JavaScript Internal Architecture

Non-blocking I/O

In js, aprox all Input and output operation is non-blocking including like HTTP requests, database operations and file system reads and writes etc in js single thread of execution asked at runtime to perform an operation, providing a callback function and then goes out for performing other operations . once the operation has been callback called after few mili sec. function changeHeaderDeferred() { var header = document.getElementById("header"); setTimeout(function changeHeader() { header.style.color = "red"; return false; }, 100); return false; } changeHeaderDeferred(); js is eventDriven interaction model differs from the request and response model However, this is but one approach

Event loop

The event loop is basically a loop which checks that "is rendered HTTP request is blocking or non-blocking and second work is check the call stack list .."is call stack empty or not " if event loop found any blocking HTTP request then it will transfer that Http request to thread pool with a callback and message and if event loop not found any Http request then it will compute and then response back to the Request Caller and whing checking
Lets look out the event loop code :
(queue.waitForMessage())
{
queue.processNextMessage();
}
queue.waitForMessage()

waits synchronously for a message to arrive if there is none currently.

Call Stack

Call stack is basically like a stack which contains all request from Request caller . if any Request caller hit the http request which contain any http verb like post ,get ,put or delete ..firstlt comes into the call stack

Task Queue

every callback which return from thread pool goes to firstly on task queue then every loop check that queue..if any callback will be available there then event loop task the callback ..execute code correspondingly

Web Apis

Every blocking request handled by web apis it returns callback after completion of blocking request with an callback and message