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

Use a JSON parser like JSON.parse:

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

I know i'm 3 years late to this question, but I felt like chiming in.

While Gumbo's solution works great, it doesn't handle a few cases where no exception is raised for JSON.parse({something that isn't JSON})

I also prefer to return the parsed JSON at the same time, so the calling code doesn't have to call JSON.parse(jsonString) a second time.

This seems to work well for my needs:

/** * If you don't care about primitives and only objects then this function * is for you, otherwise look elsewhere. * This function will return `false` for any valid json primitive. * EG, 'true' -> false *     '123' -> false *     'null' -> false *     '"I'm a string"' -> false */function tryParseJSONObject (jsonString){    try {        var o = JSON.parse(jsonString);        // Handle non-exception-throwing cases:        // Neither JSON.parse(false) or JSON.parse(1234) throw errors, hence the type-checking,        // but... JSON.parse(null) returns null, and typeof null === "object",         // so we must check for that, too. Thankfully, null is falsey, so this suffices:        if (o && typeof o === "object") {            return o;        }    }    catch (e) { }    return false;};

A comment first. The question was about not using try/catch.
If you do not mind to use it, read the answer below.Here we just check a JSON string using a regexp, and it will work in most cases, not all cases.

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}

EDIT: The new version of json2.js makes a more advanced parsing than above, but still based on a regexp replace ( from the comment of @Mrchief )