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| Server-side rendering |
So far, we've discussed creating Svelte components on the client, which is to say the browser. But you can also render Svelte components in Node.js. This can result in better perceived performance as it means the application starts rendering while the page is still downloading, before any JavaScript executes. It also has SEO advantages in some cases, and can be beneficial for people running older browsers that can't or won't run your JavaScript for whatever reason.
Using the compiler
If you're using the Svelte compiler, whether directly or via an integration like rollup-plugin-svelte or svelte-loader, you can tell it to generate a server-side component by passing the generate: 'ssr' option:
const { js } = svelte.compile(source, {
generate: 'ssr' // as opposed to 'dom', the default
});
Registering Svelte
Alternatively, an easy way to use the server-side renderer is to register it:
require('svelte/register.js');
Now you can require your components:
const Thing = require('./components/Thing.html');
If you prefer to use a different file extension, you can pass options like so:
require('svelte/register.js')({
extensions: ['.svelte']
});
Server-side API
Components have a different API in Node.js – rather than being a constructor that you use with the new keyword, a component is an object with a render(data, options) method:
require('svelte/register.js');
const Thing = require('./components/Thing.html');
const props = { answer: 42 };
const { html, css, head } = Thing.render(props);
Lifecycle hooks will not run, with the exception of onDestroy, because the component is never 'mounted'.
The SSR compiler will generate a CommonJS module for each of your components – meaning that
importandexportstatements are converted into theirrequireandmodule.exportsequivalents. If your components have non-component dependencies, they must also work as CommonJS modules in Node. If you're using ES2015 modules, we recommend theesmmodule for automatically converting them to CommonJS.
Rendering styles
You can also extract any scoped styles that are used by the component or its children:
const { css } = Thing.render(data);
You could put the resulting css in a separate stylesheet, or include them in the page inside a <style> tag. If you do this, you will probably want to prevent the client-side compiler from including the CSS again. For the CLI, use the --no-css flag. In build tool integrations like rollup-plugin-svelte, pass the css: false option.
Rendering <head> contents
If your component, any of its children, use the <svelte:head> component, you can extract the contents:
const { head } = Thing.render(data);