NodeJS 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 a 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 the 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 that 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

but is a 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, poll(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 a 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. To use zlib in node js, you need to install the node-zlib package. After installation below is a 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 a 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 a 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 reading and write operation.
• Transform - A type of duplex stream where the output is computed based on the input.

In Node Js all core modules, as well as most of the community-published modules, follow 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 well 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 the latter option, a file will be read by memory in a part that is sent to clients and the process continues until all the parts finish.

JIT stands for Just-in-time. A JIT compiler is a program that 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 writing a program, the compiler compiles the source language statements into bytecode instead of compiling them into the code that carries the information which is similar to the specific hardware platforms 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 it 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 the 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.

Aggregations are a set of functions that are used to manipulate the data that has been returned from a MongoDB query. In Mongoose, aggregations work as a pipeline. The aggregate function accepts the array of data transformations that are applied by data using different methods in terms of arguments.

Syntax: db.customers.aggregate([ ... aggregation steps go here ...]);

In above, aggregation is applied to data of customers.

The main difference between the put and the patch is

HTTP defines a set of request methods to perform the desired actions. These request methods are:

1. GET: The GET method asked for the representation of the specified resource. This request was used to retrieve the data.
2. POST: The POST technique is utilized to present an element to the predetermined resource, generally causing a change in state or reactions on the server.
3. HEAD: The HEAD method is similar to the GET method but asks for the response without the response body.
4. PUT: This method is used to substitute all current representations with the payload.
5. DELETE: It is used to delete the predetermined resource.
6. CONNECT: This request is used to settle the TCP/IP tunnel to the server by the target resource
7. OPTION: This method is used to give back the HTTP strategies to communicate with the target resource.
8. TRACE: This method echoes the message which tells the customer how many progressions have been made by an intermediate server.
9. PATCH: The PATCH method gives partial modifications to a resource.

The revealing module pattern is similar to the Module Pattern.IT ExposES only the properties and methods you want via a returned Object.

var greeting = Hello world
function greet() {
 console.log(greeting)
}
module.exports = {
  greet: greet
}

following is the syntax of one of the methods to read from a file:

fs.read(fd, buffer, offset, length, position, callback)

This method will use a file descriptor to read the file, if you want to read a file using the file name directly then you should use another method available.

Parameters

Here is the description of the parameters used -

• fd - This is the file descriptor returned by file fs.open() method.
• buffer - This is the buffer that the data will be written to.
• offset - This is the offset in the buffer to start writing at.
• length - This is an integer specifying the number of bytes to read.
• position - This is an integer specifying where to begin reading from in the file. If the position is null, data will be read from the current file position.
• callback - This is the callback function that gets the three arguments, (err, bytesRead, buffer).

Following is the syntax of one of the methods to write into a file:

fs.writeFile(filename, data[, options], callback)

This method will overwrite the file if the file already exists. If you want to write into an existing file then you should use another method available.

Parameters

Here is the description of the parameters used:

• path - This is a string having file name including path.
• data - This is the String or Buffer to be written into the file.
• options - A third parameter is an object which will hold {encoding, mode, flag}. By default encoding is utf8, a mode is octal value 0666 and flag is w
• callback - This is the callback function that gets a single parameter to err and is used to return an error in case of any writing error.

When a theme is activated it’s what’s controlling your site, while an installed theme is simply part of your theme library and available to activate.

Following is the syntax of one of the methods to close an opened file:

fs.close(fd, callback)

Parameters

Here is the description of the parameters used:

• fd - This is the file descriptor returned by file fs.open() method.
• callback - This is the callback function that gets no arguments other than a possible exception are given to the completion callback.

Following is the syntax of the method to get the information about a file:

fs.stat(path, callback)

Parameters

Here is the description of the parameters used:

• path - This is a string having file name including path.
• callback - This is the callback function that gets two arguments (err, stats) where stats is an object of fs.Stats type is printed below in the example.

Following is the syntax of the method to truncate an opened file -

fs.ftruncate(fd, len, callback)

Parameters

Here is the description of the parameters used:

• fd - This is the file descriptor returned by file fs.open() method.
• len - This is the length of the file after which the file will be truncated.
• callback - This is the callback function that gets no arguments other than a possible exception are given to the completion callback.

Following is the syntax of the method to delete a file -

fs.unlink(path, callback)

Parameters

Here is the description of the parameters used:

• path - This is the file name including path.
• callback - This is the callback function that gets no arguments other than a possible exception are given to the completion callback.

Following is the syntax of the method to create a directory:

fs.mkdir(path[, mode], callback)

Parameters

Here is the description of the parameters used:

• path - This is the directory name including path.
• mode - This is the directory permission to be set. Defaults to 0777.
• callback - This is the callback function that gets no arguments other than a possible exception are given to the completion callback.

Following is the syntax of the method to remove a directory:

fs.rmdir(path, callback)

29. Parameters

Here is the description of the parameters used:

• path - This is the directory name including path.
• callback - This is the callback function that gets no arguments other than a possible exception are given to the completion callback.

Following is the syntax of the method to read a directory:

fs.readdir(path, callback)

Parameters

Here is the description of the parameters used:

• path - This is the directory name including path.
• callback - This is the callback function that gets two arguments (err, files) where files are an array of the names of the files in the directory excluding . and ...

The __filename represents the filename of the code being executed. This is the resolved absolute path of this code file. For the main program, this is not necessarily the same filename used in the command line. The value inside a module is the path to that module file.