headers

@hattip/headers

Header manipulation, parsing, and content negotiation utilities for Hattip.

modifyHeaders

Some Response objects have an immutable headers property. Response.redirect(), for instance, creates such an object. When the headers are immutable, a naive middleware to modify the headers will fail:

async function naivePoweredBy(ctx) {
  const response = await ctx.next();
  // The following line will throw an error if `response.headers` is immutable
  response.headers.set("X-Powered-By", "Hattip");
  return response;
}

The modifyHeaders function solves this problem by trying to modify the headers in place, and if that fails, creating a new Response object and trying again:

async function correctPoweredBy(ctx) {
  let response = await ctx.next();
  response = modifyHeaders(response, (headers) => {
    headers.set("X-Powered-By", "Hattip");
  });

  return response;
}

parseHeaderValue

parseHeaderValue parses a header value and returns an array of objects with a value and directives property. Each object represents a value (comma-separated) and its directives (semicolon-separated). It understands basic quoting and comments between parentheses.

const parsed = parseHeaderValue(`en-US, en;q=0.9, fr;q=0.8`);

assert.deepStrictEqual(parsed, [
  { value: "en-US", directives: {} },
  { value: "en", directives: { q: "0.9" } },
  { value: "fr", directives: { q: "0.8" } },
]);

accept

Performs content negotiation on the Accept header:

const handler = accept("text/html, application/json", {
  "text/html": () => html("<h1>Hello world!</h1>"),
  "application/json": () => json({ message: "Hello world!" }),
  "*": () => new Response("Unacceptable", { status: 406 }),
});

const response = handler();
console.assert(response.headers.get("content-type") === "text/html");

q values and subtype and type wildcards requests are supported.

acceptLanguage

Performs content negotiation on the Accept-Language header:

const handler = acceptLanguage("en-US, fr;q=0.8", {
  "en-US": () => "Hello!",
  fr: () => "Bonjour!",
  "*": () => "Hello!",
});

const result = handler();
console.assert(result === "Hello!");