使用转义引号将只读字符串解析为JSON

Question

The title of my question can be somewhat confusing, so I'll try to explain my situation:

From an external source (in this case a Web Socket connection), I receive a JSON string that needs to be parsed to an object in JavaScript. I do not have the possibility to modify the fashion how the string is built up in the external source. I know that there are several options to parse, like using jQuery, eval() (not always recommended), etc ... In this case, I just use JSON.parse() because it seems to work just fine.

Today I ran into a problem with double quotes being escaped in the string. Take the following example:

var readonly = '{"nam":"jso\\"n"}';

Suppose that readonly is the string that is received from the external source (as the name suggests, I can't modify how the string is formatted at this level). This is actually valid JSON, to there's really no reason for me to complain to the developers of the external source that they should modify theirs. Now I would like to parse this string to an object. As already mentioned, I use JSON.parse() for this:

var my_object = JSON.parse(readonly);

This unfortunately fails, and the browser's console throws an error message.

After research, I found that I need to add another backslash to escape the double quotes in order for this to work. So I need to find a way to change the readonly string into this:

'{"nam":"jso\\\\"n"}'

My question now is: How can I add these extra backslashes to strings of which I don't know their content up front?

EDIT: Based on the comments, here's some extra info: I actually receive a string that contains more than just the json. A fixed number of characters are in front an behind of the json. This could be an example:

'xxxx{"nam":"jso\\"n"}xxxxxx'

I use substring to extract the actual json from this string.

Answer 1

The string is incorrect - it needs to be double escaped.

var readonly = '{"nam":"jso\\"n"}';
                           ^^

Remember that you're defining this string as a javascsript string, which means it's parsed/evaluated by Javascript **FIRST*. That means the characters literally stored in the var will simply be {"nam":"json"n"} - the JS parser will have removed the backslash. And now it's corrupted/invalid json.

By doubling up the backslashes, you ensure that ONE of the backslashes will survive and be present to make it valid JSON, which can then be parsed.