Nuxt Lifecycle

Understanding the lifecycle of Nuxt applications can help you gain deeper insights into how the framework operates, especially for both server-side and client-side rendering.

The goal of this guide is to provide you with a high-level overview of the different parts of the framework, their execution order, and how they work together.

Server Lifecycle

On the server, the following steps are executed for every initial request to your application:

1. Server Plugins

   Nuxt is powered by Nitro, a modern server engine.

   When Nitro starts, it initializes and executes the plugins under the /server/plugins directory. These plugins can:

   • Capture and handle application-wide errors
   • Register hooks that execute when Nitro shuts down
   • Register hooks for request lifecycle events, such as modifying responses

   Nitro plugins are executed only once when the server starts. In a serverless environment, the server boots on each incoming request, and so do the Nitro plugins. However, they are not awaited.

2. Server Middleware

   After initializing the Nitro server, middleware under server/middleware/ is executed for every request. You can use middleware for tasks such as authentication, logging, or request transformation.

   Returning a value from middleware will terminate the request and send the returned value as the response. You should generally avoid this behavior to ensure proper request handling.

3. App Plugins

   The Vue and Nuxt instances are created first. Afterward, Nuxt executes its app plugins. This includes:

   • Built-in plugins, such as Vue Router and unhead
   • Custom plugins located in the app/plugins/ directory, including those without a suffix (e.g., myPlugin.ts) and those with the .server suffix (e.g., myServerPlugin.server.ts)

   Plugins execute in a specific order and may have dependencies on one another. For more details, including execution order and parallelism, refer to the Plugins documentation.

   After this step, Nuxt calls the app:created hook, which you can use to execute additional logic.

4. Route Validation

   After initializing plugins and before executing middleware, Nuxt calls the validate method if it is defined in the definePageMeta function. The validate method, which can be synchronous or asynchronous, is often used to validate dynamic route parameters.

   • The validate function should return true if the parameters are valid
   • If validation fails, it should return false or an object containing a status and/or statusText to terminate the request

   For more information, see the Route Validation documentation.

5. App Middleware

   Middleware allows you to run code before navigating to a particular route. You often use it for tasks such as authentication, redirection, or logging.

   In Nuxt, there are three types of middleware:

   • Global route middleware
   • Named route middleware
   • Anonymous (or inline) route middleware

   Nuxt executes all global middleware on the initial page load (both on server and client) and then again before any client-side navigation. Named and anonymous middleware are executed only on the routes specified in the middleware property of the page(route) meta defined in the corresponding page components.

   For details about each type and examples, see the Middleware documentation.

   Any redirection on the server will result in a Location: header being sent to the browser; the browser then makes a fresh request to this new location. All application state will be reset when this happens, unless persisted in a cookie.

6. Page and Components

   Nuxt renders the page and its components and fetches any required data with useFetch and useAsyncData during this step. Since there are no dynamic updates and no DOM operations occur on the server, Vue lifecycle hooks such as onBeforeMount, onMounted, and subsequent hooks are NOT executed during SSR.

   By default, Vue pauses dependency tracking during SSR for better performance.

   There is no reactivity on the server side because Vue SSR renders the app top-down as static HTML, making it impossible to go back and modify content that has already been rendered.

   You should avoid code that produces side effects that need cleanup in root scope of <script setup>. An example of such side effects is setting up timers with setInterval. In client-side only code we may setup a timer and then tear it down in onBeforeUnmount or onUnmounted. However, because the unmount hooks will never be called during SSR, the timers will stay around forever. To avoid this, move your side-effect code into onMounted instead.

7. HTML Output

   After all required data is fetched and the components are rendered, Nuxt combines the rendered components with settings from unhead to generate a complete HTML document. This HTML, along with the associated data, is then sent back to the client to complete the SSR process.

   After rendering the Vue application to HTML, Nuxt calls the app:rendered hook. Before finalizing and sending the HTML, Nitro will call the render:html hook. This hook allows you to manipulate the generated HTML, such as injecting additional scripts or modifying meta tags.

Client Lifecycle

This part of the lifecycle is fully executed in the browser, no matter which Nuxt mode you’ve chosen.

1. App Plugins

   This step is similar to the server-side execution and includes both built-in and custom plugins.

   Custom plugins in the app/plugins/ directory, such as those without a suffix (e.g., myPlugin.ts) and with the .client suffix (e.g., myClientPlugin.client.ts), are executed on the client side.

   After this step, Nuxt calls the app:created hook, which you can use to execute additional logic.

2. Route Validation

   This step is the same as the server-side execution and includes the validate method if defined in the definePageMeta function.

3. App Middleware

   Nuxt middleware runs on both the server and the client. If you want certain code to run in specific environments, consider splitting it by using import.meta.client for the client and import.meta.server for the server.

4. Mount Vue App and Hydrate

   Calling app.mount('#__nuxt') mounts the Vue application to the DOM. If your application uses SSR or SSG mode, Vue performs a hydration step to make the client-side application interactive. During hydration, Vue recreates the application (excluding Server Components), matches each component to its corresponding DOM nodes, and attaches DOM event listeners.

   To ensure proper hydration, it’s important to maintain consistency between the data on the server and the client. For API requests, use useAsyncData, useFetch, or other SSR-friendly composables. These methods ensure that the data fetched on the server side is reused during hydration, avoiding repeated requests. Any new requests should only be triggered after hydration, preventing hydration errors.

   Before mounting the Vue application, Nuxt calls the app:beforeMount hook. After mounting the Vue application, Nuxt calls the app:mounted hook.

5. Vue Lifecycle

   Unlike on the server, the browser executes the full Vue lifecycle.