ketting/src/http/fetcher.ts

Opinionated HATEOAS / Rest client.

import problemFactory from './error.js';
import packageJson from 'ketting/package.json' with { type: 'json' };

export type FetchMiddleware =
  (request: Request, next: (request: Request) => Promise<Response>) => Promise<Response>;

/**
 * The fetcher object is responsible for calling fetch()
 *
 * This is wrapped in an object because we want to support
 * 'fetch middlewares'. These middlewares are similar to server-side
 * middlewares and can intercept requests and alter requests/responses.
 */
export class Fetcher {

  middlewares: [RegExp, FetchMiddleware][] = [];

  advertiseKetting: boolean = true;

  /**
   * A wrapper for MDN fetch()
   *
   * This wrapper supports 'fetch middlewares'. It will call them
   * in sequence.
   */
  fetch(resource: string|Request, init?: RequestInit): Promise<Response> {

    const request = new Request(resource, init);

    const origin = new URL(request.url).origin;
    const mws = this.getMiddlewaresByOrigin(origin);
    mws.push((innerRequest: Request) => {

      if (!innerRequest.headers.has('User-Agent') && this.advertiseKetting) {
        // eslint-disable-next-line @typescript-eslint/no-require-imports
        innerRequest.headers.set('User-Agent', `Ketting/${packageJson.version}`);
      }

      return fetch(innerRequest);
    });

    return invokeMiddlewares(mws, request);

  }

  /**
   * Returns the list of middlewares that are applicable to
   * a specific origin
   */
  getMiddlewaresByOrigin(origin: string): FetchMiddleware[] {

    return this.middlewares.filter( ([regex]) => {
      return regex.test(origin);

    }).map( ([, middleware]) => {
      return middleware;
    });

  }

  /**
   * Add a middleware
   */
  use(mw: FetchMiddleware, origin: string = '*'): void {

    const matchSplit = origin.split('*');
    const matchRegex = matchSplit.map(
      part =>
        part.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
    ).join('(.*)');

    const regex = new RegExp('^' + matchRegex + '$');
    this.middlewares.push([regex, mw]);

  }

  /**
   * Does a HTTP request and throws an exception if the server emitted
   * a HTTP error.
   *
   * @see https://developer.mozilla.org/en-US/docs/Web/API/Request/Request
   */
  async fetchOrThrow(resource: string|Request, init?: RequestInit): Promise<Response> {

    const response = await this.fetch(resource, init);

    if (response.ok) {
      return response;
    } else {
      throw await problemFactory(response);
    }

  }

}
export default Fetcher;

function invokeMiddlewares(mws: FetchMiddleware[], request: Request): Promise<Response> {

  return mws[0](
    request,
    (nextRequest: Request) => {
      return invokeMiddlewares(mws.slice(1), nextRequest);
    }
  );

}