fix(nuxt): prevent duplicate `set-cookie` headers (#28211)

packages/nuxt/src/app/composables/cookie.ts

@@ -126,6 +126,16 @@ export function useCookie<T = string | null | undefined> (name: string, _opts?:
     const nuxtApp = useNuxtApp()
     const writeFinalCookieValue = () => {
       if (opts.readonly || isEqual(cookie.value, cookies[name])) { return }
+      nuxtApp._cookies ||= {}
+      if (name in nuxtApp._cookies) {
+        // do not append a second `set-cookie` header
+        if (isEqual(cookie.value, nuxtApp._cookies[name])) { return }
+        // warn in dev mode
+        if (import.meta.dev) {
+          console.warn(`[nuxt] cookie \`${name}\` was previously set to \`${opts.encode(nuxtApp._cookies[name] as any)}\` and is being overridden to \`${opts.encode(cookie.value as any)}\`. This may cause unexpected issues.`)
+        }
+      }
+      nuxtApp._cookies[name] = cookie.value
       writeServerCookie(useRequestEvent(nuxtApp)!, name, cookie.value, opts as CookieOptions<any>)
     }
     const unhook = nuxtApp.hooks.hookOnce('app:rendered', writeFinalCookieValue)

packages/nuxt/src/app/nuxt.ts

@@ -112,6 +112,8 @@ interface _NuxtApp {
 
   [key: string]: unknown
 
+  /** @internal */
+  _cookies?: Record<string, unknown>
   /** @internal */
   _id?: number
   /** @internal */

test/fixtures/basic/pages/cookies.vue

@@ -9,6 +9,8 @@ useCookie<string | null>('set-to-null-with-default', { default: () => 'default'
 useCookie('browser-accessed-but-not-used')
 useCookie('browser-accessed-with-default-value', { default: () => 'default' })
 useCookie('browser-set').value = 'set'
+// confirm that it only sets one `set-cookie` header
+useCookie('browser-set').value = 'set'
 useCookie('browser-set-to-null').value = null
 useCookie<string | null>('browser-set-to-null-with-default', { default: () => 'default' }).value = null
 
@@ -17,7 +19,6 @@ const objectCookieSecond = useCookie('browser-object-default', {
   default: () => ({ foo: 'bar' }),
 })
 function changeCookie () {
-  console.log(objectCookie.value, objectCookieSecond.value)
   if (objectCookie.value!.foo === 'baz') {
     objectCookie.value!.foo = 'bar'
   } else {