Uncaught Error: The property or field has not been initialized.
It has not been requested or the request has not been executed.
It may need to be explicitly requested.

var CustomAction = function(){

    var clientContext = new SP.ClientContext.get_current();
    var web = clientContext.get_web();
    this.oList = web.get_lists().getByTitle("Classification");

    // .load() tells CSOM to load the properties of this object
    // multiple .load()s can be stacked
    clientContext.load(oList);

    // now start the asynchronous call and perform all commands
    clientContext.executeQueryAsync(Function.createDelegate(this, this.onSuccess), Function.createDelegate(this, this.onFailure));

    // method will exit here and onSuccess or OnFail will be called asynchronously

};

function onSuccess(sender, args) {
    alert('No of rows: ' + oList.get_itemCount());
};

function onFail(sender, args) {
    alert('Request failed.n' + args.get_message() + 'n' + args.get_stackTrace());
};