> This package is auto-generated. For pull requests please see [src/entries/web-server-renderer.js](https://github.com/vuejs/vue/blob/dev/src/entries/web-server-renderer.js).
Creates a `bundleRenderer` instance using pre-bundled application code (see [Creating the Server Bundle](#creating-the-server-bundle)). For each render call, the code will be re-run in a new context using Node.js' `vm` module. This ensures your application state is discrete between requests, and you don't need to worry about structuring your application in a limiting pattern just for the sake of SSR.
Render the bundled app to a string. Same callback interface with `renderer.renderToString`. The optional context object will be passed to the bundle's exported function.
Render the bundled app to a stream. Same stream interface with `renderer.renderToStream`. The optional context object will be passed to the bundle's exported function.
``` js
bundleRenderer
.renderToStream({ url: '/' })
.pipe(writableStream)
```
## Renderer Options
### directives
Allows you to provide server-side implementations for your custom directives:
``` js
const renderer = createRenderer({
directives: {
example (vnode, directiveMeta) {
// transform vnode based on directive binding metadata
Note that the cache object should at least implement `get` and `set`. In addition, `get` and `has` can be optionally async if they accept a second argument as callback. This allows the cache to make use of async APIs, e.g. a redis client:
In a typical Node.js app, the server is a long-running process. If we directly require our application code, the instantiated modules will be shared across every request. This imposes some inconvenient restrictions to the application structure: we will have to avoid any use of global stateful singletons (e.g. the store), otherwise state mutations caused by one request will affect the result of the next.
Instead, it's more straightforward to run our app "fresh" for each request, so that we don't have to think about avoiding state contamination across requests. This is exactly what `bundleRenderer` helps us achieve.
The application bundle can be generated by any build tool, so you can easily use Webpack + `vue-loader` with the bundleRenderer. You do need to use a slightly different webpack config and entry point for your server-side bundle, but the difference is rather minimal:
1. add `target: 'node'`, and use `output: { libraryTarget: 'commonjs2' }` for your webpack config. Also, it's probably a good idea to [externalize your dependencies](#externals).
2. In your server-side entry point, export a function. The function will receive the render context object (passed to `bundleRenderer.renderToString` or `bundleRenderer.renderToStream`), and should return a Promise, which should eventually resolve to the app's root Vue instance:
``` js
// server-entry.js
import Vue from 'vue'
import App from './App.vue'
const app = new Vue(App)
// the default export should be a function
// which will receive the context of the render call
When using the `bundleRenderer`, we will by default bundle every dependency of our app into the server bundle as well. This means on each request these depdencies will need to be parsed and evaluated again, which is unnecessary in most cases.
We can optimize this by externalizing dependencies from your bundle. During the render, any raw `require()` calls found in the bundle will return the actual Node module from your rendering process. With Webpack, we can simply list the modules we want to externalize using the [`externals` config option](https://webpack.github.io/docs/configuration.html#externals):
``` js
// webpack.config.js
module.exports = {
// this will externalize all modules listed under "dependencies"
Since externalized modules will be shared across every request, you need to make sure that the dependency is **idempotent**. That is, using it across different requests should always yield the same result - it cannot have global state that may be changed by your application. Interactions between externalized modules are fine (e.g. using a Vue plugin).
Note that cachable component **must also define a unique "name" option**. This is necessary for Vue to determine the identity of the component when using the
bundle renderer.
With a unique name, the cache key is thus per-component: you don't need to worry about two components returning the same key. A cache key should contain sufficient information to represent the shape of the render result. The above is a good implementation if the render result is solely determined by `props.item.id`. However, if the item with the same id may change over time, or if render result also relies on another prop, then you need to modify your `getCacheKey` implementation to take those other variables into account.
If the renderer hits a cache for a component during render, it will directly reuse the cached result for the entire sub tree. So **do not cache a component containing child components that rely on global state**.
In most cases, you shouldn't and don't need to cache single-instance components. The most common type of components that need caching are ones in big lists. Since these components are usually driven by objects in database collections, they can make use of a simple caching strategy: generate their cache keys using their unique id plus the last updated timestamp:
In server-rendered output, the root element will have the `server-rendered="true"` attribute. On the client, when you mount a Vue instance to an element with this attribute, it will attempt to "hydrate" the existing DOM instead of creating new DOM nodes.
In development mode, Vue will assert the client-side generated virtual DOM tree matches the DOM structure rendered from the server. If there is a mismatch, it will bail hydration, discard existing DOM and render from scratch. **In production mode, this assertion is disabled for maximum performance.**
### Hydration Caveats
One thing to be aware of when using SSR + client hydration is some special HTML structures that may be altered by the browser. For example, when you write this in a Vue template:
``` html
<table>
<tr><td>hi</td></tr>
</table>
```
The browser will automatically inject `<tbody>` inside `<table>`, however, the virtual DOM generated by Vue does not contain `<tbody>`, so it will cause a mismatch. To ensure correct matching, make sure to write valid HTML in your templates.
