Induce and Resolve Vue SSR Hydration Mismatch

This challenge focuses on understanding and fixing a common issue in Vue.js Server-Side Rendering (SSR): hydration mismatches. You will intentionally create a hydration mismatch in a Vue application and then implement the necessary changes to resolve it, demonstrating a practical grasp of SSR principles in Vue.

Problem Description

Server-Side Rendering (SSR) in Vue allows for improved performance and SEO by rendering your application on the server before sending it to the client. Hydration is the process where the client-side Vue application "takes over" the server-rendered HTML, attaching event listeners and making it interactive. A hydration mismatch occurs when the client-side rendered DOM structure or data differs from the server-rendered DOM, leading to errors and unexpected behavior.

Your task is to:

1. Create a scenario in a Vue 3 SSR application that intentionally causes a hydration mismatch.
2. Identify the root cause of the mismatch.
3. Modify the application to ensure successful hydration, meaning the client-side and server-side renderings align perfectly.

Key Requirements:

• The application should be a standard Vue 3 SSR setup (using @vue/server-renderer and @vue/compiler-sfc or similar).
• The mismatch should be reproducible on both the server and client.
• The solution should involve correcting the mismatch, not bypassing it (e.g., by disabling SSR for that component).

Expected Behavior:

• Initially, the application should exhibit a hydration mismatch error in the browser console (e.g., "Hydration completed but contains mismatches").
• After your fix, the application should hydrate successfully without any console warnings or errors related to hydration mismatches.

Edge Cases to Consider:

• Dynamic data that changes between server and client rendering.
• Differences in browser APIs available on the server vs. client.
• Conditional rendering logic that behaves differently.

Examples

Example 1: Dynamic Content based on Time

Imagine a component that displays the current time. The server renders the time at the moment of server-side rendering, while the client renders it when the page loads.

Input: A Vue 3 SSR application with a component that renders the current time using new Date().

<!-- MyClock.vue -->
<template>
  <div>
    <p>Current time: {{ currentTime }}</p>
  </div>
</template>

<script setup lang="ts">
import { ref } from 'vue';

const currentTime = ref(new Date().toLocaleTimeString());
</script>

Output (Initial State - Mismatch):

• Browser console: "Hydration completed but contains mismatches."
• The displayed time on the client might be slightly different from what was initially rendered on the server.

Explanation: The server renders new Date() at its rendering time. By the time the client hydrates, new Date() will have advanced, causing a mismatch.

Example 2: Client-Specific APIs

A component that uses navigator.userAgent to conditionally render something. navigator is not available on the Node.js server.

Input: A Vue 3 SSR application with a component that checks navigator.userAgent.

<!-- UserAgentChecker.vue -->
<template>
  <div>
    <p v-if="isMobile">You are on a mobile device.</p>
    <p v-else>You are on a desktop device.</p>
  </div>
</template>

<script setup lang="ts">
import { ref, onMounted } from 'vue';

const isMobile = ref(false);

// This will cause issues on the server because 'navigator' is undefined
if (typeof navigator !== 'undefined') {
  isMobile.value = /Mobi|Android/i.test(navigator.userAgent);
}
</script>

Output (Initial State - Mismatch):

• Browser console: "Hydration completed but contains mismatches."
• The initial rendering might be incorrect or throw errors on the server.

Explanation: On the server, navigator is undefined, so isMobile will be false (or might throw an error if not guarded). On the client, navigator is available, and isMobile might be true, leading to different DOM structures.

Constraints

• The Vue.js version used must be Vue 3.
• The solution must be implemented in TypeScript.
• The core of the problem should be addressed using standard Vue 3 SSR patterns, not by falling back to client-only rendering for the entire application.
• The application should consist of at least a server entry point, a client entry point, and at least one Vue component.

Notes

• Consider the lifecycle hooks available in Vue 3 and how they differ between server and client rendering.
• The navigator object, window object, and document object are typically not available or behave differently on the server.
• Data that is fetched asynchronously on the server and then passed to the client needs careful handling to ensure consistency.
• This challenge is designed to test your understanding of the fundamental differences between server and client environments and how Vue handles SSR hydration.