Strange JSON.parse() error in node.js

Question

I'm retrieving some stringifyed JSON via TCP in node.js an want to parse it. So my approach is similiar to this. I shortend and simplified it, so you don't have to know the surrounding logic.

socket.on("data", function(data) {
    console.log(data.toString());               // Shows the original stringifyed version
    console.log(JSON.parse(data.toString()));   // Doesn't work
});

The complete (beautified) JSON is this. As you can see, there are no errors.

{
    "result": "success",
    "source": "chat",
    "success": {
        "message": "test",
        "time": 1331770513,
        "player": "silvinci"
    }
}

But JSON.parse(data.toString()) always throws me this dumb error:

{"result":"success","source":"console","success":{"time":1331762264,"line":"20
^
SyntaxError: Unexpected token {
    at Object.parse (native)
    at Socket.<anonymous> (/home/node/api.js:152:35)    // irrelevant from here on
    at Socket.emit (events.js:67:17)
    at TCP.onread (net.js:347:14)

So I thougt: "What could be wrong with the JSON-String. Let's try it directly. Should not work." Surprise, Surprise! It worked. Why does it work when I directly enter the String?

Maybe the stream contains some weird control characters... is data a string or already the parsed object (or something else)? — Felix Kling
– Felix Kling, Commented Mar 15, 2012 at 1:40
@FelixKling has a good point. What is data.toString().charCodeAt(0)? — Matthew Flaschen
– Matthew Flaschen, Commented Mar 15, 2012 at 1:41
You are right, and I'm dumb. There is one line break hidden inside the JSON, that wasn't encoded the right way. — buschtoens
– buschtoens, Commented Mar 15, 2012 at 1:46
@silvinci, ah. Regular CR LF are okay (RFC 4627 2.), but it might have been something like a BOM. — Matthew Flaschen
– Matthew Flaschen, Commented Mar 15, 2012 at 1:54

Community · Accepted Answer · 2017-05-23 11:53:35Z

18

Thanks to @Felix Kling I found my bug. It's very important to filter unescaped characters, especially outside the stringified JSON. I didn't and overlooked an invisible linebreak right after the stringified JSON.

This is the fix:

socket.on("data", function(data) {
    console.log(data.toString());                          // Shows the original stringified version
    console.log(JSON.parse(data.toString().slice(0, -4))); // Trim the sequence "\r\n" off the end of the string
});

Please note, that this only works for me, as I have a very specialized case. The server is always responding in lines of JSON, that are terminated with an \r\n - not the whitespace characters, but literally backslash r and backslash n.
Your code may (or probably) fail due to other errors. But checking the server's response is a good starting point when you get parsing errors.

As @Zack correctly pointed out, this is a more general fix for removing unintended whitespace:

JSON.parse(data.toString().trim());

edited May 23, 2017 at 11:53

CommunityBot

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answered Mar 15, 2012 at 1:55

buschtoens

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4 Comments

MarkHu Over a year ago

Not knowing the context, it seems a little dangerous to unconditionally truncate the string. Maybe you should add a test for the offending characters before slicing.

buschtoens Over a year ago

In my case every response is terminated with \r\n. But you're right in general, so I appended a notice.

Zack Over a year ago

If you're seeing this issue and would like a more general solution use something like: JSON.parse(data.toString().trim()); String.prototype.trim will remove leading and trailing whitespace.

buschtoens Over a year ago

@Zack yep! Added it to the answer. :)

user3413723 · Accepted Answer · 2015-07-13 22:13:12Z

4

I had a similar issue. For a more general solution, this will work too. It takes off all whitespace before and after the string so you don't have to do the specific substring length.

JSON.parse(data.trim());

answered Jul 13, 2015 at 22:13

user3413723

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Comments

Mr.X · Accepted Answer · 2021-07-12 10:20:33Z

1

Instead of blindly removing last 4 chars, I suggest to remove the offending chars only:

socket.on("data", function(data) {
    console.log(data.toString()); // Shows the original stringified version
    console.log(JSON.parse(data.toString().replace('\t','').replace('\r','').replace('\n',''))); // Trim the sequence "\r\n" off the end of the string
});

edited Jul 12, 2021 at 10:20

answered Apr 25, 2018 at 7:57

Mr.X

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1 Comment

Raeisi Over a year ago

I've replaced \t too. conf = JSON.parse(confData.replace('\r','').replace('\n','').replace('\t',''));

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