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?

You might be better off using the Properties Service as you can use these as a kind of persistent global variable.

click 'file > project properties > project properties' to set a key value, or you can use

PropertiesService.getScriptProperties().setProperty('mykey', 'myvalue');

The data can be retrieved with

var myvalue = PropertiesService.getScriptProperties().getProperty('mykey');

In GAS global variables are not what they are in other languages. They are not constants nor variables available in all routines.

I thought I could use global variables for consistency amongst functions and efficiency as well. But I was wrong as pointed out by some people here at SO.

Global variable will be evaluated at each execution of a script, so not just once every time you run your application.
Global variables CAN be changed in a script (so they are not constants that cannot be changed by accident), but will be reinitialized when another script will be invoked.
There is also a speed penalty on using global variables. If within a function you use the same global variable two or more times, it will be faster to assign a local variable and use that instead.

If you want to preserve variables between all functions in your application, it might be using a cacheService will be best. I found out that looping through all files and folders on a drive takes a LOT of time. But you can store info about files and folders within cache (or even properties) and speed up at least 100 times.

The only way I use global variables now is for some prefixes and for naming widgets.

I'm using a workaround by returning a function with an object of my global variables:

function globalVariables(){
  var variables = {
    sheetName: 'Sheet1',
    variable1: 1,
    variable2: 2
  };
  return variables;
}

function functionThatUsesVariable (){
  var sheet =   SpreadsheetApp.getActiveSpreadsheet().getSheetByName(globalVariables().sheetName);
}

Global variables certainly do exist in GAS, but you must understand the client/server relationship of the environment in order to use them correctly - please see this question: https://stackoverflow.com/questions/17375360/global-variables-in-google-script-spreadsheet

However this is not the problem with your code; the documentation indicates that the function to be executed by the menu must be supplied to the method as a string, right now you are supplying the output of the function: https://developers.google.com/apps-script/reference/spreadsheet/spreadsheet#addMenu%28String,Object%29

function MainMenu_Init() {
    Logger.log('init');  
};

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

I use this: if you declare var x = 0; before the functions declarations, the variable works for all the code files, but the variable will be declare every time that you edit a cell in the spreadsheet

    var userProperties = PropertiesService.getUserProperties();


function globalSetting(){
  //creating an array
  userProperties.setProperty('gemployeeName',"Rajendra Barge");
  userProperties.setProperty('gemployeeMobile',"9822082320");
  userProperties.setProperty('gemployeeEmail'," [email protected]");
  userProperties.setProperty('gemployeeLastlogin',"03/10/2020");
  
  
  
}


var userProperties = PropertiesService.getUserProperties();
function showUserForm(){

  var templete = HtmlService.createTemplateFromFile("userForm");
  
  var html = templete.evaluate();
  html.setTitle("Customer Data");
  SpreadsheetApp.getUi().showSidebar(html);


}

function appendData(data){
 globalSetting();
 
  var ws = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Data");
  ws.appendRow([data.date,
                data.name,
                data.Kindlyattention,
                data.senderName,
                data.customereMail,
                userProperties.getProperty('gemployeeName'),
                ,
                ,
                data.paymentTerms,
                ,
                userProperties.getProperty('gemployeeMobile'),
                userProperties.getProperty('gemployeeEmail'),
                Utilities.formatDate(new Date(), "GMT+05:30", "dd-MM-yyyy HH:mm:ss")
                
                
  ]);
  
}

function errorMessage(){

Browser.msgBox("! All fields are mandetory");

}

I needed something similar like the question, you can store and fetch from the cache https://developers.google.com/apps-script/reference/cache/cache

Example:

// call cache service
 var cache = CacheService.getScriptCache();
// get an item from the cache
  var cached = cache.get("somekey");
// if exists in the cache use it
  if (cached != null) {
   // use it whatever you like. 
  }else{
   // calculate/assign your data to cache
   cache.put("somekey","somevalueorobject");

  // you can even put cache data on TTL (time to live)  in seconds.
   cache.put("somekey","somevalueorobject",60);

For constants I am using function arrow expressions. The footprint is similar to a variable declaration. Just add the () => when declaring, and () when calling the (function) variable.

var currentSheet = () => SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
var maxAttempts = () => 10;

function myFunction(){
    var sheetName = currentSheet().getName();
    for (var i=0; i< maxAttempts(); i++){
        trySomething(i);
    }
}