Node.js – Wikipedia
Node.js has an event-driven architecture capable of asynchronous I/O. These design choices aim to optimize throughput and scalability in Web applications with many input/output operations, as well as for real-time Web applications (e.g., real-time communication programs and browser games).
Corporate users of Node.js software include GoDaddy,Groupon,IBM,LinkedIn,Microsoft,Netflix,PayPal,Rakuten, SAP,Voxer,Walmart,Yahoo!, and Cisco Systems.
Dahl was inspired to create Node.js after seeing a file upload progress bar on Flickr. The browser did not know how much of the file had been uploaded and had to query the Web server. Dahl desired an easier way.
Dahl criticized the limited possibilities of the most popular web server in 2009, Apache HTTP Server, to handle a lot of concurrent connections (up to 10,000 and more) and the most common way of creating code (sequential programming), when code either blocked the entire process or implied multiple execution stacks in the case of simultaneous connections. 
In January 2010, a package manager was introduced for the Node.js environment called npm. The package manager makes it easier for programmers to publish and share source code of Node.js libraries and is designed to simplify installation, updating and uninstallation of libraries.
In January 2012, Dahl stepped aside, promoting coworker and npm creator Isaac Schlueter to manage the project. In January 2014, Schlueter announced that Timothy J. Fontaine would lead the project.
In February 2015, the intent to form a neutral Node.js Foundation was announced. By June 2015, the Node.js and io.js communities voted to work together under the Node.js Foundation.
In September 2015, Node.js v0.12 and io.js v3.3 were merged back together into Node v4.0. This brought V8 ES6 features into Node.js, and a long-term support release cycle. As of 2016, the io.js website recommends that developers switch back to Node.js and that no further releases of io.js are planned due to the merger.
Node.js is primarily used to build network programs such as Web servers. The biggest difference between Node.js and PHP is that most functions in PHP block until completion (commands execute only after previous commands have completed), while functions in Node.js are designed to be non-blocking (commands execute in parallel, and use callbacks to signal completion or failure).
People have built thousands of open-source libraries for Node.js – most of them hosted on the npm website. The Node.js developer community has two main mailing lists and the IRC channel #node.js on freenode. There is an annual Node.js developer conference, called NodeConf.
The open-source community has developed server frameworks to accelerate the development of applications. Such frameworks include Connect, Express.js, Socket.IO, Koa.js, Hapi.js, Sails.js, Meteor, Derby, and many others.
Modern desktop IDEs provide editing and debugging features specifically for Node.js applications. Such IDEs include Atom, Brackets, JetBrains WebStorm,Microsoft Visual Studio (with Node.js Tools for Visual Studio, or TypeScript with Node definitions), NetBeans,Nodeclipse Enide Studio (Eclipse-based) and Visual Studio Code. Certain online web-based IDEs also support Node.js, such as Codeanywhere, Codenvy, Cloud9 IDE and Koding.
Node.js operates on a single thread, using non-blocking I/O calls, allowing it to support tens of thousands of concurrent connections without incurring the cost of thread context switching. The design of sharing a single thread between all the requests that uses the observer pattern is intended for building highly concurrent applications, where any function performing I/O must use a callback. In order to accommodate the single-threaded event loop, Node.js utilizes the libuv library that in turn uses a fixed-sized threadpool that is responsible for some of the non-blocking asynchronous I/O operations.
A downside of this single-threaded approach is that Node.js doesn’t allow vertical scaling by increasing the number of CPU cores of the machine it is running on without using an additional module, such as cluster, StrongLoop Process Manager or pm2. However, developers can increase the default number of threads in the libuv threadpool; these threads are likely to be distributed across multiple cores by the server operating system.
Execution of parallel tasks in Node.js is handled by a thread pool. The main thread call functions post tasks to the shared task queue that threads in the thread pool pull and execute. Inherently non-blocking system functions like networking translates to kernel-side non-blocking sockets, while inherently blocking system functions like file I/O run in a blocking way on its own thread. When a thread in the thread pool completes a task, it informs the main thread of this that in turn wakes up and execute the registered callback. Since callbacks are handled in serial on the main thread, long lasting computations and other CPU-bound tasks will freeze the entire event-loop until completion.
Node.js uses libuv to handle asynchronous events. Libuv is an abstraction layer for network and file system functionality on both Windows and POSIX-based systems like Linux, macOS, OSS on NonStop and Unix.
npm is the pre-installed package manager for the Node.js server platform. It is used to install Node.js programs from the npm registry, organizing the installation and management of third-party Node.js programs. npm is not to be confused with the CommonJS require() statement. It is not used to load code; instead, it is used to install code and manage code dependencies from the command line. The packages found in the npm registry can range from simple helper libraries like Underscore.js to task runners like Grunt.
Node.js registers itself with the operating system so that it is notified when a connection is made, and the operating system will issue a callback. Within the Node.js runtime, each connection is a small heap allocation. Traditionally, relatively heavyweight OS processes or threads handled each connection. Node.js uses an event loop for scalability, instead of processes or threads. In contrast to other event-driven servers, Node.js’s event loop does not need to be called explicitly. Instead callbacks are defined, and the server automatically enters the event loop at the end of the callback definition. Node.js exits the event loop when there are no further callbacks to be performed.
Similar open source event-driven server frameworks for other platforms include:
Node.js may utilize code written in other programming languages using:
- Edge.js allows Microsoft .NET applications to run Node.js scripts in-process, and allows Node.js servers to utilize .NET compiled code via async callbacks.
- Luvit implements the Node.js APIs for the language Lua
- Node-julia allows using Julia with Node.js/io.js
- The COBOL bridge for Node.js allows using COBOL with Node.js
Source: Node.js – Wikipedia