From 4c3e73426c4bc78127bca5393b05be4fec65f369 Mon Sep 17 00:00:00 2001
From: Daniel Roe <daniel@roe.dev>
Date: Wed, 1 Sep 2021 14:55:10 +0200
Subject: [PATCH] fix: preserve reactive objects on `useFetch` hydration (#541)

---
 src/runtime/composables/fetch.ts | 59 ++++++++++++++++++++------------
 test/fixture/pages/ttfb.vue      | 12 ++++---
 2 files changed, 45 insertions(+), 26 deletions(-)

diff --git a/src/runtime/composables/fetch.ts b/src/runtime/composables/fetch.ts
index f4bfc7ac..88dc2ff4 100644
--- a/src/runtime/composables/fetch.ts
+++ b/src/runtime/composables/fetch.ts
@@ -1,5 +1,6 @@
 import {
   isRef,
+  isReactive,
   onBeforeMount,
   onServerPrefetch,
   reactive,
@@ -129,6 +130,41 @@ const setFetchState = (vm: AugmentedComponentInstance) => {
     })
 }
 
+const mergeDataOnMount = (data: Record<string, any>) => {
+  const vm = getCurrentInstance() as AugmentedComponentInstance | undefined
+  if (!vm) throw new Error('This must be called within a setup function.')
+
+  onBeforeMount(() => {
+    // Merge data
+    for (const key in data) {
+      try {
+        // Assign missing properties
+        if (key in vm) {
+          const _key = key as keyof typeof vm
+          // Skip functions (not stringifiable)
+          if (typeof vm[_key] === 'function') continue
+          // Preserve reactive objects
+          if (isReactive(vm[_key])) {
+            // Unset keys that do not exist in incoming data
+            for (const k in vm[_key]) {
+              if (!(k in data[key])) {
+                delete vm[_key][k]
+              }
+            }
+            Object.assign(vm[_key], data[key])
+            continue
+          }
+        }
+        set(vm, key, data[key])
+      } catch (e) {
+        if (process.env.NODE_ENV === 'development')
+          // eslint-disable-next-line
+          console.warn(`Could not hydrate ${key}.`)
+      }
+    }
+  })
+}
+
 const loadFullStatic = (vm: AugmentedComponentInstance) => {
   vm._fetchKey = getKey(vm)
   // Check if component has been fetched on server
@@ -151,12 +187,7 @@ const loadFullStatic = (vm: AugmentedComponentInstance) => {
     return
   }
 
-  onBeforeMount(() => {
-    // Merge data
-    for (const key in data) {
-      set(vm, key, data[key])
-    }
-  })
+  mergeDataOnMount(data)
 }
 
 async function serverPrefetch(vm: AugmentedComponentInstance) {
@@ -319,21 +350,7 @@ export const useFetch = (callback: Fetch) => {
     return result()
   }
 
-  onBeforeMount(() => {
-    // Merge data
-    for (const key in data) {
-      try {
-        if (key in vm && typeof vm[key as keyof typeof vm] === 'function') {
-          continue
-        }
-        set(vm, key, data[key])
-      } catch (e) {
-        if (process.env.NODE_ENV === 'development')
-          // eslint-disable-next-line
-          console.warn(`Could not hydrate ${key}.`)
-      }
-    }
-  })
+  mergeDataOnMount(data)
 
   return result()
 }
diff --git a/test/fixture/pages/ttfb.vue b/test/fixture/pages/ttfb.vue
index cc628820..d70f7116 100644
--- a/test/fixture/pages/ttfb.vue
+++ b/test/fixture/pages/ttfb.vue
@@ -1,27 +1,29 @@
 <template>
-  <main>TTFB: {{ ttfb }}ms</main>
+  <main>TTFB: {{ dataset.ttfb }}ms</main>
 </template>
 
 <script>
 import {
   defineComponent,
-  ref,
+  reactive,
   useFetch,
   onMounted,
 } from '@nuxtjs/composition-api'
 
 export default defineComponent({
   setup() {
+    const dataset = reactive({
+      ttfb: -1
+    })
     useFetch(() => {})
 
-    const ttfb = ref(-1)
     onMounted(() => {
-      ttfb.value =
+      dataset.ttfb =
         globalThis.performance.getEntriesByType('navigation')[0].responseStart
     })
 
     return {
-      ttfb,
+      dataset,
     }
   },
 })