The Tool Desk
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What is a global variable in JavaScript?
“Global variable” is often used for two related but different things: a binding available in a global scope, and a property stored on the global object. Those are not interchangeable. A top-level let in a classic script is globally scoped, for example, but does not become globalThis.name. The behavior depends on how the code is run: as a classic script, a CommonJS module, or a native ECMAScript module.
In browsers, the global object is commonly accessible through window; in other JavaScript hosts, use globalThis for the standard cross-environment reference. Host behavior can still differ, and a host-provided globalThis value is not guaranteed to be simply its global object. See MDN’s globalThis reference.
How declarations behave at the top level
| Code context | Top-level declaration | Global-object property? |
|---|---|---|
| Classic script | var |
Yes. A global var binding is represented as a non-configurable property of the global object. |
| Classic script | function declaration |
Yes, in the traditional script environment. |
| Classic script | let, const, or class |
No. These create global lexical bindings, not properties on the global object. |
| CommonJS module or native ECMAScript module | Any top-level declaration | No. Declarations stay within the module scope. |
These distinctions are documented in MDN’s references for var and JavaScript grammar and types.
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Classic browser scripts
In a non-module <script>, top-level var and function declarations can be seen as properties on globalThis (and usually window in a browser). Top-level let and const are still global lexical bindings in that script environment, but reading globalThis.name for them does not retrieve the binding.
// Classic browser script, not <script type="module">
var legacyShared = 1;
let scriptBinding = 2;
console.log(globalThis.legacyShared); // 1
console.log(globalThis.scriptBinding); // undefinedDo not use window as a synonym for every global binding. It is a browser-specific global-object reference, and it will not be present in environments such as Node.js workers or server-side execution.
CommonJS and native ECMAScript modules
Top-level names in CommonJS files and native ES modules belong to their module, not to the global object. This is a useful boundary: another file cannot silently access a module’s local names just because they were declared at the top level. Share a value by exporting it and importing it where needed. MDN notes that module features are imported into the scope of a single script rather than made available in global scope; see JavaScript modules.
// settings.js
export const apiBase = "/api";
// app.js
import { apiBase } from "./settings.js";
console.log(apiBase);Here apiBase is available in app.js because it is explicitly imported, not because it became global.
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How accidental globals happen
In sloppy-mode code, assigning to a name that has no declared binding can create a property on the global object. A misspelling can therefore leak state across code that was never meant to share it:
// Avoid in sloppy-mode code: no declaration exists
// total = 4;In strict mode, the same undeclared assignment throws a ReferenceError instead of creating an implicit global. Native ECMAScript modules are automatically strict, as MDN explains in its strict mode reference. Strict mode makes this class of mistake visible; it does not replace declaring variables correctly.
Best practices for variables and shared state
Keep bindings in the narrowest useful scope
Declare a value inside the function or block that uses it when possible. Local state makes dependencies easier to find and reduces collisions with unrelated scripts and libraries.
Declare every binding; choose const or let
Use const when you will not reassign the binding and let when reassignment is required. const prevents rebinding, not mutation of an object held by the binding:
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const settings = { theme: "light" };
settings.theme = "dark"; // allowed: the object was mutated
// settings = {}; // TypeError: the binding cannot be reassignedPrefer these block-scoped declarations to var in new code. The choice between const and let does not itself determine whether a name is global; execution context does.
Share through module interfaces
Use named exports and imports for application code that needs to communicate across files. This makes the sharing relationship explicit instead of placing state into a namespace visible to unrelated code.
Use strict behavior and linting
Modules are strict automatically. For classic scripts, a "use strict"; directive can make undeclared assignment fail rather than silently creating a global. ESLint’s no-implicit-globals rule can flag global declarations or assignments that may not be intended; check its behavior against the project’s script and module configuration.
If a global is genuinely required, make it explicit
Some host integrations or compatibility layers need a globally reachable value. Give it a project-specific name, document who owns it and how long it should live, and write it explicitly rather than relying on sloppy assignment:
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globalThis.AppBridge = { version: 1 };This deliberately sets a property on the host’s global-this value. Reserve the pattern for intentional integration, not ordinary application state.
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Why is my name on window?
Check whether the code runs as a classic script and whether the name was declared with top-level var or as a function declaration. Those declarations traditionally create global-object properties in that context. A top-level let or const in a classic script does not.
Why is a name missing from globalThis?
It may be a global lexical binding rather than a global-object property, or it may be top-level in a module. In either case, it is not expected to appear as globalThis.name. If another module needs the value, export and import it.
Why does undeclared assignment work in one file but fail in another?
The files may execute under different strictness rules. Modules are strict automatically; a classic script without strict mode may allow an undeclared assignment to create an accidental global. Declare the name explicitly rather than depending on that difference.
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Frequently Asked Questions
Does globalThis work in browsers and Node.js?
It is the standardized way to reference the global-this value across JavaScript environments. The host determines what that value represents; browser-only names such as window are not universal.
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No. A global lexical binding can be available to code in a classic script without appearing as a property such as globalThis.name.
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