fix(jsdom): keep File bytes in Request/Response body and formData on Node 24

packages/vitest/src/integrations/env/jsdom.ts

@@ -38,7 +38,9 @@ function catchWindowErrors(window: DOMWindow) {
 
 let NodeFormData_!: typeof FormData
 let NodeBlob_!: typeof Blob
+let NodeFile_!: typeof File
 let NodeRequest_!: typeof Request
+let NodeResponse_!: typeof Response
 
 interface LegacyResourceLoader {
   new (options?: { userAgent?: string }): NonNullable<ConstructorOptions['resources']>
@@ -75,7 +77,9 @@ export default <Environment>{
     // delay initialization because it takes ~1s
     NodeFormData_ = globalThis.FormData
     NodeBlob_ = globalThis.Blob
+    NodeFile_ = globalThis.File
     NodeRequest_ = globalThis.Request
+    NodeResponse_ = globalThis.Response
 
     const jsdomModule = await import('jsdom')
     const { CookieJar, JSDOM, VirtualConsole } = jsdomModule
@@ -127,6 +131,7 @@ export default <Environment>{
     dom.window.jsdom = dom
     dom.window.Request = createCompatRequest(utils)
     dom.window.URL = createJSDOMCompatURL(utils)
+    const restoreFormDataParsing = patchFormDataParsing(utils)
 
     // inject web globals if they are missing in JSDOM but otherwise available in Nodejs
     // https://nodejs.org/dist/latest/docs/api/globals.html
@@ -173,6 +178,7 @@ export default <Environment>{
         return dom.getInternalVMContext()
       },
       teardown() {
+        restoreFormDataParsing()
         clearAddEventListenerPatch()
         clearWindowErrors()
         dom.window.close()
@@ -184,7 +190,9 @@ export default <Environment>{
     // delay initialization because it takes ~1s
     NodeFormData_ = globalThis.FormData
     NodeBlob_ = globalThis.Blob
+    NodeFile_ = globalThis.File
     NodeRequest_ = globalThis.Request
+    NodeResponse_ = globalThis.Response
 
     const jsdomModule = await import('jsdom')
     const { CookieJar, JSDOM, VirtualConsole } = jsdomModule
@@ -237,9 +245,11 @@ export default <Environment>{
     global.jsdom = dom
     global.Request = createCompatRequest(utils)
     global.URL = createJSDOMCompatURL(utils)
+    const restoreFormDataParsing = patchFormDataParsing(utils)
 
     return {
       teardown(global) {
+        restoreFormDataParsing()
         clearAddEventListenerPatch()
         clearWindowErrors()
         dom.window.close()
@@ -298,6 +308,40 @@ interface CompatUtils {
   window: DOMWindow
   makeCompatBlob: (blob: Blob) => Blob
   makeCompatFormData: (formData: FormData) => FormData
+  makeWindowFormData: (formData: FormData) => Promise<FormData>
+}
+
+function patchFormDataParsing(utils: CompatUtils) {
+  // undici builds each part with the global File and then brand checks it, so
+  // the parse needs Node's File on the global. The swap spans an await and the
+  // global is shared, so calls are serialized through one lock: overlapping
+  // `formData()` calls (including across Request and Response) run one at a
+  // time and never observe each other's swap.
+  let lock: Promise<unknown> = Promise.resolve()
+  const restores = [NodeRequest_, NodeResponse_].map((Class) => {
+    const original = Class.prototype.formData
+    Class.prototype.formData = function () {
+      const run = async () => {
+        const savedFile = globalThis.File
+        globalThis.File = NodeFile_
+        let formData: FormData
+        try {
+          formData = await original.call(this)
+        }
+        finally {
+          globalThis.File = savedFile
+        }
+        return utils.makeWindowFormData(formData)
+      }
+      const result = lock.then(run, run)
+      lock = result.catch(() => {})
+      return result
+    }
+    return () => {
+      Class.prototype.formData = original
+    }
+  })
+  return () => restores.forEach(restore => restore())
 }
 
 function createCompatUtils(window: DOMWindow): CompatUtils {
@@ -312,14 +356,31 @@ function createCompatUtils(window: DOMWindow): CompatUtils {
       const nodeFormData = new NodeFormData_()
       formData.forEach((value, key) => {
         if (value instanceof window.Blob) {
-          nodeFormData.append(key, utils.makeCompatBlob(value as any) as any)
+          // Node's FormData wraps a Blob with the global File, which is jsdom's here
+          const compatBlob = utils.makeCompatBlob(value as any)
+          nodeFormData.append(key, new NodeFile_([compatBlob], value.name, { type: value.type, lastModified: value.lastModified }))
         }
         else {
           nodeFormData.append(key, value)
         }
       })
       return nodeFormData
     },
+    async makeWindowFormData(formData: FormData) {
+      const windowFormData = new window.FormData()
+      const entries: [string, FormDataEntryValue][] = []
+      formData.forEach((value, key) => entries.push([key, value]))
+      for (const [key, value] of entries) {
+        if (typeof value === 'string') {
+          windowFormData.append(key, value)
+        }
+        else {
+          const bytes = await value.arrayBuffer()
+          windowFormData.append(key, new window.File([bytes], value.name, { type: value.type, lastModified: value.lastModified }))
+        }
+      }
+      return windowFormData as FormData
+    },
     makeCompatBlob(blob: Blob) {
       const impl = (blob as any)[implSymbol]
       // jsdom 28 renamed `_buffer` to `_bytes`

test/unit/test/environments/jsdom.spec.ts

@@ -189,6 +189,57 @@ describe('FormData', () => {
 
     expect(await request.text()).toBe('hello world')
   })
+
+  test('keeps File bytes inside a multipart Request body', async () => {
+    const form = new FormData()
+    form.append('file', new File(['file content'], 'report.csv', { type: 'text/csv' }))
+    const request = new Request('http://localhost/', { method: 'POST', body: form })
+
+    const body = await request.text()
+    const partBody = body.split('\r\n\r\n')[1].split('\r\n')[0]
+    expect(partBody).toBe('file content')
+  })
+
+  test('parses a multipart Request body back into FormData', async () => {
+    const form = new FormData()
+    form.append('file', new File(['test'], 'test.txt', { type: 'text/plain' }))
+    form.append('field', 'value')
+    const request = new Request('http://localhost/', { method: 'POST', body: form })
+
+    const parsed = await request.formData()
+    expect(parsed).toBeInstanceOf(FormData)
+    const file = parsed.get('file') as File
+    expect(file).toBeInstanceOf(File)
+    expect(file.name).toBe('test.txt')
+    expect(file.type).toBe('text/plain')
+    expect(await file.text()).toBe('test')
+    expect(parsed.get('field')).toBe('value')
+  })
+
+  test('parses a multipart Response body back into FormData', async () => {
+    const form = new FormData()
+    form.append('file', new File(['test'], 'test.txt'))
+    const request = new Request('http://localhost/', { method: 'POST', body: form })
+    const response = new Response(request.body, { headers: request.headers })
+
+    const parsed = await response.formData()
+    const file = parsed.get('file') as File
+    expect(file).toBeInstanceOf(File)
+    expect(await file.text()).toBe('test')
+  })
+
+  test('concurrent formData() calls restore the global File', async () => {
+    const make = () => {
+      const form = new FormData()
+      form.append('file', new File(['data'], 'f.txt'))
+      return new Request('http://localhost/', { method: 'POST', body: form })
+    }
+    const before = globalThis.File
+    const [a, b] = await Promise.all([make().formData(), make().formData()])
+    expect(await (a.get('file') as File).text()).toBe('data')
+    expect(await (b.get('file') as File).text()).toBe('data')
+    expect(globalThis.File).toBe(before)
+  })
 })
 
 test('DOM APIs accept AbortController', () => {