jQuery.parseJSON returns null

Question

I am passing valid JSON on the server side (using node.js) but not able to parse on the client side (using jQuery). jQuery.parseJSON returns null. Any assistance will be much appreciated.

Server side-

var message = [{key:"1", count:1},{key:"2", count:2}];
client.publish('update', JSON.stringify(message));

Client side -

socket.on("update", function(data) {
    var obj = jQuery.parseJSON(data);
    alert (obj); // returns null
    // do something
}

@FabrícioMatté thanks for editing my question. alert(data) returns Object. — harishvc
– harishvc, Commented Sep 8, 2013 at 4:33
Well, JSON.parse takes a string, so there's the problem. If you're on Chrome (or Firefox with Firebug), use console.log(data) instead of alert then press F12 to inspect the object properties. Either your JSON string is inside a property of the data object or data is the parsed object already. — Fabrício Matté
– Fabrício Matté, Commented Sep 8, 2013 at 4:48
Exactly! I tried the technique suggested by @JonathanLonowski . See my response in the thread below. Any ideas? — harishvc
– harishvc, Commented Sep 8, 2013 at 5:51
Problem solved! socket.io emits a key value pair (not an array of key value pair). I changed my implimentation and it works now! Thanks. — harishvc
– harishvc, Commented Sep 8, 2013 at 19:29

Jonathan Lonowski · Accepted Answer · 2013-09-08 01:44:20Z

2

[...] now I get a new error message "Uncaught SyntaxError: Unexpected token o"

That error usually means that data is already the Array parsed from the JSON. You can verify this with typeof:

console.log(typeof data);
// if `string`, then it has yet to be parsed
// if `object`, then it's already been parsed

So, you may not need to use $.parseJSON() or JSON.parse() and may be able to iterate data:

for (var i = 0, l = data.length; i < l; i++) {
    console.log('Key = %s, Count = %s', data[i].key, data[i].count);
}

And, the error comes from JSON.parse() expecting a String:

Let JText be ToString(text)

So, passing an Array of Objects to it will use .toString() to produce:

"[object Object],[object Object]"

It'll start trying to parse this as another Array due to the leading [, but will find object invalid since JSON doesn't support identities.

answered Sep 8, 2013 at 1:44

Jonathan Lonowski

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1 Comment

harishvc Over a year ago

Thanks for the explanation. Much appreciated. Applied changes suggested but still having problems. Below is more information. alert(typeof data); // returns Object alert (data.length); // returns undefined // Hard-coding l=2 for now for (var i = 0, l = 2; i < l; i++) { alert(data[i].key); //Console error: Uncaught TypeError: Cannot read property 'key' of undefined } Intresting. Any ideas?

DevlshOne · Accepted Answer · 2013-09-08 00:56:02Z

1

What version of jQuery are you using? From the $.parseJSON documentation:

Prior to jQuery 1.9, $.parseJSON returned null instead of throwing an error if it was passed an empty string, null, or undefined, even though those are not valid JSON.

answered Sep 8, 2013 at 0:56

DevlshOne

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5 Comments

harishvc Over a year ago

I was using 1.7.1 . I just changed to 1.9.1 now I get a new error message "Uncaught SyntaxError: Unexpected token o"

DevlshOne Over a year ago

I have a feeling that socket is not a valid jQuery object.

harishvc Over a year ago

socket libraries are included from node.js .

DevlshOne Over a year ago

Ah, okay. I don't use node.js

harishvc Over a year ago

@JonathanLonowski How can I get the key value pairs?

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