This is an example showing how Deno's Oak can be tested without network connections

mock-oak-test.ts

import type {
  Server,
  ServerRequest,
  ServerResponse,
} from "https://deno.land/x/[email protected]/types.d.ts";

import {
  Application,
  ListenOptions,
} from "https://deno.land/x/[email protected]/mod.ts";

class MockServer {
  serverClosed = false;
  requestStack: ServerRequest[];

  constructor(requestStack: ServerRequest[]) {
    this.requestStack = requestStack;
  }

  close(): void {
    this.serverClosed = true;
  }

  async *[Symbol.asyncIterator]() {
    for await (const request of this.requestStack) {
      yield request;
    }
  }
}

/**
 * Make a mock Oak application that can process requests
 * and middleware for testing purposes.
 *
 * ```js
 * const app = new MockApplication();
 * app.use(middleware);
 * app.makeRequest("/"
 * ```
 */
export class MockApplication extends Application {
  requestStack: ServerRequest[] = [];
  responseStack: ServerResponse[] = [];

  constructor() {
    super({
      serve: (addr: string | ListenOptions): Server => {
        return new MockServer(this.requestStack) as Server;
      }
    });
  }

  makeRequest(
    url = "/",
    headersInit: string[][] = [["host", "example.com"]]
  ) {
    this.requestStack.push({
      url,
      headers: new Headers(headersInit),
      respond: (response: ServerResponse) => {
        this.responseStack.push(response);
        return Promise.resolve();
      },
      proto: "HTTP/1.1"
    } as any);
  }

  async listen() {
    // this port doesn't actually get used
    await super.listen(":1337")
    while (this.requestStack.pop()) {};
  }
}


Deno.test("MockApplication can be used to test middleware.", async function() {
  const app = new MockApplication();
  let middlewareCalled = 0;
  app.use((ctx: Context, next: () => Promise<void>) => {
    middlewareCalled += 1;
  })

  app.makeRequest();
  await app.listen();
  assertEquals(middlewareCalled, 1);
  app.makeRequest();
  await app.listen();
  assertEquals(middlewareCalled, 2);
})