How to check if a string is a valid JSON string in JavaScript without using Try/Catch

Question

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.

Answer 1

Use a JSON parser like JSON.parse:

function IsJsonString(str) {
    try {
        JSON.parse(str);
    } catch (e) {
        return false;
    }
    return true;
}

Answer 2

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();

Answer 3

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.

Answer 4

Have a look around the line 450 in http://www.json.org/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

}