fusion-core

Guides

• What is FusionJS
• Getting started
• Framework comparison

Core concepts

• Universal code
• Creating a plugin
  • Dependencies
  • Configuring plugins
  • Creating endpoints
  • Creating providers
  • Modifying the HTML template
  • Working with secrets

fusion-core

The fusion-core package provides a generic entry point class for FusionJS applications that is used by the FusionJS runtime.

If you're using React, you should use the fusion-react package instead.

This package also exposes utilities for developing plugins.

Example

// main.js
import React from 'react';
import {render} from 'react-dom';
import {renderToString} from 'react-dom/server';
import App from 'fusion-core';

const Hello = () => <div>Hello</div>;

const render = el => __NODE__ ? renderToString(el) : render(el, document.getElementById('root'));

export default function() {
  return new App(<Hello />, render);
}

API

import App from 'fusion-core';

const app = new App(element, render);

• element: T - Required. The root element of the application. Typically this should be a React/Preact element
• render: T => string|undefined - A function that can render element

Creates an application that can be registered into the Fusion server.

The application is responsible for rendering (both virtual dom and server-side rendering)

An application can receive any number of plugins, which can augment the behavior of the application.

Typically a plugin works the same way as a Koa middleware.

Instance members

app.plugin

const plugin = app.plugin(factory, dependencies);

• factory: (dependencies: Object) => Plugin - Required. The function that is exported by a plugin package
• dependencies: Object - Optional. A map of dependencies and configuration for the plugin
• plugin - a plugin

Call this method to register a plugin into a FusionJS application. Returns a plugin, which can be passed as a dependency to other plugins.

Plugin

There are two types of plugins: middleware plugins and service plugins.

When writing a plugin you should always export a function that returns either a middleware or a instance of the Plugin class.

Middleware plugins

A middleware plugin is a Koa middleware, a function that takes two argument: a ctx object that has some FusionJS-specific properties, and a next callback function.

const middleware = (ctx, next) => {
  return next();
}

In FusionJS, the next() call represents the time when virtual dom rendering happens. Typically, you'll want to run all your logic before that, and simply have a return next() statement at the end of the function. Even in cases where virtual DOM rendering is not applicable, this pattern is still the simplest way to write a middleware.

In a few more advanced cases, however, you might want to do things after virtual dom rendering. In that case, you can call await next() instead:

export default () => __NODE__ && async (ctx, next) => {
  // this happens before virtual dom rendering
  const start = new Date();

  await next();

  // this happens after virtual rendeing, but before the response is sent to the browser
  console.log('timing: ', new Date() - start);
}

Services

Often we want to encapsulate some functionality into a single coherent package that exposes a programmatic API that can be consumed by others.

In FusionJS, any class can be a service.

import {Plugin} from 'fusion-core';

export default () => {
  return new Plugin({
    Service: class SomeService {
      /* ... */
    },
  });
}

Singleton services

In some cases, it's desirable to enforce that only a single instance of a service exists in an application. To do this, simply use the SingletonPlugin instead of the Plugin class:

import {SingletonPlugin} from 'fusion-core';

export default () => {
  return new SingletonPlugin({
    Service: class {
      constructor() {
        console.log('only gets instantiated once');
      }
    },
  })
}

The singleton service instance can be acquired using the .of method. Calling .of(ctx) from a middleware returns the same instance for all requests.

const Thing = app.plugin(MyThing);
const instance = Thing.of();

Examples

Implementing HTTP endpoints

A plugin can be used to implement a RESTful HTTP endpoint. To achieve this, simply run code conditionally based on the url of the request

export default () => async (ctx, next) => {
  if (ctx.method === 'GET' && ctx.path === '/api/v1/users') {
    ctx.body = await getUsers();
  }
  return next();
}

Serialization and hydration

A plugin can be atomically responsible for serialization/deserialization of data from the server to the client.

The example below shows a plugin that grabs the project version from package.json and logs it in the browser:

// plugins/version-plugin.js
import util from 'util';
import fs from 'fs';
import {html} from 'fusion-core'; // html sanitization

export default () => {
  if (__NODE__) {
    const read = util.promisify(fs.readFile);

    return async (ctx, next) => {
      const data = read('package.json');
      const {version} = JSON.parse(data);
      ctx.body.head.push(html`<meta id="app-version" content="${version}">`);
      return next();
    }
  }
  else {
    return async (ctx, next) => {
      const version = document.getElementById('app-version').content;
      console.log(`Version: ${version}`);
      return next();
    }
  }
}

We can then consume the plugin like this:

// main.js
import React from 'react';
import App from 'fusion-core';
import VersionPlugin from './plugins/version-plugin';

const root = <div>Hello world</div>;

const render = el => __NODE__ ? renderToString(el) : render(el, document.getElementById('root'));

export default function() {
  const app = new App(root, render);
  app.plugin(VersionPlugin);
  return app;
}

API

Middleware plugins

export default () => (ctx, next) => {
  return next()
}

• ctx: {element: T} - An object with a property called element. Plugins can compose element in order to add providers into a React tree, for example:
  ctx.element = <SomeProvider>{ctx.element}</SomeProvider>

  The ctx object also exposes properties that server-specific properties in the server. See context

• next: () => Promise - Every plugin must call await next() (or next().then(...)) exactly once. Code before this call happens before virtual dom rendering, and code after it runs after. In the server, flushing the response to the client happens after

Service plugins

import {Plugin} from 'fusion-core';

export default () => {
  const plugin = new Plugin({Service, middleware});
  return plugin;
}

• Service: class - Optional. A class that provides a programmatic API
• middleware: (ctx: Object, next: () => Promise) => Promise - Optional. A Koa middleware
• plugin: {Service, middleware, of}
  • Service: class
  • middleware: (ctx: Object, next: () => Promise) => Promise
  • of: (ctx: Object|null|undefined) => Service - returns an instance of the service, memoized using ctx as the memoization key.

Context

Middlewares receive a ctx object as their first argument. This object has a property called element in both server and client.

• ctx: Object
  • element: Object

In the server, ctx also exposes the same properties as a Koa context

• ctx: Object
  • header: Object - alias of ctx.headers
  • headers: Object - map of parsed HTTP headers
  • method: string - HTTP method
  • url: string - request URL
  • originalUrl: string - same as url, except that url may be modified (e.g. for url rewriting)
  • path: string - request pathname
  • query: Object - parsed querystring as an object
  • querystring: string - querystring without ?
  • host: string - host and port
  • hostname: string
  • origin: string - request origin, including protocol and host
  • href: string - full URL including protocol, host and url
  • fresh: boolean - check for cache negotiation
  • stale: boolean - inverse of fresh
  • socket: Socket - request socket
  • protocol: string
  • secure: boolean
  • ip: string - remote IP address
  • ips: Array<string> - proxy IPs
  • subdomains: Array<string>
  • is: (...types: ...string) => boolean - response type check
  • accepts: (...types: ...string) => boolean - request MIME type check
  • acceptsEncoding: (...encodings: ...string) => boolean
  • acceptsCharset: (...charsets: ...string) => boolean
  • acceptsLanguage: (...languages: ...string) => boolean
  • get: (name: String) => string - returns a header
  • req: http.IncomingMessage - Node's request object
  • res: Response - Node's response object
  • request: Request - Koa's request object
  • response: Response - Koa's response object
  • state: Object - A state bag for Koa middlewares
  • app: Object - a reference to the Koa instance
  • cookies: {get, set}
    • get: (name: string, options: ?Object) => string - get a cookie
      • name: string
      • options: {signed: boolean}
    • set: (name: string, value: string, options: ?Object)
      • name: string
      • value: string
      • options: Object - Optional
        • maxAge: number - a number representing the milliseconds from Date.now() for expiry
        • signed: boolean - sign the cookie value
        • expires: Date - a Date for cookie expiration
        • path: string - cookie path, /' by default
        • domain: string - cookie domain
        • secure: boolean - secure cookie
        • httpOnly: boolean - server-accessible cookie, true by default
        • overwrite: boolean - a boolean indicating whether to overwrite previously set cookies of the same name (false by default). If this is true, all cookies set during the same request with the same name (regardless of path or domain) are filtered out of the Set-Cookie header when setting this cookie.
  • throw: (status: number, message: ?string, properties: ?Object) => void - throws an error
    • status: number - HTTP status code
    • message: string - error message
    • properties: Object - is merged to the error object
  • assert: (value: any, status: ?number, message: ?string, properties) - throws if value is falsy
    • value: any
    • status: number - HTTP status code
    • message: string - error message
    • properties: Object - is merged to the error object
  • respond: boolean - set to true to bypass Koa's built-in response handling. You should not use this flag.

Additionally, when server-side rendering a page, FusionJS sets ctx.body to an object with the following properties:

• ctx: Object
  • body: Object
    • htmlAttrs: Object - attributes for the <html> tag. For example {lang: 'en-US'} turns into <html lang="en-US">. Default: empty object
    • title: string - The content for the <title> tag. Default: empty string
    • head: Array - A list of sanitized HTML strings. Default: empty array
    • body: Array - A list of sanitized HTML strings. Default: empty array

When a request does not require a server-side render, ctx.body follows regular Koa semantics.

HTML sanitization

Default-on HTML sanitization is important for preventing security threats such as XSS attacks.

Fusion automatically sanitizes htmlAttrs and title. When pushing HTML strings to head or body, you must use the html template tag to mark your HTML as sanitized:

import {html} from 'fusion-core';

export default () => (ctx, next) => {
  if (ctx.element) {
    const userData = await getUserData();
    // userData can't be trusted, and is automatically escaped
    ctx.body.body.push(html`<div>${userData}</div>`)
  }
  return next();
}

If userData above was <script>alert(1)</script>, the string would be automatically turned into <div>\u003Cscript\u003Ealert(1)\u003C/script\u003E</div>. Note that only userData is escaped, but the HTML in your code stays intact.

If your HTML is complex and needs to be broken into smaller strings, you can also nest sanitized HTML strings like this:

const notUserData = html`<h1>Hello</h1>`
const body = html`<div>${notUserData}</div>`

Note that you cannot mix sanitized HTML with unsanitized strings:

ctx.body.body.push(html`<h1>Safe</h1>` + 'not safe') // will throw an error when rendered

Also note that only template strings can have template tags (i.e. html`<div></div>`). The following are NOT valid Javascript: html"<div></div>" and html'<div></div>'.

If you get an Unsanitized html. You must use html`[your html here]` error, remember to prepend the html template tag to your template string.

If you have already taken steps to sanitize your input against XSS and don't wish to re-sanitize it, you can use dangerouslySetHTML(string) to let Fusion render the unescaped dynamic string.

Serialization and deserialization

Here's how to serialize JSON data in the server:

ctx.body.body.push(html`<script id="__MY_DATA__" type="text/plain">${JSON.stringify(data)}</script>`);

Here's how to deserialize it in the browser:

import {unescape} from 'fusion-core';

const data = JSON.parse(unescape(document.getElementById('__MY_DATA__').innerHTML));