Challenge 3: Add Request Correlation — Possible Solution
====================================================================

interface LogFields {
  [key: string]: string | number | boolean | undefined;
}

interface Logger {
  debug(event: string, fields?: LogFields): void;
  info(event: string, fields?: LogFields): void;
  warn(event: string, fields?: LogFields): void;
  error(event: string, fields?: LogFields): void;
}

interface RequestContext {
  requestId: string;
  userId?: string;
}

function logWithContext(
  logger: Logger,
  ctx: RequestContext,
  level: "debug" | "info" | "warn" | "error",
  event: string,
  fields: LogFields = {}
): void {
  const enriched: LogFields = {
    requestId: ctx.requestId,
    userId: ctx.userId,
    ...fields,
  };
  logger[level](event, enriched);
}

// --- Usage across a simulated request lifecycle ---

class ConsoleLogger implements Logger {
  debug(event: string, fields?: LogFields) { console.log(JSON.stringify({ level: "debug", event, ...fields })); }
  info(event: string, fields?: LogFields) { console.log(JSON.stringify({ level: "info", event, ...fields })); }
  warn(event: string, fields?: LogFields) { console.log(JSON.stringify({ level: "warn", event, ...fields })); }
  error(event: string, fields?: LogFields) { console.log(JSON.stringify({ level: "error", event, ...fields })); }
}

const logger = new ConsoleLogger();
const ctx: RequestContext = { requestId: "req-8f3a", userId: "u42" };

logWithContext(logger, ctx, "info", "request.received", { path: "/orders" });
// {"level":"info","event":"request.received","requestId":"req-8f3a","userId":"u42","path":"/orders"}

logWithContext(logger, ctx, "info", "order.placed", { orderId: "o1", total: 49.99 });
// {"level":"info","event":"order.placed","requestId":"req-8f3a","userId":"u42","orderId":"o1","total":49.99}

logWithContext(logger, ctx, "info", "request.completed", { statusCode: 200, durationMs: 42 });
// {"level":"info","event":"request.completed","requestId":"req-8f3a","userId":"u42","statusCode":200,"durationMs":42}

// A log aggregator query for requestId:"req-8f3a" now returns all three
// lines above, in order, even if they were logged from different modules
// or (in a real distributed system) different services.

WHY THIS WORKS
--------------
- ctx fields are spread in BEFORE the caller's own fields, so a caller can
  still override requestId/userId in an edge case, but in normal use every
  event automatically carries the same requestId without each call site
  having to remember to add it.
- Accepting `level` as a parameter and indexing into `logger[level](...)`
  avoids writing four nearly-identical wrapper functions
  (logInfoWithContext, logErrorWithContext, ...) for each log level.
- Because RequestContext is created once per incoming request (typically in
  middleware) and threaded through every downstream call, every log line
  produced while handling that request shares the same requestId — the
  simplest form of tracing, with no external library required.