Modern browsers support JSON.parse().

var arr_from_json = JSON.parse( json_string );

In browsers that don't, you can include the json2 library.

You could create a function that initializes those objects for you. Here's one I quickly drafted:

function parseJSONToObject(str) {
    var json = JSON.parse(str);

    var name = null;
    for(var i in json) { //Get the first property to act as name
        name = i;
        break;
    }

    if (name == null)
        return null;

    var obj = new window[name]();
    for(var i in json[name])
        obj[i] = json[name][i];

    return obj;
}

This creates an object of the type represented by the name of the first attribute, and assigns it's values according to the attributes of the object of the first attribute. You could use it like that:

var identifier = parseJSONToObject('{"Identifier": { "name":"uno","contextId":"dos"}}');
console.log(identifier);

Live example