Global object

JavaScriptJunior~2 min read

What the global object is

The global object is a special object that always exists, regardless of the environment (browser, Node.js, and so on). It represents the global scope, where all global variables and functions live.

Simply put:

The global object is a "container" that holds all the variables and functions declared outside functions and modules.

Examples of global objects in different environments

Environment	Global object
Browser	`window` or `self`
Web Worker	`self`
Node.js	`global`
Modern universal way	`globalThis`

Example: browser

javascript

var x = 10;

console.log(window.x); // 10
console.log(this.x);   // 10 (in the global scope)

The variable x, declared with var, automatically becomes a property of the global object window.

Example: Node.js

javascript

var y = 5;

console.log(global.y); // undefined

In Node.js, var, let, and const variables are not added to global, because Node.js uses a module system (each file is a separate module, not a global script).

The universal way: globalThis

ES2020 introduced a new standard, globalThis, which works the same way in every environment.

javascript

console.log(globalThis);

It works:

in the browser -> window
in Node.js -> global
in a web worker -> self

Relationship to the execution context

When the global execution context is created, the JS engine:

creates the global object;
sets this as a reference to that object (unless strict mode is used);
places all global declarations (var, function) into it.

Example:

javascript

var a = 42;

function sayHi() {
  console.log("Hi");
}

console.log(globalThis.a); // 42
globalThis.sayHi();        // "Hi"

The difference between let/const and var

Variables declared with let and const:

do not become properties of the global object,
but still exist in the global scope.

Example:

javascript

let a = 10;
var b = 20;

console.log(window.a); // undefined
console.log(window.b); // 20

Summary

Feature	Description
What it is	The top-level object that always exists
Where it lives	In the global execution context
Automatically contains	Global functions (`setTimeout`, `fetch`, `console`, ...)
How to access it	`window` (browser), `global` (Node.js), `globalThis` (universal)
Specifics of `let`/`const`	They do not become properties of the global object

Practical example using globalThis

javascript

globalThis.appName = "DemoApp";

function printName() {
  console.log(globalThis.appName);
}

printName(); // "DemoApp"

Works in the browser, in Node.js, and in any environment.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.