var jsonObj;

/**
 * Reads the .json file and parses it to a JS object (jsonObj).
 * @param callback Asynchronous function.
 */
function loadJSON( callback ) {
    // new Http request object (asynchronous), both the web page and the XML file it tries to load, must be located on the same server.
    var xobj = new XMLHttpRequest();

    xobj.overrideMimeType( "application/json" );

    // Specifies the request
    /* method: the request type GET or POST
    url: the file location
    async: true (asynchronous) or false (synchronous)
    user: optional user name
    psw: optional password */
    xobj.open( 'GET', 'plain.json', true ); // Path to the file

    // Defines a function to be called when the readyState property changes
    xobj.onreadystatechange = function GetResponseText() {
        if( xobj.readyState == 4 && xobj.status == "200" ) {
            // Required use of an anonymous callback as .open will NOT return a value but simply returns undefined in asynchronous mode
            // Returns the response data as a string
            callback( xobj.responseText );
        }
    };
    // Sends the request to the server - Used for GET requests
    xobj.send( null );
}

loadJSON(
    function JSONparse( response ) {
        // Parse JSON string into object
        jsonObj = JSON.parse( response );
        console.log( jsonObj );
    }
);