Modifying the HTML template

The HTML template consists of the HTML tag that gets served from the server which contains metadata such as the <title> and <meta> tags.

The easiest way to edit those tags is via fusion-plugin-react-helmet-async.

Add the Helmet component to your React tree and add HTML elements inside of it to add/modify the respective HTML tag.

import React from 'react';
import {Helmet} from 'fusion-plugin-react-helmet-async';

const Root = () => (
      <title>Hello World</title>
      <link rel="canonical" href="" />
    <h1>Hello World</h1>

Modifying HTML without React

It's also possible to modify the HTML template of a Fusion.js app by using middleware plugins.

When a page is server-side rendered, the ctx.template property in the middleware is an object:

ctx.template = {
  htmlAttrs: {}
  title: '',
  head: [],
  body: []

To modify the title, use:

app.middleware((ctx, next) => {
  ctx.template.title = 'the new title';
  return next();

Similarly, to add attributes to the <html> tag, use:

app.middleware((ctx, next) => {
  ctx.template.htmlAttrs.lang = 'en-US';
  return next();

To add arbitrary HTML to <head> and <body>, however, you must sanitize the HTML to ensure that there's no risk of an XSS attack from unsanitized user data.

To sanitize HTML, use the html template tag:

import {html} from 'fusion-core';

app.middleware((ctx, next) => {
    html`<link href="" rel="stylesheet" />`
  return next();

The html template tag sanitizes template string interpolations and return a safe string.

Note that pushing unsanitized strings to either head or body will result in an error.

When is a page server-side rendered

Fusion.js assumes a page requires server-side rendering if the HTTP request has an Accept header containing text/html. This header value exists when URLs that are requested from a browser address bar, but it does not for programmatic requests such as XHR/fetch or assets (e.g. <link href="...">, <img src="...">)

A caveat of this approach is that hitting an endpoint from the browser address bar (as one might do during development) will include the header on an endpoint that isn't meant to be server-side rendered, and similarly, hitting a page via curl or similar tool without the header will not trigger server-side rendering correctly.

Serialization and deserialization

Sometimes it's useful to share information from the server to the client.

The mechanism to do it is similar to the one above: use the html template tag to sanitize the interpolated data.

On the browser, we use unescape to get the original data.

import {html, unescape} from 'fusion-core';

app.middleware((ctx, next) => {
  if (__NODE__) {
    const data = {
      /* some data */
      html`<meta id="__MY_DATA__" content="${JSON.stringify(data)}">`
  } else {
    const data = JSON.parse(
  return next();