compose

@hattip/compose

Middleware system for Hattip.

compose

The compose function composes multiple handlers into a single one, creating a simple but powerful middleware system. Each handler is called in sequence until one returns a response. A handler can pass control to the next handler either by not returning anything or calling context.next(). The latter allows the handler to modify the response before returning. Handler arrays passed to compose is flattened and falsy values are filtered.

import { compose } from "@hattip/compose";

// Example of making things available in `ctx`
// Middleware to parse the URL into a URL object
const urlParser = (ctx) => {
  ctx.url = new URL(ctx.request.url);
};

// Example of modifying the response
// Middleware to add an X-Powered-By header
const poweredBy = async (ctx) => {
  const response = await ctx.next();
  response.headers.set("X-Powered-By", "Hattip");
  return response;
};

const homeHandler = (ctx) => {
  if (ctx.url.pathname === "/") {
    return new Response("Home");
  }
};

const fooHandler = (ctx) => {
  if (ctx.url.pathname === "/foo") {
    return new Response("Foo");
  }
};

const barHandler = (ctx) => {
  if (ctx.url.pathname === "/bar") {
    return new Response("Bar");
  }
};

export default compose(
  urlParser,
  poweredBy,
  homeHandler,
  fooHandler,
  barHandler,
);

compose ends the handler chain with a final handler that calls context.passThrough and returns a 404 response.

Handler

Handlers passed to compose can return or throw a Request object or any other object with a toResponse method (which in turn returns a Response) synchronously or asynchronously. Returning a falsy value implicitly passes the control to the next handler. Calling context.next() does the same but explicitly and allows the handler to modify the response before returning.

RequestContext

Handlers are passed a single argument, an object representing the request context:

interface RequestContext {
  /**
   * The request. @see https://developer.mozilla.org/en-US/docs/Web/API/Request
   */
  request: Request;
  /**
   * IP address that generated the request. Check with the platform adapter
   * documentation to understand how it is generated.
   */
  ip: string;
  /**
   * Platform specific stuff. Check with the platform adapter documentation for
   * more information.
   */
  platform: unknown;
  /**
   * Signal that the request hasn't been handled and the returned response is
   * a placeholder (usually a 404). In this case the adapter should handle the
   * request itself if it has a way to do that. For example, an Express
   * middleware adapter may call next() to let the next middleware handle the
   * request. An edge adapter may pass through the request to the origin
   * server. Other adapters may return the placeholder and ignore this call.
   */
  passThrough(): void;
  /**
   * Some platforms (e.g. Cloudflare Workers) require this to be called to
   * keep running after the response is returned when streaming responses.
   * This is a no-op if the platform adapter doesn't need to do anything.
   */
  waitUntil(promise: Promise<any>): void;
  /** Parsed request URL */
  url: URL;
  /** Request method */
  method: string;
  /** App-local stuff */
  locals: Locals;
  /** Call the next handler in the chain */
  next(): Promise<Response>;
  /** Redefine to handle errors by generating a response from an error */
  handleError(error: unknown): Response | Promise<Response>;
}