Lets start with the json string:

var jsonString = '{name:John}';

you can easily determine its length:

alert(The string has +jsonString.length+ characters); // will alert 15

Then parse it to an object:

var jsonObject = JSON.parse(jsonString);

A JavaScript Object is not an Array and has no length. If you want to know how many properties it has, you will need to count them:

var propertyNames = Object.keys(jsonObject);

alert(There are +propertyNames.length+ properties in the object); // will alert 1

If Object.keys, the function to get an Array with the (own) property names from an Object, is not available in your environment (older browsers etc.), you will need to count manually:

var props = 0;

for (var key in jsonObject) {

    // if (j.hasOwnProperty(k))

    /* is only needed when your object would inherit other enumerable

       properties from a prototype object */

        props++;

}

alert(Iterated over +props+ properties); // will alert 1