The node:fs module is Node.js’s interface to the file system. It provides methods for reading, writing, renaming, deleting, and inspecting files and directories. Every method exists in three forms: synchronous, callback-based, and promise-based. The synchronous forms block the event loop and are intended for startup and scripts. The callback forms use the error-first convention and…
The EventEmitter class is the backbone of Node.js’s event-driven architecture. It provides a simple mechanism: objects emit named events, and listeners subscribe to those events and run when they are emitted. The http.Server, net.Socket, fs.ReadStream, and process objects are all EventEmitter instances, and the pattern is available to any code that extends the class. Understanding…
Callbacks are the original asynchronous pattern in Node.js. Before promises and async/await, every asynchronous operation was expressed by passing a function that would be invoked when the operation completed. The convention that emerged is error-first: the callback receives an error as its first argument and the result as its second. If the error is null…
Promises and async/await are the modern way to write asynchronous code in Node.js. A promise represents the eventual result of an asynchronous operation: it is either pending, fulfilled with a value, or rejected with an error. The await keyword pauses an async function until the promise settles, and the value is returned or the error…
Non-blocking I/O is the principle that lets Node.js serve thousands of concurrent connections with a single JavaScript thread. A blocking operation stops the thread until it completes; a non-blocking operation starts the work, returns immediately, and notifies when the result is ready. Node.js is built around the second approach: the JavaScript thread never waits for…
COPY and ADD are the two Dockerfile instructions that bring files from the build context into the image. They look interchangeable, and for the simple case of copying a local file they behave identically. The difference is that ADD does two extra things: it auto-extracts local tar archives and it can fetch remote URLs. Those…
CMD and ENTRYPOINT are the two Dockerfile instructions that determine what a container runs. They are similar enough to be confused and different enough that the confusion causes real problems: containers that ignore their configuration, applications that never receive SIGTERM, and images that cannot be used as a base for anything else. The confusion is…
A Dockerfile is a text file with instructions that the image builder reads in order. Each instruction creates a layer, and the final layer stack becomes the image. The three instructions that most define an image’s behavior are FROM, which sets the base, RUN, which executes commands during the build, and CMD and ENTRYPOINT, which…
Docker images are portable only when they are stored in a registry. The local daemon holds images built or pulled on a machine, but those images are not available to other machines, to CI runners, or to a Kubernetes cluster until they are pushed to a registry. Docker Hub is the default public registry, and…
A Docker image is not a single file. It is a manifest that references a configuration and an ordered list of layers, and each layer is stored as a content-addressed blob. This architecture is what makes images distributable, shareable, and verifiable. When you pull an image, Docker does not download a monolithic tarball; it fetches…