up vote 16 down vote favorite
1

Something like:

var jsonString = '{ "Id": 1, "Name": "Coke" }';

//should be true
IsJsonString(jsonString);

//should be false
IsJsonString("foo");
IsJsonString("<div>foo</div>")

EDIT: The solution should not contain try/catch. Some of us turn on "break on all errors" and they don't like the debugger breaking on those invalid Json strings.

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5 Answers

up vote 28 down vote accepted

Use a JSON parser like JSON.parse:

function IsJsonString(str) {
    try {
        JSON.parse(str);
    } catch (e) {
        return false;
    }
    return true;
}
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Thank you, but I just ran this with the team and they want something that doesn't use try/catch. The question is edited along with a new title. Sorry about that. – Chi Chan Sep 14 '10 at 15:26
1  
Get it at json.org/json2.js. It does not use try / catch. You can modify the code to replace the throw statements. – Mark Lutton Sep 14 '10 at 15:27
Good call Mark! json2.js is free to copy and modify too! – Chi Chan Sep 14 '10 at 15:47
Hm... Strange... I used this example and it works fine for me (JSON object from PHP's json_encode passes as valid), but it doesn't work for the example in question: { "Id": 1, "Name": "Coke" }. It returns false for me. – trejder Sep 29 '12 at 9:16
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up vote 0 down vote

Maybe it will useful:

    function parseJson(code)
{
    try {
        return JSON.parse(code);
    } catch (e) {
        return code;
    }
}
function parseJsonJQ(code)
{
    try {
        return $.parseJSON(code);
    } catch (e) {
        return code;
    }
}

var str =  "{\"a\":1,\"b\":2,\"c\":3,\"d\":4,\"e\":5}";
alert(typeof parseJson(str));
alert(typeof parseJsonJQ(str));
var str_b  = "c";
alert(typeof parseJson(str_b));
alert(typeof parseJsonJQ(str_b));

output:

IE7: string,object,string,string

CHROME: object,object,string,string

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up vote 3 down vote

Have a look around the line 450 in https://github.com/douglascrockford/JSON-js/blob/master/json2.js

There is a regexp that check for a valid JSON, something like:

if (/^[\],:{}\s]*$/.test(text.replace(/\\["\\\/bfnrtu]/g, '@').
replace(/"[^"\\\n\r]*"|true|false|null|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/g, ']').
replace(/(?:^|:|,)(?:\s*\[)+/g, ''))) {

  //the json is ok

}else{

  //the json is not ok

}
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Thanks Mic, This is what I ended up doing, except I left the "if (cx.test(text))" before the regexp check. – Chi Chan Sep 14 '10 at 15:55
2  
This is only checking if the code is safe for eval to use. For example the following string '2011-6-27' would pass that test. – SystemicPlural Jul 27 '11 at 16:22
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up vote 1 down vote

You can use the javascript eval() function to verify if it's valid.

e.g.

var jsonString = '{ "Id": 1, "Name": "Coke" }';
var json;

try {
  json = eval(jsonString);
} catch (exception) {
  //It's advisable to always catch an exception since eval() is a javascript executor...
  json = null;
}

if (json) {
  //this is json
}

Alternatively, you can use JSON.parse function from json.org:

try {
  json = JSON.parse(jsonString);
} catch (exception) {
  json = null;
}

if (json) {
  //this is json
}

Hope this helps.

WARNING eval() is Dangerous if someone adds malicious JS code, since it will execute it. Make sure the JSON String is trustworthy, i.e. you got it from a trusted source.

Edit For my 1st solution, it's recommended to do this.

 try {
      json = eval("{" + jsonString + "}");
    } catch (exception) {
      //It's advisable to always catch an exception since eval() is a javascript executor...
      json = null;
    }

To guarantee json-ness. If the jsonString isn't pure JSON, the eval will throw an exception.

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First example using eval says that "<div>foo</div>" is valid JSON. It may work differently in different browsers, but it appears that in FireFox, eval() accepts XML. – Mark Lutton Sep 14 '10 at 15:26
Thank you, but I just ran this with the team and they want something that doesn't use try/catch. The question is edited along with a new title. Sorry about that. – Chi Chan Sep 14 '10 at 15:26
@Mark Lutton, the object type won't be of JSON but of XML Dom Document (I forgot what the exact type in firefox is). – Buhake Sindi Sep 14 '10 at 15:30
@Chi Chan. You can use option 2 without using try/catch. Without using try/catch you basically allowing harm to come to your program. – Buhake Sindi Sep 14 '10 at 15:32
eval also accepts valid JavaScript, like "alert(5);" and strings in single quotes, which are not valid JSON. – Mark Lutton Sep 14 '10 at 15:48
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up vote 1 down vote

in prototype js we have method isJSON. try that

http://api.prototypejs.org/language/string/prototype/isjson/

even http://www.prototypejs.org/learn/json

"something".isJSON();
// -> false
"\"something\"".isJSON();
// -> true
"{ foo: 42 }".isJSON();
// -> false
"{ \"foo\": 42 }".isJSON();
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3  
Thanks, but I think using the prototype library to do this is a little overkilled. – Chi Chan Sep 14 '10 at 15:29
1  
You gave FOUR examples but only THREE results. What is the result for "{ foo: 42 }".isJSON()? If false, as I assume (result should follow function it document), then good question is, why it is false? { foo: 42 } seems to be perfectly valid JSON. – trejder Sep 29 '12 at 9:58
1  
@trejder Unfortunately, the JSON spec requires quoted keys. – mikermcneil Oct 8 '12 at 1:27
And "2002-12-15".isJSON returns true, while JSON.parse("2002-12-15") throws an error. – ychaouche Nov 13 '12 at 17:14
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