Node.js Lifecycle

 

1. Initialization

   • Node.js starts the event loop and initializes the environment. This includes loading the required modules, setting up the global objects, and preparing the execution context.

2. Execution of Scripts

   • Node.js executes the provided script or file. It parses the code, executes the top-level code, and sets up event listeners for asynchronous operations.

3. Event Loop

   • The heart of Node.js's asynchronous nature. It processes the events and callbacks generated by asynchronous operations. The event loop runs in cycles, called ticks, and it has several phases:

Event Loop Phases

1. Timers Phase

   • Executes callbacks scheduled by setTimeout() and setInterval().
   • Node.js checks the timer queue and executes any timer callbacks that are due.

2. Pending Callbacks Phase

   • Executes I/O callbacks deferred to the next loop iteration.
   • This includes some system operations like TCP errors, which require more time than the timers phase allows.

3. Idle, Prepare Phase

   • Only used internally by Node.js for performing tasks that need to be executed before the poll phase.
   • This phase is not directly accessible by developers.

4. Poll Phase

   • The most crucial phase where the event loop retrieves new I/O events.
   • Node.js blocks here when waiting for new events, processing I/O callbacks as they arrive.
   • When the poll queue is empty, the event loop will either end the poll phase or continue to wait for callbacks.

5. Check Phase

   • Executes callbacks scheduled by setImmediate().
   • Node.js processes all setImmediate() callbacks, which are often used for breaking up long operations.

6. Close Callbacks Phase

   • Executes close event callbacks, such as when a socket or handle is closed.
   • This includes cleanup or shutdown functions for resources.

Interaction between Call Stack, Callback Queue, and Event Loop

1. Call Stack

   • Synchronous code execution context.
   • Functions are called and executed here in a LIFO (Last In, First Out) order.
   • When an asynchronous function is called, it sends the callback to the appropriate queue.

2. Callback Queue

   • Stores callbacks of asynchronous operations.
   • Includes queues for different phases (e.g., timer queue, I/O queue, check queue).

3. Event Loop

   • Continuously checks the call stack and the callback queue.
   • If the call stack is empty, it picks the next callback from the callback queue and pushes it onto the call stack for execution.