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Javascript Pattern to access the global object [duplicate]

I want to check in a script if a certain other module is already loaded.
if (ModuleName) {
// extend this module
}
But if ModuleName doesn't exist, that throws.
If I knew what the Global Object was I could use that.
if (window.ModuleName) {
// extend this module
}
But since I want my module to work with both browsers and node, rhino, etc., I can't assume window.
As I understand it, this doesn't work in ES 5 with "use strict";
var MyGLOBAL = (function () {return this;}()); // MyGlobal becomes null
This will also fail with a thrown exception
var MyGLOBAL = window || GLOBAL
So it seems like I'm left with
try {
// Extend ModuleName
}
catch(ignore) {
}
None of these cases will pass JSLint.
Am I missing anything?

Well, you can use the typeof operator, and if the identifier doesn't exist in any place of the scope chain, it will not throw a ReferenceError, it will just return "undefined":
if (typeof ModuleName != 'undefined') {
//...
}
Remember also that the this value on Global code, refers to the global object, meaning that if your if statement is on the global context, you can simply check this.ModuleName.
About the (function () { return this; }()); technique, you are right, on strict mode the this value will simply be undefined.
Under strict mode there are two ways to get a reference to the Global object, no matter where you are:
Through the Function constructor:
var global = Function('return this')();
Functions created with the Function constructor don't inherit the strictness of the caller, they are strict only if they start their body with the 'use strict' directive, otherwise they are non-strict.
This method is compatible with any ES3 implementation.
Through an indirect eval call, for example:
"use strict";
var get = eval;
var global = get("this");
The above will work because in ES5, indirect calls to eval, use the global environment as both, the variable environment and lexical environment for the eval code.
See details on Entering Eval Code, Step 1.
But be aware that the last solution will not work on ES3 implementations, because an indirect call to eval on ES3 will use the variable and lexical environments of the caller as the environments for the eval code itself.
And at last, you may find useful to detect if strict mode is supported:
var isStrictSupported = (function () { "use strict"; return !this; })();

Update 2019
With all of today's Webpacks and Broccolis, and Gulps and Grunts, and TypeScripts and AltScripts, and create-react-apps, etc, this is pretty useless, but if you're just working with plain, old, VanillaJS and you want to make it isomorphic, this is probably your best option:
var global
try {
global = Function('return this')();
} catch(e) {
global = window;
}
The Function constructor invocation will work even when using --use_strict in node, as the Function constructor always executes in a global non-strict scope.
If the Function constructor fails, it's because you're in a browser with eval disabled by CSP headers.
Of course, with Deno on the way (the node replacement), they may also disallow the Function constructor, in which case it's back to enumerating objects like global, module, exports, globalThis and window, and then duck-type checking which is the global exhaustively... :-/
Crazy one-line solution (Original):
var global = Function('return this')() || (42, eval)('this');
.
.
.
Works
in every environment (that I tested)
in strict mode
and even in a nested scope
Update 2014-Sept-23
This can now fail if HTTP headers in the latest browsers explicitly forbid eval.
A workaround would be to try / catch the original solution as only browsers are known to run this type of subset of JavaScript.
var global;
try {
global = Function('return this')() || (42, eval)('this');
} catch(e) {
global = window;
}
Example:
---
(function () {
var global = Function('return this')() || (42, eval)('this');
console.log(global);
// es3 context is `global`, es5 is `null`
(function () {
"use strict";
var global = Function('return this')() || (42, eval)('this');
console.log(global);
}());
// es3 and es5 context is 'someNewContext'
(function () {
var global = Function('return this')() || (42, eval)('this');
console.log(global);
}).call('someNewContext');
}());
Tested:
---
* Chrome v12
* Node.JS v0.4.9
* Firefox v5
* MSIE 8
Why:
---
In short: it's some weird quirk. See the comments below (or the post above)
In `strict mode` `this` is never the global, but also in `strict mode` `eval` operates in a separate context in which `this` *is* always the global.
In non-strict mode `this` is the current context. If there is no current context, it assumes the global. An anonymous function has no context and hence in non-strict mode assumes the global.
Sub Rant:
There's a silly misfeature of JavaScript that 99.9% of the time just confuses people called the 'comma operator'.
var a = 0, b = 1;
a = 0, 1; // 1
(a = 0), 1; // 1
a = (0, 1); // 1
a = (42, eval); // eval
a('this'); // the global object

Why just don't simply use this in a global scope as param to a wrapper function, as follows?
(function (global) {
'use strict';
// Code
}(this));

Here you go :)
var globalObject = (function(){return this;})();
This should work from anywhere, for example from within another closure.
Edit - just read your post more carefully and saw the part about ES5 strict mode. Can anyone shed some more light on that? This has been the accepted way to get the the global object for as long as I can remember... I sure hope it doesn't end up getting broken.
Edit 2 - CMS' answer has more info on ES5 strict mode's treatment of this.

I think this is pretty much okay in rhino, node, browser and with jslint (without additional workaround flags) - would this help? Am I missing something?
x = 1;
(function(global){
"use strict";
console.log(global.x);
}(this));
Though I myself tend to use the window object and if I do need headless testing I can use env.js (rhino) or Phantom (node).

ECMAScript will be adding this to its standard soon:
https://github.com/tc39/proposal-global
Until its done, this is what's recommended:
var getGlobal = function () {
// the only reliable means to get the global object is
// `Function('return this')()`
// However, this causes CSP violations in Chrome apps.
if (typeof self !== 'undefined') { return self; }
if (typeof window !== 'undefined') { return window; }
if (typeof global !== 'undefined') { return global; }
throw new Error('unable to locate global object');
};

This is not passing jslint: var Fn = Function, global = Fn('return this')();
Try it yourself: http://www.jslint.com/
this will: var Fn = Function, global = new Fn('return this')();
But effectively those are same thing according to MDN:
Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

This following solution works in:
Chrome
Node.JS
Firefox
MSIE
Web Workers
The code is:
(function (__global) {
// __global here points to the global object
})(typeof window !== "undefined" ? window :
typeof WorkerGlobalScope !== "undefined" ? self :
typeof global !== "undefined" ? global :
Function("return this;")());
You just need to change X for the name of the variable that you would like to

I had this problem before, I'm not happy with the solution, but it works and passes JSLint (assume browser|assume node):
"use strict";
var GLOBAL;
try{
/*BROWSER*/
GLOBAL = window;
}catch(e){
/*NODE*/
GLOBAL = global;
}
if(GLOBAL.GLOBAL !== GLOBAL){
throw new Error("library cannot find the global object");
}
once you have the GLOBAL var you can do your checking, and at the end of the script type
delete GLOBAL.GLOBAL;

Here's what I am using:
"use strict";
if(this && this.hasOwnProperty && !this.hasOwnProperty('globalScope')){
try {
globalScope = Function('return this')();
}catch(ex){
if(this.hasOwnProperty('window')){
globalScope = window;
}else{
throw 'globalScope not found';
}
}
}

Ways to break JavaScript Scope

JavaScript normally follows the function scope i.e. variables are accessible only within the function in which they are declared.
One of the ways to break this convention and make the variable accessible outside the function scope is to use the global window object
e.g.
window.myVar = 123;
My question is are there any other ways in JavaScript/jQuery to make the variable accessible outside the function scope?

Not with variable declarations, no. You can obviously declare a variable in an outer scope so that it's accessible to all descendant scopes:
var a; // Available globally
function example() {
a = "hello"; // References a in outer scope
}
If you're not in strict mode you can simply remove the var keyword. This is equivalent to your example:
// a has not been declared in an ancestor scope
function example() {
a = "hello"; // a is now a property of the global object
}
But this is very bad practice. It will throw a reference error if the function runs in strict mode:
function example() {
"use strict";
a = "hello"; // ReferenceError: a is not defined
}

As you wrote, variables are only visible inside the function in which they were defined (unless they are global).
What you can do is assign variables to the function object itself:
function foo() {
foo.myVar = 42;
}
console.log(foo.myVar); // outputs "undefined"
foo();
console.log(foo.myVar); // outputs "42"
But I would advise against it. You should really have a very good reason to do something like this.

You can define them as part of a global object, then add variables later
// Define your global object
var myObj = {};
// Add property (variable) to it
myObj.myVar = 'Hello world';
// Add method to it
myObj.myFunctions = function() {
// Do cool stuff
};
See the link below:
Variable declaration
Also, you can declare it without the var keyword. However, this may not be a good practice as it pollutes the global namespace. (Make sure strict mode is not on).
Edit:
I didn't see the other comments before posting. #JamesAllardice answer is also good.

defining an undefined global variable inside a function

This is messy stuff (not my code but I'm stuck to it). A function depends on a globally defined variable.
function variableIssues(){
alert(someGlobalString); // alerts "foo"
}
Sometimes this globally defined variable is, undefined. In this case we want to cast it for further processing. The function is modified.
function variableIssues(){
alert(someGlobalString); // undefined
if (!someGlobalString){
var someGlobalString = "bar";
}
}
However, if this function is now called with a defined someGlobalString, because of javascript evaluation the variable is set to undefined and always get set to bar.
function variableIssues(){
alert(someGlobalString); // "should be foo, but javascript evaluates a
// variable declaration it becomes undefined"
if (!someGlobalString){
var someGlobalString = "bar";
}
}
I would like to get some suggestions on how to handle undefined global variable. Any ideas?

Global variables are properties of the window object, so you can access them explicitly with window:
if (!window.someGlobalString) {
// depending on possible values, you might want:
// if (typeof window.someGlobalString === 'undefined')
window.someGlobalString = "bar";
}
If you are using global variables, then this is better style, since it is clear what you are doing and assigning to undefined global variables wouldn't throw an error in strict mode.

What is the correct way to check if a global variable exists?

JSLint is not passing this as a valid code:
/* global someVar: false */
if (typeof someVar === "undefined") {
var someVar = "hi!";
}
What is the correct way?

/*global window */
if (window.someVar === undefined) {
window.someVar = 123456;
}
if (!window.hasOwnProperty('someVar')) {
window.someVar = 123456;
}

/**
* #param {string} nameOfVariable
*/
function globalExists(nameOfVariable) {
return nameOfVariable in window
}
It doesn't matter whether you created a global variable with var foo or window.foo — variables created with var in global context are written into window.

If you are wanting to assign a global variable only if it doesn't already exist, try:
window.someVar = window.someVar || 'hi';
or
window['someVar'] = window['someVar'] || 'hi';

try
variableName in window
or
typeof window[variableName] != 'undefined'
or
window[variableName] !== undefined
or
window.hasOwnProperty(variableName)

I think this is actually a problem with JSLint. It will issue the following error:
Unexpected 'typeof'. Compare directly with 'undefined'.
I believe this is bad advice. In JavaScript, undefined is a global variable that is, usually, undefined. But some browsers allow scripts to modify it, like this: window.undefined = 'defined'. If this is the case, comparing directly with undefined can lead to unexpected results. Fortunately, current ECMA 5 compliant browsers do not allow assignments to undefined (and will throw an exception in strict mode).
I prefer typeof someVar === "undefined", as you posted, or someVar in window as Susei suggested.

if (typeof someVar === "undefined") {
var someVar = "hi!";
}
will check if someVar (local or global) is undefined.
If you want to check for a global variable you can use
if(window['someVar'] === undefined) {
...
}
assuming this is in a browser :)

As of ES6 most of other answers, including the accepted answer, are incorrect, because global variables defined by let or const, or resulting from a class declaration, do not have corresponding properties on the global object (window in a browser, or global in node.js). Several of them—mainly the ones which use typeof—can also be fooled by global variables which exist but which are set to undefined.
The only fully general way to test to see if a global variable exists—regardless of whether it has been declared using var, let or const, created via a function or class declaration, created by assignment (i.e., myVar = value at the top level of a program without any declaration for myVar) or by creating a property on the global object (i.e., window.myVar = value)—is to attempt to access it via a global eval and see if TypeError is thrown.
(This builds on an idea presented by Ferran Maylinch, but with a trick to ensure that it will work properly even when encapsulated in a function.)
function globalExists(varName) {
// Calling eval by another name causes evalled code to run in a
// subscope of the global scope, rather than the local scope.
const globalEval = eval;
try {
globalEval(varName);
return true;
} catch (e) {
return false;
}
}
undeclared = undefined;
const myConst = undefined;
let myLet;
var myVar;
globalExists('undeclared') // => true
globalExists('myConst') // => true
globalExists('myLet') // => true
globalExists('myVar') // => true
globalExists('nonexistent') // => false
globalExists('globalExists') // => true - can see itself.
globalExists('varName') // => false - not fooled by own parameters.
globalExists('globalEval') // => false - not fooled by local variable.
Note that this makes use of eval, so all the usual caveats apply: you should not supply an untrusted value as the parameter, and if you must use an untrusted value you should check to make sure that varName is a valid JavaScript identifier. Doing so is out of scope for this question, but it can be done using a (rather complex) regular expression—just beware that the correct regexp depends on the version of ECMAScript you are using, whether the code is a script or (ES6) module, whether it is in an async function, etc. etc.

bfavaretto is incorrect.
Setting the global undefined to a value will not alter tests of objects against undefined. Try this in your favorite browsers JavaScript console:
var udef; var idef = 42;
alert(udef === undefined); // Alerts "true".
alert(idef === undefined); // Alerts "false".
window.undefined = 'defined';
alert(udef === undefined); // Alerts "true".
alert(idef === undefined); // Alerts "false".
This is simply due to JavaScript ignoring all and any values attempted to be set on the undefined variable.
window.undefined = 'defined';
alert(window.undefined); // Alerts "undefined".

This would be a simple way to perform the check .
But this check would fail if variableName is declared and is assigned with the boolean value: false
if(window.variableName){
}

I think the best solution is the following:
if(window.hasOwnProperty('foo')) {
console.log('Variable is not declared');
}
The following solution will not work if the variables is declared but is not assigned (var foo;).
typeof foo === 'undefined'

If you are not sure whether a global variable is defined, you can always try accessing it and see what happens.
function node_env(name) {
try {
return process.env[name];
} catch (ignore) {}
}

this.globalvar vs window.globalvar

I recently read somewhere (I am really sorry I can't provide the source) that you can use this.varname to access globals in replacement to window.varname to save 2 chars
var myVar = "global";
function myFunc() {
var myVar = "notGlobal";
alert( this.myVar ); //global
}
It seems to work, but I want to know if:
it is safe to use in old browsers
it is cross-browser compatible
it will fail under some weird circumstances

I don't think I'd do it, but it's completely cross-browser compatible if this is referring to the global object (window). Whether it is will depend on how the function in question was called (at global scope, this does indeed refer to the global object), and whether the code in question is in "strict mode" or not. (In strict mode, this does not refer to the global object. Kudos and upvotes to Esailija for pointing that out.)
In non-strict code:
So at global scope:
console.log(this === window); // true if not in strict mode
And similarly, if you have a function you call directly:
function foo() {
console.log(this === window);
}
foo(); // Logs "true"
But, in JavaScript, this is set entirely by how a function is called. So we could call foo setting this to something else:
var obj = {};
foo.call(obj); // Now it logs "false", `this` === `obj` during the call
Similarly:
var obj = {};
obj.f = foo;
obj.f(); // Also logs "false", `this` === `obj` during the call
So in conclusion, at global scope (not in any function call), yes, this is reliably pointing to the global object, and if you control how the function gets called and you call it without setting this to anything else (via call or apply, or by using it from an object property a'la obj.f above), then, again, it will reliably refer to the global object. This is covered by sections 10.4.1 (Entering Global Code) and 10.4.3 (Entering Function Code) of the specification. This is how it's been from the very beginning, I believe; certainly for the last 15 years, so you're unlikely to find a non-compliant environment.
More reading:
Mythical methods
You must remember this

It will not work in modern browsers if someone slaps "use strict" above your code as this will be undefined inside the function.
<script type="text/javascript">
"use strict";
...
function test(){
console.log(this);
}
test(); // undefined
</script>
Note that you can save much more characters just by assigning window to some variable at the top of your code and using that as it will be shortened to some one character variable by a minimizer.
(function(global){
global.foo();
global.bar();
})(window);
would be minimized to (without whitespace):
(function(a){
a.foo();
a.bar();
})(window);
Where as this would not be touched as it's a keyword.

this always has a value. And if this hasnt been overridden somehow, then it will be window where all your globals are.
It will work in all JS implementations, but beware! This is more brittle, and it won't always point to a global variable.
For instance:
var myVar = "global";
var obj = {
myVar: "property",
fn: function() { return this.myVar; }
}
console.log(obj.fn()); // "property"
This fails because in this context this is that object. But when executing a function that is NOT a property of an object, this will default to window, like your example.