feat: add beforeSerialize

Related to nuxt/nuxt#9332

content/en/guides/concepts/context-helpers.md

@@ -73,7 +73,7 @@ function (context) { // Could be asyncData, nuxtServerInit, ...
 
   // Only available on the Server-side
   if (process.server) {
-    const { req, res, beforeNuxtRender } = context
+    const { req, res, beforeNuxtRender, beforeSerialize } = context
   }
 
   // Only available on the Client-side

content/en/guides/internals-glossary/context.md

@@ -28,7 +28,7 @@ function (context) {
   } = context
   // Server-side
   if (process.server) {
-    const { req, res, beforeNuxtRender } = context
+    const { req, res, beforeSerialize } = context
   }
   // Client-side
   if (process.client) {
@@ -147,6 +147,35 @@ Response from the Node.js server. If Nuxt is used as a middleware, the res objec
 
 Use this method to update `__NUXT__` variable rendered on client-side, the `fn` (can be asynchronous) is called with `{ Components, nuxtState }`, see [example](https://github.com/nuxt/nuxt.js/blob/cf6b0df45f678c5ac35535d49710c606ab34787d/test/fixtures/basic/pages/special-state.vue).
 
+### beforeSerialize
+
+`beforeSerialize(fn)` (_Function_)
+
+Use this method to update `__NUXT__` variable rendered on client-side, the `fn` (must be synchronous) is called with `nuxtState` as argument. This method is called inside the `rendered` method of Vue SSR, allowing you to use it in components.
+
+Not this usage is advanced and mostly used for Nuxt modules.
+
+```js
+export default {
+  // Using asyncData
+  asyncData({ beforeSerialize }) {
+    if (process.server) {
+      beforeSerialize(nuxtState => {
+        nuxtState.hello = 'world'
+      })
+    }
+  },
+  // Using fetch
+  fetch() {
+    if (process.server) {
+      this.$root.context.beforeSerialize(nuxtState => {
+        nuxtState.hello = 'world'
+      })
+    }
+  }
+}
+```
+
 ## Client-side keys
 
 These keys are available only on client-side.