global.navigator = dom.window.navigator is ignored on Node 21+ (or throws) — your jsdom test runs with Node's navigator

Symptom

A test file that builds its own DOM:

const { JSDOM } = require('jsdom');
const dom = new JSDOM('<!doctype html><html><body></body></html>');
global.window = dom.window;
global.document = dom.window.document;
global.navigator = dom.window.navigator;

• Without 'use strict': no error. navigator.userAgent is Node's (Node.js/24 on Node 24, in general Node.js/<major>), not the jsdom user agent, and navigator.onLine is undefined. Tests that do not read those properties stay green, so nothing tells you.
• With 'use strict' (or the same setup in an ES module, using import):

TypeError: Cannot set property navigator of #<Object> which has only a getter

Cause

Node.js 21.0.0 added a global navigator (documented as active development; since 21.2.0 it can be switched off with --no-experimental-global-navigator). On Node 24.16 its property descriptor is an accessor: get present, set missing, configurable: true. Plain assignment to an accessor without a setter is a no-op in sloppy mode and a TypeError in strict mode.

The two objects are not interchangeable. Measured on Node 24.16 with jsdom 24.1:

So a component that sniffs userAgent, checks onLine or reads cookieEnabled sees a different world than the test author assumed.

Fix

Replace the assignment with a property definition (the existing property is configurable, so this is allowed):

Object.defineProperty(globalThis, 'navigator', {
  value: dom.window.navigator,
  configurable: true,
  writable: true
});
if (navigator !== dom.window.navigator) throw new Error('test setup: navigator is not the jsdom one');

configurable: true keeps delete globalThis.navigator in a teardown working; afterwards there is no navigator at all – Node's own object does not come back. The alternative is to start Node with --no-experimental-global-navigator (it can also be set once through NODE_OPTIONS); then Node exposes no navigator at all and the plain assignment works again — but every place that runs the tests (package scripts, CI) must get the flag.

Notes

• Check Object.getOwnPropertyDescriptor(globalThis, name) before overwriting any global that Node now provides itself – they differ: on Node 24.16 crypto is also a getter without a setter, performance has a setter, and fetch, URL or Blob are plain writable properties.
• A line that "cannot fail" deserves an assertion when the test depends on it — the silent version of this trap left 18 test files in one monorepo running with Node's navigator (17 through the ignored assignment, one through an if (!global.navigator) guard that never fired); it surfaced only when a new test file with 'use strict' threw.