feat(validator): Introduce responseValidator

src/context.ts

@@ -862,4 +862,11 @@
     this.#notFoundHandler ??= () => new Response()
     return this.#notFoundHandler(this)
   }
+
+  validateData: unknown
+
+  validate = <T>(data: T): T => {
+    this.validateData = data
+    return data
+  }
 }

src/types.ts

@@ -1933,6 +1933,26 @@ export type ValidationTargets<T extends FormValue = ParsedFormValue, P extends s
   cookie: Record<string, string>
 }
 
+////////////////////////////////////////////////
+//////                                     /////
+//////      ResponseValidationTargets      /////
+//////                                     /////
+////////////////////////////////////////////////
+
+export type ResponseValidationTargets = {
+  body: ReadableStream<any> | null
+  text: string
+  json: any
+  html: string
+  header: Record<RequestHeader | CustomHeader, string>
+  cookie: Record<string, string>
+  status: {
+    ok: boolean
+    status: StatusCode
+    statusText: string
+  }
+}
+
 ////////////////////////////////////////
 //////                            //////
 //////      Path parameters       //////

src/validator/index.ts

@@ -5,3 +5,6 @@
 
 export { validator } from './validator'
 export type { ValidationFunction } from './validator'
+
+export { responseValidator } from './response-validator'
+export type { ResponseValidationFunction } from './response-validator'

src/validator/response-validator.sandbox.ts

@@ -0,0 +1,30 @@
+import { Hono } from '../preset/quick'
+import { validator, responseValidator } from '.'
+
+const app = new Hono<{
+  Bindings: {
+    foo: string
+  }
+}>()
+
+app
+  .get(
+    '*',
+    validator('query', (value, c) => {
+      console.log(value, c)
+    }),
+    responseValidator('text', (value, c) => {
+      console.log(value)
+      if (!/\d/.test(value)) {
+        return c.text('Invalid!', 400)
+      }
+    })
+  )
+  .get('/', (c) => {
+    return c.text(c.validate('hello world at ' + Date.now()))
+  })
+  .get('/invalid', (c) => {
+    return c.text(c.validate('hello world at ' + NaN))
+  })
+
+export default app

src/validator/response-validator.ts

@@ -0,0 +1,90 @@
+import type { Context } from '../context'
+import type { Env, MiddlewareHandler, ResponseValidationTargets } from '../types'
+
+type ResponseValidationTargetKeys = keyof ResponseValidationTargets
+
+export type ResponseValidationFunction<
+  U extends ResponseValidationTargetKeys,
+  E extends Env = {},
+  P extends string = string
+> = (
+  value: ResponseValidationTargets[U],
+  c: Context<E, P>
+) => undefined | Response | Promise<Response>
+
+const textRegex = /^text\/([a-z-\.]+\+)?(;\s*[a-zA-Z0-9\-]+\=([^;]+))*$/
+const jsonRegex = /^application\/([a-z-\.]+\+)?json(;\s*[a-zA-Z0-9\-]+\=([^;]+))*$/
+const htmlRegex = /^text\/html(;\s*[a-zA-Z0-9\-]+\=([^;]+))*$/
+
+export const responseValidator = <
+  P extends string,
+  U extends ResponseValidationTargetKeys,
+  P2 extends string = P,
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  E extends Env = any
+>(
+  target: U,
+  validationFunc: ResponseValidationFunction<U, E, P2>
+): MiddlewareHandler<E, P> => {
+  return async (c, next) => {
+    await next()
+
+    if (!c.finalized) {
+      return
+    }
+
+    let value: unknown
+
+    const contentType = c.res.headers.get('Content-Type')
+
+    switch (target) {
+      case 'body':
+        if (!c.res.body) {
+          break
+        }
+        value = c.res.body
+        break
+      case 'text':
+        if (!contentType || !textRegex.test(contentType) || typeof c.validateData !== 'string') {
+          break
+        }
+        value = c.validateData
+        break
+      case 'json':
+        if (!contentType || !jsonRegex.test(contentType) || typeof c.validateData !== 'object') {
+          break
+        }
+        value = c.validateData
+        break
+      case 'html':
+        if (!contentType || !htmlRegex.test(contentType) || typeof c.validateData !== 'string') {
+          break
+        }
+        value = c.validateData
+        break
+      case 'header':
+        value = Object.fromEntries(c.res.headers.entries()) as Record<string, string>
+        break
+      case 'cookie':
+        value = c.res.headers.getSetCookie().reduce((record, cookie) => {
+          const [name, ...rest] = cookie.split('=')
+          record[name] = rest.join('=').split(';')[0]
+          return record
+        }, {} as Record<string, string>)
+        break
+      case 'status':
+        value = {
+          ok: c.res.ok,
+          status: c.res.status,
+          statusText: c.res.statusText,
+        }
+        break
+    }
+
+    const res = await validationFunc(value, c)
+
+    if (res instanceof Response) {
+      c.res = res
+    }
+  }
+}