A global object is an
object
that always exists in the
global scope
.
In JavaScript, there's always a global object defined. In a web browser, when scripts create global variables defined with the
var
keyword, they're created as members of the global object. (In
Node.js
this is not the case.) The global object's
interface
depends on the execution context in which the script is running. For example:
- In a web browser, any code which the script doesn't specifically start up as a background task has a
Window
as its global object. This is the vast majority of JavaScript code on the Web.
- Code running in a
Worker
has a
WorkerGlobalScope
object as its global object.
- Scripts running under
Node.js
have an object called
global
as their global object.
The
globalThis
global property allows one to access the global object regardless of the current environment.
var
statements and
function declarations
at the top level create properties of the global object. On the other hand,
let
and
const
declarations never create properties of the global object.
The properties of the global object are automatically added to the
global scope
.
In JavaScript, the global object always holds a reference to itself:
console
.
log
(
globalThis
===
globalThis
.
globalThis
)
;
console
.
log
(
window
===
window
.
window
)
;
console
.
log
(
self
===
self
.
self
)
;
console
.
log
(
frames
===
frames
.
frames
)
;
console
.
log
(
global
===
global
.
global
)
;