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Plugins

Plugins can dynamically add new functionality to Fresh without exposing significant complexity to the user. Users can add plugins by importing and initializing them in their main.ts file:

main.ts
import { start } from "$fresh/server.ts";
import manifest from "./fresh.gen.ts";

import twindPlugin from "$fresh/plugins/twind.ts";
import twindConfig from "./twind.config.js";

await start(manifest, {
  plugins: [
    // This line configures Fresh to use the first-party twind plugin.
    twindPlugin(twindConfig),
  ],
});

Currently, the only available first-party plugin is the Twind plugin. Third-party plugins are also supported - they can be imported from any HTTP server, like any other Deno module.

Plugin hooks are executed in the order that the plugins are defined in the plugins array. This means that the first plugin in the array will be executed first, and the last plugin in the array will be executed last. For many plugins, this does not matter, but for some plugins it may.

Creating a plugin

Fresh plugins are in essence a collection of hooks that allow the plugin to hook into various systems inside of Fresh. Currently only a render hook is available (explained below).

A Fresh plugin is just a JavaScript object that conforms to the Plugin interface. The only required property of a plugin is its name. Names must only contain the characters a-z, and _.

import { Plugin } from "$fresh/server.ts";

const plugin: Plugin = {
  name: "my_plugin",
};

A plugin containing only a name is technically valid, but not very useful. To be able to do anything with a plugin, it must register some hooks, middlewares, or routes.

Hook: render

The render hook allows plugins to:

  • Control timing of the synchronous render of a page.
  • Inject additional CSS and JS into the rendered page.

This is commonly used to set thread local variables for the duration of the render (for example preact global context, preact option hooks, or for style libraries like Twind). After render is complete, the plugin can inject inline CSS and JS modules (with attached state) into the page.

The render hook is called with the PluginRenderContext object, which contains a render() method. This method must be invoked during the render hook to actually render the page. It is a terminal error to not call the render() method during the render hook.

The render() method returns a PluginRenderFunctionResult object which contains the HTML text of the rendered page, as well as a boolean indicating whether the page contains any islands that will be hydrated on the client.

The render hook needs to synchronously return a PluginRenderResult object. Additional CSS and JS modules can be added to be injected into the page by adding them to styles, links and scripts arrays in this object. The plugin can also replace the the HTML in side the <body>-element of the page by including a htmlText string in this object.

styles are injected into the <head> of the page as inline CSS. Each entry can define the CSS text to inject, as well as an optional id for the style tag, and an optional media attribute for the style tag.

links are injected into the <head> of the page as <link> tags. A link tag is created for each entry, with attributes from the entry’s properties.

scripts define JavaScript/TypeScript modules to be injected into the page. The possibly loaded modules need to be defined up front in the Plugin#entrypoints property. Each defined module must be a JavaScript/TypeScript module that has a default export of a function that takes one (arbitrary) argument, and returns nothing (or a promise resolving to nothing). Fresh will call this function with the state defined in the scripts entry. The state can be any arbitrary JSON serializable JavaScript value.

For an example of a plugin that uses the render hook, see the first-party Twind plugin.

Hook: renderAsync

This hook is largely the same as the render hook, with a couple of key differences to make asynchronous style and script generation possible. It must asynchronously return its PluginRenderResult, either from an async/await function or wrapped within a promise.

The render hook is called with the PluginAsyncRenderContext object, which contains a renderAsync() method. This method must be invoked during the render hook to actually render the page. It is a terminal error to not call the renderAsync() method during the render hook.

This is useful for when plugins are generating styles and scripts with asynchronous dependencies based on the htmlText. Unlike the synchronous render hook, async render hooks for multiple pages can be running at the same time. This means that unlike the synchronous render hook, you can not use global variables to propagate state between the render hook and the renderer.

The renderAsync hooks start before any page rendering occurs, and finish after all rendering is complete — they wrap around the underlying JSX->string rendering, plugin render hooks, and the RenderFunction that may be provided to Fresh’s start entrypoint in the main.ts file.

Hook: buildStart

This hook is run at the start of the Fresh ahead-of-time build task. It may be synchronous or asynchronous.

The build start hook is called with the ResolvedFreshConfig object, which contains the full Fresh configuration.

This hook may be used to generate precompiled static assets. Any files saved to the static subfolder of config.build.outDir (typically _fresh) will be served the same as other static files.

Hook: buildEnd

This hook is run at the end of the Fresh ahead-of-time build task. It may be synchronous or asynchronous.

Routes and Middlewares

You can create routes and middlewares that get loaded and rendered like the normal routes and middlewares.

The plugin routes and middlewares need a defined path in the format of a file name without a filetype inside the routes directory(E.g. blog/index, blog/[slug]).

For more examples see the Concepts: Routing page.

To create a middleware you need to create a MiddlewareHandler function.

And to create a route you can create both a Handler and/or component.

A very basic example can be found here.

Islands

Islands from plugins can be loaded by specifying a list of file paths in your plugin. Those files will be treated by Fresh as if they had been placed inside the islands/ directory. They will be processed and bundled for the browser in the same way.

my-island-plugin.ts
import { Plugin } from "$fresh/server.ts";

export default function myIslandPlugin(): Plugin {
  return {
    name: "my-island-plugin",
    islands: {
      baseLocation: import.meta.url,
      paths: [
        "./plugin/MyPluginIsland.tsx",
        "./plugin/OtherPluginIsland.tsx",
      ],
    },
  };
}