AJAX: Check if a string is JSON?

Question

My JavaScript sometimes crashes on this line:

var json = eval('(' + this.responseText + ')');

Crashes are caused when the argument of eval() is not JSON. Is there any way to check if the string is JSON before making this call?

I don't want to use a framework - is there any way to make this work using just eval()? (There's a good reason, I promise.)

You could attempt to JSON.parse() in a try/catch... if you get to catch, its not valid JSON markup. Of course, thats sorta inefficient, heh... Could you give me an example of the invalid JSON markup you're getting? — Warty, Feb 22 '10 at 19:50
Possible duplicate of [How to check if a string is a valid JSON string in JavaScript without using Try/Catch](https://stackoverflow.com/questions/3710204/how-to-check-if-a-string-is-a-valid-json-string-in-javascript-without-using-try) — Donald Duck, Jun 21 '19 at 15:47

score 158 · Answer 1 · edited Aug 15 '20 at 10:48

158

If you include the JSON parser from json.org, you can use its parse() function and just wrap it in a try/catch, like so:

try
{
   var json = JSON.parse(this.responseText);
}
catch(e)
{
   alert('invalid json');
}

Something like that would probably do what you want.

edited Aug 15 '20 at 10:48

Ry-

218,210
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answered Feb 22 '10 at 19:50

brettkelly

27,655
8
56
72

10

using jQuery.parseJSON(..) you wouldn't need to include json.org – RayLoveless Dec 27 '12 at 05:57
1

@Raymo OP didn't mention using jQuery and json2.js less than half the size of jQuery (in terms of file size). – brettkelly Dec 27 '12 at 17:27
Parsing an entire string is bad practice, and throwing an exception can cause lag – redolent Jun 29 '16 at 23:20
it will not throw an exception if you send a number string – Hesham Yassin Jul 30 '18 at 11:20

score 22 · Answer 2 · answered Dec 27 '12 at 05:54

22

Hers's the jQuery alternative...

try
{
  var jsonObject = jQuery.parseJSON(yourJsonString);
}
catch(e)
{
  // handle error 
}

answered Dec 27 '12 at 05:54

RayLoveless

19,880
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94

score 16 · Answer 3 · answered Feb 22 '10 at 19:50

I highly recommend you use a javascript JSON library for serializing to and from JSON. eval() is a security risk which should never be used unless you are absolutely certain that its input is sanitized and safe.

With a JSON library in place, just wrap the call to its parse() equivalent in a try/catch-block to handle non-JSON input:

try
{
  var jsonObject = JSON.parse(yourJsonString);
}
catch(e)
{
  // handle error 
}

score 2 · Answer 4 · edited Oct 24 '18 at 11:01

Maybe this helps: With this code, you can get directly your data…

<!DOCTYPE html>
<html>
<body>

<h3>Open console, please, to view result!</h3>
<p id="demo"></p>

<script>
var tryJSON = function (test) {
 try {
     JSON.parse(test);
 }
 catch(err) {
     // maybe you need to escape this… (or not)
     test = '"'+test.replace(/\\?"/g,'\\"')+'"';
 }
 eval('test = '+test);
 console.debug('Try json:', test);
};

// test with string…
var test = 'bonjour "mister"';
tryJSON(test);
// test with JSON…
var test = '{"fr-FR": "<p>Ceci est un texte en français !</p>","en-GB": "<p>And here, a text in english!</p>","nl-NL": "","es-ES": ""}';
tryJSON(test);
</script>

</body>
</html>

There are a lot of alternative ways to achieve the same result, using eval() is probably be the least appropriate one. — David, Apr 26 '19 at 14:48

score 1 · Answer 5 · answered Jul 30 '18 at 11:33

The problem with depending on the try-catch approach is that JSON.parse('123') = 123 and it will not throw an exception. Therefore, In addition to the try-catch, we need to check the type as follows:

function isJsonStr(str) {
    var parsedStr = str;
    try {
        parsedStr = JSON.parse(str);
    } catch (e) {
        return false;
    }
    return typeof parsedStr == 'object'
}

score 0 · Answer 6 · answered Aug 29 '19 at 21:45

0

Why you can't just check what is the response? It is more more efficient.

var result;

if (response.headers['Content-Type'] === 'application/json')
    result = JSON.parse(this.responseText);
else
    result = this.responseText;

screen1

answered Aug 29 '19 at 21:45

ADM-IT

3,719
1
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26

score 0 · Answer 7 · answered Aug 27 '21 at 22:12

0

jQuery $.ajax() will add the responseJSON property to the response object, and to test if the response is JSON, you can use:

if (xhr.hasOwnProperty('responseJSON')) {}

answered Aug 27 '21 at 22:12

rémy

1,026
13
18

score -1 · Answer 8 · answered Feb 23 '19 at 00:20

There is a tiny library that checks JavaScript types: is.js

is.json({foo: 'bar'});
=> true

// functions are returning as false
is.json(toString);
=> false

is.not.json([]);
=> true

is.all.json({}, 1);
=> false

is.any.json({}, 2);
=> true

// 'all' and 'any' interfaces can also take array parameter
is.all.json([{}, {foo: 'bar'}]);
=> true

Actually is.js is much more then this, some honorable mentions:

var obj = document.createElement('div');
is.domNode(obj);
=> true

is.error(new Error());
=> true

is.function(toString);
=> true

is.chrome();
=> true if current browser is chrome

AJAX: Check if a string is JSON?

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