew line and a \"quote\" ";

but if you're just wrapping that existing variable in quote character, ie to give it to some other system that requ开发者_高级运维ires quoted input, there's some confusion.

Obviously you have to escape any potential double quote characters that are in the string.

var doubleQuoteRe = /\"/g;
var quoted = "\"" + unquoted.replace(escaper, '\\\"') + "\"";

But according to some you also now have to worry about escaping literal backslash characters in the variable. In other words using much bigger hammer than my little regex. However i dont see why.

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You might want to avoid escaping quotes you already escaped-

String.prototype.inquotes=function(){
 return '"'+this.replace(/(^|[^\\])"/g,'$1\\"')+'"';
}

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The answer is that yes, you have to do two things:

1. replace literal backslash characters in the string, with two backslashes,
2. THEN, you proceed replacing any occurrences of " with \".

the simplest explanation for why step 1 is essential, is to consider the 5-character string :

foo\"   

After the first 3 characters (foo), there is a literal backslash character in the string, and then there is a literal double quote character.

(Put another way, as a string literal this would look like "foo\"")

If I were to only replace the quote character, i'd end up with a quoted string whose value was

foo\\"     

But the two backslashes here will be interpreted as a single backslash. So when I wrap this value in quotes, I end up with unbalanced quotes.

"foo\\""

on the other hand, if I do step 1 first -- replacing all backslashes with double backslashes gives

foo\\"

and then step 2 -- replacing the quote with slash-quote gives

foo\\\"

Now when i wrap my value in quote characters i finally get

"foo\\\""

which is correct.

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There is a non standard str.quote() in FF

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/quote They suggest the following polyfill

    if(!String.prototype.quote){
  // oop version - no dependencies
  String.prototype.quote = (function(){
    // prepare fallback
    // ----------------
    // backslash escape double quotes and backslashes
    var escp_regex = /[\\"]/g,
      escp_callback = '\\$&',
      // escape control characters
      ctrl_map = {
        '\b': '\\b', // backspace
        '\t': '\\t', // tab
        '\n': '\\n', // new line
        '\f': '\\f', // form feed
        '\r': '\\r'  // carriage return
      },
      // don't rely on `Object.keys(ctrl_map).join('')`
      ctrl_regex = new RegExp('[\b\t\n\f\r]', 'g'),
      ctrl_callback = function(match){
        return ctrl_map[match];
      },
      // hex-escape, spare out control characters and ASCII printables
      // [0-7,11,14-31,127-255]
      xhex_regex = /[\x00-\x07\x0B\x0E-\x1F\x7F-\xFF]/g,
      xhex_callback = function(match, char_code){
        char_code = match.charCodeAt(0);
        return '\\x' + (char_code < 16 ? '0' : '') + char_code;
      },
      // hex-escape all others
      uhex_regex = /[\u0100-\uFFFF]/g,
      uhex_callback = function(match, char_code){
        char_code = match.charCodeAt(0);
        return '\\u' + (char_code < 4096 ? '0' : '') + char_code;
      },
      // delegate to native `JSON.stringify` if available
      stringify = typeof JSON !== 'undefined' && JSON.stringify;

    // return actual polyfill
    // ----------------------
    return function(){
      var self = this; // promote compression
      if(self == null) throw new TypeError('can\'t convert ' + self + ' to object');
      if(stringify) return stringify(self);
      return '"' + self
        .replace(escp_regex, escp_callback)
        .replace(ctrl_regex, ctrl_callback)
        .replace(xhex_regex, xhex_callback)
        .replace(uhex_regex, uhex_callback) + '"';
    }
  }());

  // generic version - requires Function#bind
  String.quote = Function.call.bind(''.quote);
}

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You might want to escape other characters aside from quotes, eg whitespace characters (newlines!) and/or non-ASCII characters. There's Crockford's quote(), and my own implementation can be found at mercurial.intuxication.org.