fix(bypass): support modifying the bypassed request (#2093)

src/core/bypass.test.ts

@@ -45,3 +45,25 @@ it('returns bypassed request given request instance', async () => {
     'x-msw-intention': 'bypass',
   })
 })
+
+it('allows modifying the bypassed request instance', async () => {
+  const original = new Request('http://localhost/resource', {
+    method: 'POST',
+    body: 'hello world',
+  })
+  const request = bypass(original, {
+    method: 'PUT',
+    headers: { 'x-modified-header': 'yes' },
+  })
+
+  expect(request.method).toBe('PUT')
+  expect(Object.fromEntries(request.headers.entries())).toEqual({
+    'x-msw-intention': 'bypass',
+    'x-modified-header': 'yes',
+  })
+  expect(original.bodyUsed).toBe(false)
+  expect(request.bodyUsed).toBe(false)
+
+  expect(await request.text()).toBe('hello world')
+  expect(original.bodyUsed).toBe(false)
+})

src/core/bypass.ts

@@ -15,7 +15,15 @@ export type BypassRequestInput = string | URL | Request
  * @see {@link https://mswjs.io/docs/api/bypass `bypass()` API reference}
  */
 export function bypass(input: BypassRequestInput, init?: RequestInit): Request {
-  const request = input instanceof Request ? input : new Request(input, init)
+  // Always create a new Request instance.
+  // This way, the "init" modifications will propagate
+  // to the bypass request instance automatically.
+  const request = new Request(
+    // If given a Request instance, clone it not to exhaust
+    // the original request's body.
+    input instanceof Request ? input.clone() : input,
+    init,
+  )
 
   invariant(
     !request.bodyUsed,