@fastify/early-hints

Draft proposal of plugin handling the HTTP 103 code. Based on : https://github.com/fastify/fastify/issues/2683

Install

npm i @fastify/early-hints

Options

You can pass the following options during the plugin registration:

await fastify.register(import('@fastify/early-hints'), {
  warn: true // default: false
})

• warn : indicates if the plugin should log warnings if invalid values are supplied as early hints

Usage

Reply.writeEarlyHints

This method is used to write early hints with any header you need. It accepts either object or Array of headers and return Promise.

const Fastify = require("fastify");
const eh = require("@fastify/early-hints");

const fastify = Fastify({ logger: true });
fastify.register(eh);

fastify.get("/", async (request, reply) => {
  // object
  await reply.writeEarlyHints({
    'Content-Security-Policy': 'style-src: self;',
    Link: ['</style.css>; rel=preload; as=style', '</script.js>; rel=preload; as=script']
  })
  // array
  await reply.writeEarlyHints([
    { name: 'Content-Security-Policy', value: 'style-src: self;' },
    { name: 'Link', value: '</style.css>; rel=preload; as=style' },
    { name: 'Link', value: '</script.js>; rel=preload; as=script' },
  ])
  return { hello: "world" };
});

const start = async () => {
  try {
    await fastify.listen({ port: 3000 });
    fastify.log.info(`server listening on ${fastify.server.address().port}`);
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
};
start();

Result

$ curl -D - http://localhost:3000    
HTTP/1.1 103 Early Hints
Content-Security-Policy: style-src: self;
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script

HTTP/1.1 103 Early Hints
Content-Security-Policy: style-src: self;
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script

HTTP/1.1 200 OK
content-type: application/json; charset=utf-8
content-length: 17
Date: Thu, 12 Nov 2020 22:45:54 GMT
Connection: keep-alive

{"hello":"world"}

Reply.writeEarlyHintsLinks

This method used to write only the Link header. It accepts an Array and return Promise.

const Fastify = require("fastify");
const eh = require("@fastify/early-hints");

const fastify = Fastify({ logger: true });
fastify.register(eh);

fastify.get("/", async (request, reply) => {
  await reply.writeEarlyHintsLinks([
    "Link: </style.css>; rel=preload; as=style",
    "Link: </script.js>; rel=preload; as=script",
  ])
  await reply.writeEarlyHintsLinks([
    { href: "//example.com", rel: "preload", as: "style" },
    { href: "//example.com", rel: "preload", as: "style", cors: true },
    { href: "//example.com", rel: "preconnect" },
    { href: "//example2.com", rel: "preconnect", cors: true },
    { href: "//example3.com", rel: "preconnect", cors: "use-credentials" },
  ])
  return { hello: "world" };
});

const start = async () => {
  try {
    await fastify.listen({ port: 3000 });
    fastify.log.info(`server listening on ${fastify.server.address().port}`);
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
};
start();

Result

$ curl -D - http://localhost:3000    
HTTP/1.1 103 Early Hints
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script

HTTP/1.1 103 Early Hints
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script
Link: <//example.com>; rel=preload; as=style
Link: <//example.com>; rel=preload; as=style; crossorigin
Link: <//example.com>; rel=preconnect
Link: <//example2.com>; rel=preconnect; crossorigin
Link: <//example3.com>; rel=preconnect; crossorigin=use-credentials

HTTP/1.1 200 OK
content-type: application/json; charset=utf-8
content-length: 17
Date: Thu, 12 Nov 2020 22:45:54 GMT
Connection: keep-alive

{"hello":"world"}

Browser Limitation

Currently (2022-09-29), only Chrome 103 is supporting 103 Early Hints and Chrome will ignore 103 Early Hints in the following situations.

• Early Hints sent on subresource requests
• Early Hints sent on iframe navigation
• Early Hints sent on HTTP/1.1 or earlier
• Second and following Early Hints

Read more on https://chromium.googlesource.com/chromium/src/+/master/docs/early-hints.md#103-early-hints

References

• https://httpwg.org/specs/rfc8297.html
• https://www.w3.org/TR/resource-hints/

License

Licensed under MIT.