fix: remove undefined values in options (#2281)

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>
Resolves #2153

packages/vitest/src/node/plugins/index.ts

@@ -2,7 +2,7 @@ import type { UserConfig as ViteConfig, Plugin as VitePlugin } from 'vite'
 import { relative } from 'pathe'
 import { configDefaults } from '../../defaults'
 import type { ResolvedConfig, UserConfig } from '../../types'
-import { deepMerge, ensurePackageInstalled, notNullish } from '../../utils'
+import { deepMerge, ensurePackageInstalled, notNullish, removeUndefinedValues } from '../../utils'
 import { resolveApiConfig } from '../config'
 import { Vitest } from '../core'
 import { generateScopedClassName } from '../../integrations/css/css-modules'
@@ -36,7 +36,12 @@ export async function VitestPlugin(options: UserConfig = {}, ctx = new Vitest('t
         // preliminary merge of options to be able to create server options for vite
         // however to allow vitest plugins to modify vitest config values
         // this is repeated in configResolved where the config is final
-        const preOptions = deepMerge({}, configDefaults, options, viteConfig.test ?? {})
+        const preOptions = deepMerge(
+          {},
+          configDefaults,
+          options,
+          removeUndefinedValues(viteConfig.test ?? {}),
+        )
         preOptions.api = resolveApiConfig(preOptions)
 
         if (viteConfig.define) {

packages/vitest/src/utils/index.ts

@@ -66,6 +66,14 @@ export function getFullName(task: Task) {
   return getNames(task).join(c.dim(' > '))
 }
 
+export function removeUndefinedValues<T extends Record<string, any>>(obj: T): T {
+  for (const key in Object.keys(obj)) {
+    if (obj[key] === undefined)
+      delete obj[key]
+  }
+  return obj
+}
+
 export async function ensurePackageInstalled(
   dependency: string,
   root: string,