How to define global variable in Google Apps Script

I see most examples from Google is they use only functions in a single giant script.

e.g. https://developers.google.com/apps-script/quickstart/macros

But in our style, we usually write all functions under a single namespace, such as

MyCompany = (MyCompany || {});
MyCompany.init = function () {
    Logger.log('init');  
};

function onOpen() {
    var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
    var menus = [{
        name: "Init",
        functionName: MyCompany.init
    }];
    spreadsheet.addMenu("Test", menus);
};

However, when I run the code above, it return

"MyCompany is not defined."

How to solve?