cannot mock spyOn namespace import in browser mode

[kwojcik]

Describe the bug

If you use spyOn on a namespace import you get this error TypeError: Cannot redefine property:. This seems to work fine in jsdom environment, but not browser mode.

Workaround that still gives you the behavior of spyOn if you need a "default call original" behavior:

import { expect, it, vi } from "vitest";
import { one } from "./basic.js";

vi.mock("./basic.js", async (importActual) => {
  const actual = await importActual<typeof import("./basic.js")>();
  return {
    ...actual,
    one: vi.fn().mockImplementation(actual.one),
  };
});
it("test", () => {
  vi.mocked(one).mockReturnValue(2);
  expect(one()).toBe(2);
});

Reproduction

import { expect, it, vi } from "vitest";
import * as thing from "./basic.js";

it("test", () => {
  vi.spyOn(thing, "one").mockReturnValue(2);
  expect(thing.one()).toBe(2);
});

basic.js

export function one() {
  return 1;
}

 FAIL  test/basic.test.tsx > test
TypeError: Cannot redefine property: one

Failure screenshot:
  - test/__screenshots__/basic.test.tsx/test-1.png

 ❯ test/basic.test.tsx:5:5
      3|
      4| it("test", () => {
      5|   vi.spyOn(thing, "one").mockReturnValue(2);
       |     ^
      6|   expect(thing.one()).toBe(2);
      7| });

System Info

System:
    OS: macOS 13.6.7
    CPU: (12) arm64 Apple M2 Max
    Memory: 4.92 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.18.2 - ~/.asdf/installs/nodejs/18.18.2/bin/node
    Yarn: 1.22.21 - /opt/homebrew/bin/yarn
    npm: 9.8.1 - ~/.asdf/plugins/nodejs/shims/npm
    pnpm: 6.32.8 - /opt/homebrew/bin/pnpm
    Watchman: 2024.04.01.00 - /opt/homebrew/bin/watchman
  Browsers:
    Chrome: 126.0.6478.127
    Firefox Nightly: 127.0
    Safari: 16.6
  npmPackages:
    @vitest/browser: 2.0.2 => 2.0.2
    @vitest/ui: 2.0.2 => 2.0.2
    vite: latest => 5.3.2
    vitest: 2.0.2 => 2.0.2

Used Package Manager

npm

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• Read the Contributing Guidelines.
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• The provided reproduction is a minimal reproducible example of the bug.

[sheremet-va]

Vitest Browser Mode previously had a mode (called slowHijackESM) when this behaviour was possible, but it made the runner slower and introduced subtle bugs that were hard to find. We effectively had to maintain a separate ESM environment. In the release version, we removed this flag in favor of using vi.mock.

Just calling vi.mock doesn't replicate the behavior though because it overrides the implementation, but maybe we can add a flag to wrap the methods instead of overriding them:

vi.mock("./basic.js", { spy: true })

[kwojcik]

I'm not necessarily requesting this feature (since there is a workaround), just encountered it while moving to browser mode. If you decide against this then just having a nice error message that says it's not supported and suggestion of what to do instead would be great.