XPath attribute quoting in JavaScript

Question

I am trying to build a function that will properly quote/escape an attribute in XPath. I have seen solutions posted in C# here and here, but my implementation in JavaScript results in an error "This expression is not a legal expression"

Here is my function:

function parseXPathAttribute(original){
            let result = null;
            /* If there are no double quotes, wrap in double quotes */
            if(original.indexOf("\"")<0){
                result = "\""+original+"\"";
            }else{
                /* If there are no single quotes, wrap in single quotes */
                if(original.indexOf("'")<0){
                    result = "'"+original+"'";
                }else{ /*Otherwise, we must use concat() */
                    result = original.split("\"")
                    for (let x = 0;x<result.length;x++){
                        result[x] = result[x].replace(/"/g,"\\\"");
                        if (x>0){
                            result[x] = "\\\""+result[x];
                        }
                        result[x] = "\""+result[x]+"\"";
                    }
                    result = result.join();
                    result = "concat("+result+")";
                }

            }

            return result;
        }

Sample failing input:

"'hi'"

Sample failing output:

concat("","\"'hi'","\"")]

I don't understand why it is an illegal expression (given that the double quotes are escaped), so I don't know how to fix the function.

Answer 1

\ is not an escape character in XPath string literals. (If it was, you could just backslash-escape one of the quotes, and never have to worry about concat!) "\" is a complete string in itself, which is then followed by 'hi..., which doesn't make sense.

So there should be no backslashes in your output, it should look something like:

concat('"', "'hi'", '"')

I suggest:

function xpathStringLiteral(s) {
    if (s.indexOf('"')===-1)
        return '"'+s+'"';
    if (s.indexOf("'")===-1)
        return "'"+s+"'";
    return 'concat("'+s.replace(/"/g, '",\'"\',"')+'")';
}

It's not quite as efficient as it might be (it'll include leading/trailing empty string segments if the first/last character is a double-quote), but that's unlikely to matter.

(Do you really mean let in the above? This is a non-standard Mozilla-only langauge feature; one would typically use var.)