A Node.js application containerized naively produces an image over a gigabyte, runs as root, ships every dev dependency and the TypeScript compiler into production, and ignores SIGTERM when Docker tries to stop it gracefully. Each of these is fixable with a few Dockerfile lines, and the fixes are not new—multi-stage builds have existed since 2017—yet…

Node.js performance problems fall into two categories: the event loop is blocked by synchronous work, or the event loop is busy but not blocked—it is processing too many callbacks, or the callbacks themselves are slow. The distinction matters because the tools are different. perf_hooks.monitorEventLoopDelay measures the first: how long a scheduled timer has to wait…

A memory leak in Node.js is not a crash. It is a slow, silent bleed. The process starts at 80 MB, climbs to 200 MB over a few hours, then 500 MB overnight, and eventually the container hits its memory limit and is killed by the OOM killer. The application “works” until it doesn’t, and…

The V8 Inspector is the debugging protocol built into Node.js. It exposes the runtime’s internal state—execution stack, scope variables, breakpoints, CPU profiles, and heap snapshots—over a WebSocket connection. Chrome DevTools speaks this protocol, which means the same debugging interface you use for browser JavaScript can attach to a Node.js process and give you breakpoints, step-through…

Mocking an external API by stubbing fetch or axios is a trap. The stub intercepts the function call, not the network request. The application’s HTTP client—with its interceptors, retry logic, serialization, and status-code handling—never runs. The test passes, but it is testing a code path that does not exist in production. Mock Service Worker (MSW)…

Supertest is a library that lets you make HTTP requests against your Express application and assert on the response, without starting a server on a fixed port and without writing manual socket code. It extends SuperAgent, an HTTP client, and adds a chainable .expect() method that validates status codes, headers, and body content in a…

Jest and Vitest both provide the same core testing vocabulary—describe, it, expect, mocks, snapshots, and coverage—but they are built on fundamentally different foundations. Jest was created in the CommonJS era and has spent years retrofitting support for ES modules and TypeScript through Babel transforms and experimental flags . Vitest was built from the start on…

The node:test module includes a mocking system built into the runtime. It provides spies, stubs, method replacement, and fake timers without requiring Sinon, Jest, or any other third-party library. The mocking API is exposed through the mock object and through the TestContext object that every test function receives. The distinction between these two entry points…

The Node.js test runner is not a third-party package. It ships with the runtime, stabilized in Node.js 20, and requires no npm install for the core functionality . You write a test file, run node –test, and the runner discovers the file, executes it in a separate child process, and reports the result. The assertion…

Prometheus is a pull-based monitoring system. Unlike agents that push metrics to a central server, Prometheus periodically requests a /metrics endpoint on your application, parses the response, and stores the values as time-series data. This means your Node.js application’s job is not to send metrics anywhere—it is to expose them in a format Prometheus understands,…