Classification of modules (core modules, file modules, third-party modules)

Node.js modules are divided into core modules, file modules, and third-party modules. Core modules are built-in foundational modules in Node.js, such as `fs` and `http`, which can be loaded directly without installation and offer optimal performance. File modules are custom modules created by developers, loaded via relative or absolute paths, and can be either a single file or a directory containing `index.js`. Third-party modules are installed via npm and stored in `node_modules`, providing specific functionalities to address domain-specific problems. Node.js follows the CommonJS specification for module loading, with a specific search order: core modules are prioritized first, followed by modules in `node_modules`. Each module has an independent scope and exposes interfaces via `module.exports`. Node.js supports the coexistence of ES modules and CommonJS and can handle circular dependencies. It also offers various debugging techniques, such as inspecting module paths and clearing the cache.

The Node.js REPL environment translates this sentence into English.

The Node.js REPL environment is an interactive interpreter that allows developers to quickly test code snippets, debug, and explore APIs. It provides instant feedback, making it ideal for learning and experimentation. REPL stands for Read-Eval-Print Loop and is Node.js's built-in interactive programming environment. When you enter `node` without any arguments in the command line, you enter REPL mode. This environment reads input, executes it, prints the result, and loops to wait for the next input. The REPL environment includes several special variables, such as `_` (which holds the result of the last expression), `.exit` (to quit), `.break` (to exit multi-line input), `.clear` (to reset the context), and `.editor` (to enter editor mode). Node.js allows the creation of custom REPL environments via the `repl` module, supporting the injection of custom variables and functions. The REPL environment supports modern JavaScript features, including `async/await`, and provides detailed error stack traces. In the REPL, you can directly `require` installed npm modules. While REPL is great for quick testing, performance considerations should be noted. It is well-suited for practical scenarios like API exploration, algorithm validation, and data transformation. Compared to browser consoles and Python REPL, the Node.js REPL has unique advantages. When launching the REPL, various options can be passed or configured in code. The REPL automatically detects multi-line input or allows entering a dedicated editor mode via the `.editor` command. When using the REPL, security should be a priority—avoid exposing it on production servers. The REPL can be used alongside debuggers, profilers, and other tools. Behavior may vary across Node.js versions. Common issues like input freezing or undefined variables have corresponding solutions. Performance can be optimized by preloading frequently used modules, disabling colors, or limiting history size.

The global object in Node.js

In the Node.js runtime environment, there are global objects that can be used directly without requiring them. These objects are shared across different modules and provide basic API functionality. `global` is the top-level global object, and all global variables (except `module`) are its properties. The `process` object provides an interface to interact with the current process, including environment variables, process control, standard I/O, and performance analysis. The `console` object offers formatted output, timing functions, and stack tracing. Timer functions include one-time, periodic timers, and immediate execution. The `Buffer` class handles binary data. `__filename` and `__dirname` provide the current module's path information. `module`, `exports`, and `require` are core components of the module system. The `URL` class, `TextEncoder`, `TextDecoder`, and `performance` are also important global objects. The module system affects the visibility of global variables, with each module having its own scope. The true global object can be accessed via `globalThis`.

Node.js version management tools (nvm/n)

Node.js version management tools like nvm and n help developers efficiently switch between different Node versions to resolve compatibility issues across multiple projects. nvm supports macOS, Linux, and Windows, and can be installed via scripts, allowing parallel management of multiple versions with quick switching and isolated environments. n, on the other hand, only supports macOS and Linux, is installed via npm, and has a more lightweight design. The two tools differ significantly in cross-platform support, installation methods, and version isolation. The article provides a detailed explanation of their installation processes, common commands, and practical use cases, such as project version synchronization and CI configuration. It also offers practical tips like mirror acceleration, global module management, and version aliasing. Finally, it covers advanced usage such as custom installation paths and auto-loading.

The difference between Node.js and browser JavaScript

Node.js and browser JavaScript, although based on the same ECMAScript standard, exhibit significant differences in runtime environment APIs, module systems, and other aspects. Node.js runs on the server side, providing OS-level APIs such as file system access, while browser JavaScript operates in a client-side sandboxed environment constrained by security policies. Their global objects differ: Node.js uses `global` and `process`, whereas browsers use `window` and `document`. In terms of module systems, Node.js adopts CommonJS, while browsers support ES modules. Their networking APIs also vary: Node.js provides the `http` module, whereas browsers offer `fetch` and `XMLHttpRequest`. The event loop mechanisms differ as well—Node.js has six distinct phases, while browsers handle it more simply, primarily managing macro-tasks and micro-tasks. Debugging tools are distinct too: Node.js uses the `inspect` parameter, while browsers rely on developer tools. Performance-wise, Node.js excels in high-concurrency I/O, whereas browsers are limited by user device capabilities. Security restrictions differ: Node.js can access local resources, while browsers adhere to the same-origin policy. Their package management ecosystems also diverge—Node.js uses npm, while browsers rely on `<script>` tags or CDNs. Both support async/await for asynchronous programming, but their traditional styles differ: Node.js leans toward callbacks, while browsers favor Promises. Memory management also varies: Node.js allows manual garbage collection triggering, whereas browsers handle it entirely automatically.

Application scenarios of Node.js

Node.js, a JavaScript runtime environment based on the Chrome V8 engine, is renowned for its non-blocking I/O and event-driven characteristics, making it suitable for high-concurrency, low-latency scenarios. Typical applications include real-time applications like chat rooms (implementing bidirectional communication via WebSocket), API services and microservice architectures (quickly building backend services with Express), server-side rendering (improving first-screen loading speed), and command-line tool development (processing files or automation scripts). It is also used in IoT applications (connecting sensors and handling data streams), efficiently processing large files, middleware services (aggregating interfaces and transforming data formats), cloud functions (supporting event-driven serverless functions), proxy servers (implementing request forwarding), and database operations (supporting mainstream databases like MongoDB and MySQL). It is ideal for rapid development of data-intensive applications.

The CommonJS module system translates this sentence into English.

The CommonJS module system is the core mechanism for organizing and managing code in Node.js, enabling module import and export through `require` and `module.exports`. It addresses the lack of native modular support in early JavaScript. Each file is treated as an independent module, with variables, functions, and classes inside the module being private by default and requiring explicit export for external access. Key features of the module system include synchronous loading, a caching mechanism, clear dependencies, and simple syntax. Exports can be done via `module.exports` or the `exports` shorthand, while imports use the `require` function, which follows specific lookup rules covering core modules, file modules, directory modules, and `node_modules` modules. The caching mechanism handles circular dependencies, and each module has an isolated scope to avoid polluting the global environment. Although primarily synchronous, dynamic loading is also possible. Differences exist between CommonJS and ES modules, such as loading methods and syntax. Practical applications include configuration file management, middleware patterns, and support for various module organization modes like singletons and factories. It also involves module version management and testing techniques.

Single-threaded and event loop

Node.js adopts a single-threaded model for executing JavaScript code, simplifying the concurrency model but introducing performance bottlenecks. The event loop is key to achieving non-blocking I/O by delegating I/O operations to the system kernel via the libuv library. Tasks are categorized into microtasks and macrotasks, with microtasks having higher priority. CPU-intensive tasks can block the event loop and should be handled using worker threads. Performance monitoring interfaces are provided to detect event loop latency. In web servers, CPU-intensive routes should use worker threads, while I/O-intensive ones can be processed directly. Advanced patterns leverage event loop features for traffic control, such as rate limiter classes.

The non-blocking I/O model

Node.js's non-blocking I/O model is its core feature, enabling efficient handling of a large number of concurrent requests. Unlike traditional blocking I/O, it does not wait for I/O operations to complete but continues executing subsequent code, processing results through callback functions. The event loop mechanism is key to implementing this feature, consisting of multiple phases such as timers, I/O callbacks, and polling. While it improves performance, it may lead to callback hell, which can be resolved using Promises and async/await. Non-blocking I/O is suitable for scenarios like HTTP servers and database operations but not ideal for CPU-intensive tasks. Attention must be paid to error handling and memory usage. Stream processing is an advanced application suitable for large files, while worker threads can handle CPU tasks. Debugging asynchronous code can be done using the async_hooks module. Best practices include avoiding blocking the event loop, using Promises wisely, error handling, stream processing, and concurrency control.

Event-driven architecture

Event-Driven Architecture (EDA) is a software design pattern centered around events, where system components communicate by producing and consuming events. Node.js is inherently well-suited for implementing EDA, with its core `events` module providing the `EventEmitter` class as a foundation. The event loop mechanism includes phases such as timers, pending callbacks, polling, and checks. The `EventEmitter` class offers rich methods for managing event listeners, and common design patterns include simple event handling, event aggregators, event buses, and the publish-subscribe pattern. In web applications, it can decouple components, such as in user registration workflows. Special attention should be paid to error handling, avoiding memory leaks, and controlling the number of listeners. In microservices environments, it enables loose coupling and can be combined with Event Sourcing and CQRS patterns. During testing, the asynchronous nature must be considered.