How would you implement structured logging in a Node.js API?

Direct answer

Use pino (Fastify default) or winston with JSON lines — log level, msg, reqId, userId, duration; redact secrets; stdout for Docker; correlate request/response in hooks; avoid console.log in production.

Scenario: debug slow GET /interviews/topics/:slug in production using searchable JSON logs (Q32, Q76).

Field	Purpose
level	info/warn/error filtering
reqId	Trace one request across services
req.method + url	Identify route
responseTime	Latency SLO monitoring
err.stack	Server-side only on 5xx

scenario-pino.mjs

import pino from "pino";

const logger = pino({
  level: process.env.LOG_LEVEL ?? "info",
  redact: ["req.headers.authorization", "req.headers.cookie"],
});

fastify.addHook("onResponse", async (req, reply) => {
  req.log.info({
    reqId: req.id,
    method: req.method,
    url: req.url,
    statusCode: reply.statusCode,
    responseTime: reply.elapsedTime,
  }, "request completed");
});