How can I mock a module that only provides type definitions?

[mysticfall]

Hi,

I'm trying to write unit tests for a library that uses skyrim-platform.

This package is rather peculiar in that it only provides a type definition file and has an empty entry point (i.e., the main field in package.json). It's because the package is meant to run on an embedded Node environment, in which a native plugin provides its implementation.

Understandably, unit tests fail with the following error:

Error: Cannot find package '/home/mysticfall/workspace/eldermind/node_modules/.pnpm/skyrim-effect@file+..+skyrim-effect_@[email protected][email protected]/node_modules/@skyrim-platform/skyrim-platform' imported from /home/mysticfall/workspace/eldermind/node_modules/.pnpm/skyrim-effect@file+..+skyrim-effect_@[email protected][email protected]/node_modules/skyrim-effect/dist/esm/game/Form.js
    at legacyMainResolve (node:internal/modules/esm/resolve:204:26)
    at packageResolve (node:internal/modules/esm/resolve:778:12)
    at moduleResolve (node:internal/modules/esm/resolve:854:18)
    at defaultResolve (node:internal/modules/esm/resolve:984:11)
    at ModuleLoader.defaultResolve (node:internal/modules/esm/loader:654:12)
    at ModuleLoader.#cachedDefaultResolve (node:internal/modules/esm/loader:603:25)
    at ModuleLoader.resolve (node:internal/modules/esm/loader:586:38)
    at ModuleLoader.getModuleJobForImport (node:internal/modules/esm/loader:242:38)
    at ModuleJob._link (node:internal/modules/esm/module_job:135:49)

However, if I try to mock it using vi.mock("@skyrim-platform/skyrim-platform"), it also fails with a different error:

Error: Failed to resolve entry for package "@skyrim-platform/skyrim-platform". The package may have incorrect main/module/exports specified in its package.json.
    at packageEntryFailure (file:///home/mysticfall/workspace/eldermind/node_modules/.pnpm/[email protected]_@[email protected]/node_modules/vite/dist/node/chunks/dep-CB_7IfJ-.js:46637:15)
    at resolvePackageEntry (file:///home/mysticfall/workspace/eldermind/node_modules/.pnpm/[email protected]_@[email protected]/node_modules/vite/dist/node/chunks/dep-CB_7IfJ-.js:46634:3)
    at tryNodeResolve (file:///home/mysticfall/workspace/eldermind/node_modules/.pnpm/[email protected]_@[email protected]/node_modules/vite/dist/node/chunks/dep-CB_7IfJ-.js:46450:16)
    at ResolveIdContext.resolveId (file:///home/mysticfall/workspace/eldermind/node_modules/.pnpm/[email protected]_@[email protected]/node_modules/vite/dist/node/chunks/dep-CB_7IfJ-.js:46200:19)
    at PluginContainer.resolveId (file:///home/mysticfall/workspace/eldermind/node_modules/.pnpm/[email protected]_@[email protected]/node_modules/vite/dist/node/chunks/dep-CB_7IfJ-.js:49015:17)

Is there any way I can mock such a package so that I can unit-test it using Vitest?

Thanks in advance!

[sheremet-va]

This is just how Vite works. To make Vite aware of this module and not fail during transformation, define an alias. More in docs: https://vitest.dev/guide/mocking.html#virtual-modules

[mysticfall]

Thanks! I will try that.

[mysticfall]

This is just how Vite works. To make Vite aware of this module and not fail during transformation, define an alias. More in docs: https://vitest.dev/guide/mocking.html#virtual-modules

Could you tell me how I should declare an alias for the module, @skyrim-platform/skyrim-platform? I tried using the package name, regular expression, etc., to declare an alias but it didn't change the error message.

Strangely, switching from pnpm to npm somehow fixed the issue, and it didn't even require adding an alias.

[sheremet-va]

Try something like this:

import { resolve } from 'node:path'
import { defineConfig } from 'vitest/config'
export default defineConfig({
  resolve: {
    alias: [
      { find: /^@skyrim-platform\/skyrim-platform$/, replacement: resolve('./path-to-my-file.js') },
    ],
  },
})