Auto-mocking classes doesn't work in browser mode

[scottohara]

Describe the bug

Apologies if this is by design, but I couldn't find anywhere that confirms this.

When mocking a module that exports a Class, the expectation is that without passing a factory, that all exports (including the exported Class are automocked).

This seems to work as expected with --environment=jsdom, but when used in --browser=chrome it fails with:

TypeError: Class constructor <name of class> cannot be invoked without 'new'

The error in browser mode can be avoided by supplying a factory to the mock function, e.g. instead of:

vi.mock("./foo.js");

we can pass:

vi.mock("./foo.js", () => {
  return {
    Foo: vi.fn()
  };
});

But it is unclear why this additional boilerplate is only needed in browser mode?

Reproduction

https://github.com/scottohara/vitest-browser-automock-class

System Info

System:
    OS: macOS 15.0
    CPU: (8) arm64 Apple M1
    Memory: 88.84 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.11.1 - ~/.asdf/installs/nodejs/20.11.1/bin/node
    Yarn: 1.22.19 - ~/.asdf/installs/nodejs/20.11.1/bin/yarn
    npm: 10.5.0 - ~/.asdf/plugins/nodejs/shims/npm
    pnpm: 9.9.0 - ~/.asdf/installs/nodejs/20.11.1/bin/pnpm
  Browsers:
    Chrome: 129.0.6668.59
    Chrome Canary: 131.0.6737.0
    Safari: 18.0
  npmPackages:
    @vitest/browser: 2.1.1 => 2.1.1 
    vitest: 2.1.1 => 2.1.1

Used Package Manager

npm

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[sheremet-va]

Spying on classes doesn't work at all: #6160

Closing this as a duplicate of #6104