Node.js is a powerful and popular runtime environment that allows developers to run JavaScript on the server side. Understanding its architecture is crucial for leveraging its full potential. In this section, we will explore the core components and design principles that make Node.js unique.

1. Single-Threaded Event Loop:

   • Node.js operates on a single-threaded event loop model, which allows it to handle multiple concurrent operations without creating multiple threads.
   • This model is efficient for I/O-bound tasks but may not be suitable for CPU-intensive tasks.

2. Non-Blocking I/O:

   • Node.js uses non-blocking I/O operations, meaning it can handle other tasks while waiting for I/O operations to complete.
   • This is achieved through asynchronous programming, which is a core concept in Node.js.

3. Event-Driven Architecture:

   • Node.js is event-driven, meaning it uses events to handle asynchronous operations.
   • The EventEmitter class is central to this architecture, allowing objects to emit and listen for events.

4. V8 JavaScript Engine:

   • Node.js is built on the V8 JavaScript engine, developed by Google for the Chrome browser.
   • V8 compiles JavaScript directly to native machine code, providing high performance.

• Role: Executes JavaScript code.
• Features: High performance, Just-In-Time (JIT) compilation.

• Role: Provides the event loop and asynchronous I/O.
• Features: Cross-platform support, handles file system, DNS, network, child processes, and more.

• Role: Manages asynchronous operations.
• Phases:
  1. Timers: Executes callbacks scheduled by setTimeout() and setInterval().
  2. Pending Callbacks: Executes I/O callbacks deferred to the next loop iteration.
  3. Idle, Prepare: Internal use only.
  4. Poll: Retrieves new I/O events; executes I/O-related callbacks.
  5. Check: Executes setImmediate() callbacks.
  6. Close Callbacks: Executes close event callbacks, e.g., socket.on('close', ...).

• Role: Provides bindings to C++ libraries.
• Features: Allows Node.js to interface with low-level system operations.

• Role: Provides built-in modules and APIs.
• Examples: fs for file system operations, http for creating servers, path for handling file paths.

Let's look at a simple example to understand how the event loop works:

1. Synchronous Operations:

   • console.log('Start') is executed first.
   • fs.readFile is called, but it is an asynchronous operation, so it doesn't block the execution.
   • console.log('End') is executed next.

2. Asynchronous Operation:

   • Once the file reading is complete, the callback function provided to fs.readFile is executed.
   • This happens after the synchronous code has finished executing.

Write a Node.js script that demonstrates the order of execution of different asynchronous operations, such as setTimeout, setImmediate, and process.nextTick.

• process.nextTick is executed after the current operation completes, before the event loop continues.
• setImmediate is executed in the check phase of the event loop.
• setTimeout with a delay of 0 is executed in the timers phase, which comes after the check phase.

In this section, we covered the architecture of Node.js, focusing on its single-threaded event loop, non-blocking I/O, and event-driven nature. We also explored the key components like the V8 engine, libuv, and Node.js APIs. Understanding these concepts is fundamental for writing efficient and scalable Node.js applications. In the next module, we will dive deeper into the event loop and asynchronous programming with callbacks.