var escapable = /[\\"\x00-\x1f\x7f-\uffff]/g,
        meta = {    // table of character substitutions
            '\b': '\b',
            '\t': '\t',
            '\n': '\n',
            '\f': '\f',
            '\r': '\r',
            '"' : '\"',
            '\': '\\'
        };

    function quote(string) {

// If the string contains no control characters, no quote characters, and no
// backslash characters, then we can safely slap some quotes around it.
// Otherwise we must also replace the offending characters with safe escape
// sequences.

        escapable.lastIndex = 0;
        return escapable.test(string) ?
            '"' + string.replace(escapable, function (a) {
                var c = meta[a];
                return typeof c === 'string' ? c :
                    '\u' + ('0000' + a.charCodeAt(0).toString(16)).slice(-4);
            }) + '"' :
            '"' + string + '"';
    }

e.g. quote("Doppelgänger!") will be "Doppelg\u00e4nger!"

var encoded = quote("Doppelgänger!");
var back = eval(encoded);