fix(HttpResponse): narrow "body" type to "JsonBodyType"

src/core/HttpResponse.test.ts

@@ -38,6 +38,18 @@ describe('HttpResponse.json()', () => {
     })
   })
 
+  it('creates a json response given an array', async () => {
+    const response = HttpResponse.json([1, 2, 3])
+
+    expect(response.status).toBe(200)
+    expect(response.statusText).toBe('OK')
+    expect(response.body).toBeInstanceOf(ReadableStream)
+    expect(await response.json()).toEqual([1, 2, 3])
+    expect(Object.fromEntries(response.headers.entries())).toEqual({
+      'content-type': 'application/json',
+    })
+  })
+
   it('creates a json response given a plain string', async () => {
     const response = HttpResponse.json(`"hello"`)
 
@@ -77,7 +89,10 @@ describe('HttpResponse.json()', () => {
     expect(response.status).toBe(200)
     expect(response.statusText).toBe('OK')
     expect(response.body).toBeInstanceOf(ReadableStream)
-    expect(await response.json()).toEqual({ firstName: 'John' })
+    // A ReadableStream instance is not a valid body init
+    // for the "Response.json()" static method. It gets serialized
+    // into a plain object.
+    expect(await response.json()).toEqual({})
     expect(Object.fromEntries(response.headers.entries())).toEqual({
       'content-type': 'application/json',
     })

src/core/HttpResponse.ts

@@ -1,4 +1,4 @@
-import type { DefaultBodyType } from './handlers/RequestHandler'
+import type { DefaultBodyType, JsonBodyType } from './handlers/RequestHandler'
 import {
   decorateResponse,
   normalizeResponseInit,
@@ -62,14 +62,14 @@ export class HttpResponse extends Response {
    * HttpResponse.json({ firstName: 'John' })
    * HttpResponse.json({ error: 'Not Authorized' }, { status: 401 })
    */
-  static json<BodyType extends DefaultBodyType>(
+  static json<BodyType extends JsonBodyType>(
     body?: BodyType | null,
     init?: HttpResponseInit,
   ): StrictResponse<BodyType> {
     const responseInit = normalizeResponseInit(init)
     responseInit.headers.set('Content-Type', 'application/json')
     return new HttpResponse(
-      body instanceof ReadableStream ? body : JSON.stringify(body),
+      JSON.stringify(body),
       responseInit,
     ) as StrictResponse<BodyType>
   }

src/core/handlers/RequestHandler.ts

@@ -19,6 +19,14 @@ export type DefaultBodyType =
   | null
   | undefined
 
+export type JsonBodyType =
+  | Record<string, any>
+  | string
+  | number
+  | boolean
+  | null
+  | undefined
+
 export interface RequestHandlerDefaultInfo {
   header: string
 }