Mock Server Implementation

Adds a service worker that captures all outgoing `/api/*` requests, and connects back to the main thread via a MessageChannel. That way, code running in the browser can implement the APIs. All requests can be inspected in the network tab.

Author: dagimene

public/sw.js

@@ -0,0 +1,72 @@
+/**
+ * Generic mock API Service Worker.
+ *
+ * Intercepts fetch requests to /api/* and forwards them to the main thread
+ * via MessageChannel. The main thread runs registered route handlers and
+ * sends the response back through the channel port.
+ *
+ * This SW is challenge-agnostic — each challenge registers its own handlers.
+ */
+
+self.addEventListener('fetch', (event) => {
+  const url = new URL(event.request.url);
+  if (url.pathname.startsWith('/api/')) {
+    event.respondWith(handleMockRequest(event));
+  }
+});
+
+async function handleMockRequest(event) {
+  const client = await self.clients.get(event.clientId);
+  if (!client) {
+    return new Response(JSON.stringify({ error: 'No client found' }), {
+      status: 500,
+      headers: { 'Content-Type': 'application/json' },
+    });
+  }
+
+  let body = null;
+  try {
+    const text = await event.request.text();
+    if (text) body = JSON.parse(text);
+  } catch (_) {
+    // No body or non-JSON body
+  }
+
+  const { port1, port2 } = new MessageChannel();
+
+  return new Promise((resolve) => {
+    port1.onmessage = (e) => {
+      resolve(
+        new Response(JSON.stringify(e.data.body), {
+          status: e.data.status || 200,
+          headers: { 'Content-Type': 'application/json' },
+        })
+      );
+    };
+
+    const reqUrl = new URL(event.request.url);
+    client.postMessage(
+      {
+        type: 'mock-api-request',
+        url: reqUrl.pathname + reqUrl.search,
+        method: event.request.method,
+        body: body,
+      },
+      [port2]
+    );
+  });
+}
+
+// Activate immediately and claim all clients so the SW is ready without a reload
+self.addEventListener('install', () => self.skipWaiting());
+self.addEventListener('activate', (event) => {
+  event.waitUntil(self.clients.claim());
+});
+
+// Allow the main thread to re-trigger claim (e.g. after unregister + re-register
+// where activate doesn't fire again because the SW script hasn't changed).
+self.addEventListener('message', (event) => {
+  if (event.data?.type === 'claim') {
+    self.clients.claim();
+  }
+});

src/components/InterviewShell.tsx

@@ -1,36 +1,30 @@
-import React, { useState, useEffect, useRef } from "react";
-import { InterviewPattern } from "../interviews/types";
+import React, {useEffect, useRef, useState} from "react";
+import {InterviewPattern} from "@/interviews/types";
+import {setupRoutes} from "@/server";
 import Instructions from "./Instructions";
 import CodingChallengeWrapper from "./CodingChallengeWrapper";
 import CodeReviewInterface from "./CodeReviewInterface";
 import "./InterviewShell.css";
-import { Button } from "./ui/button";
-import { ArrowLeftIcon } from "lucide-react";
-import { ThemeSwitcher } from "./theme-switcher";
-import { Badge } from "./ui/badge";
+import {Button} from "./ui/button";
+import {ThemeSwitcher} from "./theme-switcher";
+import {Badge} from "./ui/badge";
 import {
   Breadcrumb,
-  BreadcrumbList,
   BreadcrumbItem,
   BreadcrumbLink,
-  BreadcrumbSeparator,
+  BreadcrumbList,
   BreadcrumbPage,
+  BreadcrumbSeparator,
 } from "@/components/ui/breadcrumb";
 import {
   Sidebar,
   SidebarContent,
-  SidebarGroup,
-  SidebarGroupLabel,
-  SidebarGroupContent,
-  SidebarMenu,
-  SidebarMenuItem,
-  SidebarMenuButton,
-  SidebarProvider,
   SidebarInset,
+  SidebarProvider,
   SidebarTrigger,
   useSidebar,
 } from "@/components/ui/sidebar";
-import { Separator } from "./ui/separator";
+import {Separator} from "./ui/separator";
 
 interface InterviewShellProps {
   pattern: InterviewPattern;
@@ -108,6 +102,15 @@ const SIDEBAR_OPEN_KEY_PREFIX = "sidebar-open-";
 const InterviewShell: React.FC<InterviewShellProps> = ({ pattern, onBack }) => {
   const [showInstructions, setShowInstructions] = useState(false);
   const [hasViewedInstructions, setHasViewedInstructions] = useState(false);
+  const [serverReady, setServerReady] = useState(!pattern.routes?.length);
+
+  // Initialize mock API server if the pattern declares routes
+  useEffect(() => {
+    if (pattern.routes?.length) {
+      setServerReady(false);
+      setupRoutes(pattern.routes).then(() => setServerReady(true));
+    }
+  }, [pattern]);
   const [sidebarWidth, setSidebarWidth] = useState(640); // 40rem in pixels
   const [isDragging, setIsDragging] = useState(false);
   const sidebarRef = useRef<HTMLDivElement>(null);
@@ -252,7 +255,11 @@ const InterviewShell: React.FC<InterviewShellProps> = ({ pattern, onBack }) => {
               </div>
             </header>
             <main className="bg-white flex-1 overflow-y-auto">
-              {showInstructions && pattern.readmes ? (
+              {!serverReady ? (
+                <div className="flex items-center justify-center h-full text-muted-foreground">
+                  Starting server...
+                </div>
+              ) : showInstructions && pattern.readmes ? (
                 <Instructions
                   readmes={pattern.readmes}
                   onClose={() => setShowInstructions(false)}

src/index.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import ReactDOM from 'react-dom/client';
 import App from './App';
 import './index.css';
+import './server'; // Register SW eagerly at startup
 
 const root = ReactDOM.createRoot(
   document.getElementById('root') as Element

src/interviews/types.ts

@@ -9,6 +9,18 @@ export interface ImplementationDetails {
   testCases?: string;
 }
 
+export interface ApiRequest {
+  params: Record<string, string>;
+  query: Record<string, string>;
+  body: unknown;
+}
+
+export interface ApiRoute {
+  method: 'GET' | 'POST' | 'PUT' | 'DELETE';
+  path: string;
+  handler: (req: ApiRequest) => Promise<unknown>;
+}
+
 export interface InterviewPattern {
   id: string;
   name: string;
@@ -21,6 +33,7 @@ export interface InterviewPattern {
   component?: React.ComponentType;  // Optional for code-review type
   type?: 'react' | 'coding-challenge' | 'code-review';  // Added code-review type
   implementationDetails?: ImplementationDetails;
+  routes?: ApiRoute[];
 }
 
 export interface InterviewConfig {

src/server/concurrency.ts

@@ -0,0 +1,25 @@
+export function createPool(maxConcurrency: number) {
+  let active = 0;
+  const queue: (() => void)[] = [];
+
+  function acquire(): Promise<void> {
+    if (active < maxConcurrency) {
+      active++;
+      return Promise.resolve();
+    }
+    return new Promise((resolve) =>
+      queue.push(() => {
+        active++;
+        resolve();
+      })
+    );
+  }
+
+  function release(): void {
+    active--;
+    const next = queue.shift();
+    if (next) next();
+  }
+
+  return { acquire, release };
+}

src/server/index.ts

@@ -0,0 +1,73 @@
+import type { ApiRoute } from "@/interviews/types";
+import { createPool } from "@/server/concurrency";
+import { compilePath, matchRoute, type CompiledRoute } from "@/server/router";
+import { ensureController } from "@/server/serviceWorkerController";
+
+let compiledRoutes: CompiledRoute[] = [];
+
+const pool = createPool(5);
+
+function mockDelay(): Promise<void> {
+  const ms = 300 + (Math.random() * 400 - 200); // 100–500ms
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}
+
+/**
+ * Registers the Service Worker and message listener.
+ * Call once at app startup — resolves when the SW is controlling the page.
+ */
+export const initServer: Promise<void> = (async () => {
+  if (!("serviceWorker" in navigator)) {
+    console.warn("[codeflow] Service Workers not supported. Server will not work");
+    return;
+  }
+
+  await ensureController();
+  console.log("[codeflow] Server started");
+
+  navigator.serviceWorker.addEventListener("message", async (event) => {
+    if (event.data?.type !== "mock-api-request") return;
+
+    const { method, url, body } = event.data;
+    const parsed = new URL(url, location.origin);
+    const matched = matchRoute(compiledRoutes, method, parsed.pathname);
+
+    if (matched) {
+      await pool.acquire();
+      try {
+        await mockDelay();
+        const result = await matched.handler({
+          params: matched.params,
+          query: Object.fromEntries(parsed.searchParams),
+          body,
+        });
+        event.ports[0].postMessage({ status: 200, body: result });
+      } catch (err) {
+        event.ports[0].postMessage({
+          status: 500,
+          body: { error: String(err) },
+        });
+      } finally {
+        pool.release();
+      }
+    } else {
+      event.ports[0].postMessage({
+        status: 404,
+        body: { error: `No handler for ${method} ${url}` },
+      });
+    }
+  });
+})();
+
+/**
+ * Replaces the active set of mock API routes.
+ * Waits for the SW to be ready before resolving.
+ */
+export async function setupRoutes(apiRoutes: ApiRoute[]): Promise<void> {
+  await initServer;
+  compiledRoutes = apiRoutes.map((route) => ({
+    method: route.method,
+    regex: compilePath(route.path),
+    handler: route.handler,
+  }));
+}

src/server/router.ts

@@ -0,0 +1,36 @@
+import type { ApiRequest } from "@/interviews/types";
+
+export interface CompiledRoute {
+  method: string;
+  regex: RegExp;
+  handler: (req: ApiRequest) => Promise<unknown>;
+}
+
+function escapeRegex(str: string): string {
+  return str.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
+export function compilePath(path: string): RegExp {
+  const pattern = path
+    .split("/")
+    .map((seg) =>
+      seg.startsWith(":") ? `(?<${seg.slice(1)}>[^/]+)` : escapeRegex(seg)
+    )
+    .join("/");
+  return new RegExp(`^${pattern}$`);
+}
+
+export function matchRoute(
+  routes: CompiledRoute[],
+  method: string,
+  pathname: string
+): { handler: CompiledRoute["handler"]; params: Record<string, string> } | null {
+  for (const route of routes) {
+    if (route.method !== method) continue;
+    const match = route.regex.exec(pathname);
+    if (match) {
+      return { handler: route.handler, params: match.groups ?? {} };
+    }
+  }
+  return null;
+}

src/server/serviceWorkerController.ts

@@ -0,0 +1,34 @@
+/**
+ * Registers the SW and ensures it controls this page.
+ * If an existing-but-unregistered SW fails to claim within a short window,
+ * it unregisters the stale registration and retries from scratch.
+ */
+export async function ensureController(): Promise<void> {
+  if (navigator.serviceWorker.controller) return;
+
+  const controllerChanged = new Promise<void>((resolve) => {
+    navigator.serviceWorker.addEventListener("controllerchange", () => resolve(), {
+      once: true,
+    });
+  });
+
+  const registration = await navigator.serviceWorker.register("/sw.js");
+
+  // If the SW is already active but not controlling (e.g. after unregister +
+  // re-register where activate doesn't re-fire), ask it to claim explicitly.
+  if (registration.active && !navigator.serviceWorker.controller) {
+    registration.active.postMessage({ type: "claim" });
+  }
+
+  // If the controller does not connect in 500ms, SW is likely stale. Unregister and retry.
+  const result = await Promise.race([
+    controllerChanged.then(() => "ok" as const),
+    new Promise<"timeout">((r) => setTimeout(() => r("timeout"), 500))
+  ]);
+
+  if (result === "timeout") {
+    console.warn("[codeflow] Server initialization failed. Retrying...");
+    await registration.unregister();
+    return ensureController();
+  }
+}