Express and Node JS Interview Questions

Node Js is one of the most popular and powerful server technologies today.
It allows you built the entire website only in one programming Language i.e Javascript. Node js is free and open source server technology that uses Javascript to create complete web software.It runs on various platforms like Windows, Linux, Unix, Mac OS X, etc.

Modules are reusable block of code whose existence does not impact other code in any way. It is not supported by Javascript. Modules are introduced in ES6. Modules are important for Maintainability, Reusability, and Namespacing of Code.

Yes, Node Js is single threaded to perform asynchronous processing. Doing async processing on a single thread could provide more performance and scalability under typical web loads than the typical thread-based implementation.

require() is used to include modules from external files in Node Js. It is the easiest way to include a module in Node. Basically require is a function that takes a string parameter which contains the location of the file that you want to include. It reads the entire javascript file, executes the file, and then proceeds to return the exports object.
Syntax:

require('path');

Node js is written in C, C++,JavaScript.It uses Google’s open source V8 Javascript Engine to convert Javascript code to C++.

Using Node Js you can build applications like:

• Internet of Things
• Real-Time Chats Applications
• Complex Single-Page Applications
• Real-Time Collaboration Tools
• Streaming apps
• Microservices / API’s

libuv is Cross-platform I/O abstraction library that supports asynchronous I/O based on event loops.It is written in C and released under MIT Licence.

libuv support Windows IOCP, epoll(4), kqueue(2), and Solaris event ports. Initially, it was designed for Node.js but later it is also used by other software projects.

Reference : https://en.wikipedia.org/wiki/Libuv

Zlib is Cross-platform data compression library. It was written by Jean-loup Gailly and Mark Adler. In Node js, you can Zlib for Threadpool, HTTP requests, and responses compression and Memory Usage Tuning. In order to use zlib in node js, you need to install node-zlib package. After installation below is sample code to use Zlib.

var Buffer = require('buffer').Buffer;
var zlib = require('zlib');

var input = new Buffer('lorem ipsum dolor sit amet');
var compressed = zlib.deflate(input);
var output = zlib.inflate(compressed);

Further Reading https://nodejs.org/api/zlib.html

https://en.wikipedia.org/wiki/Zlib

Normally NodeJs reads the content of a file in non-blocking, asynchronous way. Node Js uses its fs core API to deal with files. The easiest way to read the entire content of a file in nodeJs is with fs.readFile method. Below is sample code to read a file in NodeJs asynchronously and synchronously.

Reading a file in node asynchronously/ non-blocking

var fs = require('fs'); 
fs.readFile('DATA', 'utf8', function(err, contents) {
    console.log(contents);
});
console.log('after calling readFile');

Reading a file in node asynchronously/blocking

var fs = require('fs'); 
var contents = fs.readFileSync('DATA', 'utf8');
console.log(contents);

Streams are special types of objects in Node that allow us to read data from a source or write data to a destination continuously. There are 4 types of streams available in Node Js, they are

• Readable − For reading operation.
• Writable − For writing operation.
• Duplex − Used for both read and write operation.
• Transform − A type of duplex stream where the output is computed based on the input.

Further reading: https://www.tutorialspoint.com/nodejs/nodejs_streams.htm

In Node Js all core modules, as well as most of the community-published modules, follows a pattern where the first argument to any callback handler is an error object. this object is optional, if there is no error then in that case null or undefined is passed to the callback.

Example of the callback function

function callback(err, results) {
	// usually we'll check for the error before handling results
	if(err) {
		// handle error somehow and return
	}
		// no error, perform standard callback handling
}

• readFile load the whole file which you had marked to read whereas createReadStream reads the complete file in the parts of the size you have declared.
• The client will receive the data faster in the case of createReadStream in contrast with readFile.
• In readFile, a file will first completely read by memory and then transfers to a client but in later option, a file will be read by memory in a part which is sent to clients and the process continue until all the parts finish.

JIT stands for Just-in-time. A JIT compiler is a program which is used to send bytecode (it consists of instruction that can be interpreted) to the processor by converting it into instruction. After you have done with writing a program, the compiler compiles the source language statements into bytecode instead of compiling it into the code that carries the information which is similar to the specific hardware platform's processor.

Relation of JIT with Node: Virtual machine of Nodejs has JIT compilation which improves the execution speed of the code. The virtual machine takes the source code and converts to machine code in runtime. By this, the hot functions which are called very often are compiled to machine code and, hence increasing speed.

By writing following line of code, you can create a server in Node Js.

var http =require('http');
http.createServer(function(req,res){

res.writeHead(200,{'Content-Type':'text/plain'});
res.end('Hello World\n');

}).listen(1320,'127.0.0.3');

A Closure is a function defined within another scope that has access to all the variables within the outer scope.

Global variables can be made local (private) with closures.