Top 60+ MEAN Stack Developer Interview Questions And Answers

Angular’s route guards are interfaces which can tell the router whether or not it should allow navigation to a requested route. They make this decision by looking for a true or false return value from a class which implements the given guard interface.

There are five different types of guards and each of them is called in a particular sequence. The router’s behavior is modified differently depending on which guard is used. The guards are:

• CanActivate
• CanActivateChild
• CanDeactivate
• CanLoad
• Resolve

Consider:

import { Injectable } from '@angular/core';
import { Router, CanActivate } from '@angular/router';
import { AuthService } from './auth.service';

@Injectable()
export class AuthGuardService implements CanActivate {
  constructor(public auth: AuthService, public router: Router) {}
  canActivate(): boolean {
    if (!this.auth.isAuthenticated()) {
      this.router.navigate(['login']);
      return false;
    }
    return true;
  }
}

In JavaScript, each function gets its own scope. Scope is basically a collection of variables as well as the rules for how those variables are accessed by name. Only code inside that function can access that function's scoped variables.

A variable name has to be unique within the same scope. A scope can be nested inside another scope. If one scope is nested inside another, code inside the innermost scope can access variables from either scope.

The answer is YES. There are 4 main principles to Object Oriented Programming:

• Encapsulation,
• Inheritance,
• Abstraction, and
• Polymorphism.

TypeScript can implement all four of them with its smaller and cleaner syntax.

The Centralized Workflow uses a central repository to serve as the single point-of-entry for all changes to the project. The default development branch is called master and all changes are committed into this branch.

Developers start by cloning the central repository. In their own local copies of the project, they edit files and commit changes. These new commits are stored locally.

To publish changes to the official project, developers push their local master branch to the central repository. Before the developer can publish their feature, they need to fetch the updated central commits and rebase their changes on top of them.

Compared to other workflows, the Centralized Workflow has no defined pull request or forking patterns.

Indexes support the efficient execution of queries in MongoDB. Without indexes, MongoDB must perform a collection scan, i.e. scan every document in a collection, to select those documents that match the query statement. If an appropriate index exists for a query, MongoDB can use the index to limit the number of documents it must inspect.

Following are main benefits of using Node.js

• Aynchronous and Event Driven - All APIs of Node.js library are aynchronous that is non-blocking. It essentially means a Node.js based server never waits for a API to return data. Server moves to next API after calling it and a notification mechanism of Events of Node.js helps server to get response from the previous API call.
• Very Fast - Being built on Google Chrome's V8 JavaScript Engine, Node.js library is very fast in code execution.
• Single Threaded but highly Scalable - Node.js uses a single threaded model with event looping. Event mechanism helps server to respond in a non-bloking ways and makes server highly scalable as opposed to traditional servers which create limited threads to handle requests. Node.js uses a single threaded program and same program can services much larger number of requests than traditional server like Apache HTTP Server.
• No Buffering - Node.js applications never buffer any data. These applications simply output the data in chunks.

Containerisation is a type of virtualization strategy that emerged as an alternative to traditional hypervisor-based virtualization.

In containerization, the operating system is shared by the different containers rather than cloned for each virtual machine. For example Docker provides a container virtualization platform that serves as a good alternative to hypervisor-based arrangements.

A request for a resource (like an image or a font) outside of the origin is known as a cross-origin request. CORS (cross-origin resource sharing) manages cross-origin requests. CORS allows servers to specify who (i.e., which origins) can access the assets on the server, among many other things.

Access-Control-Allow-Origin is an HTTP header that defines which foreign origins are allowed to access the content of pages on your domain via scripts using methods such as XMLHttpRequest.

For example, if your server provides both a website and an API intended for XMLHttpRequest access on a remote websites, only the API resources should return the Access-Control-Allow-Origin header. Failure to do so will allow foreign origins to read the contents of any page on your origin.

# Allow any site to read the contents of this JavaScript library, so that subresource integrity works
Access-Control-Allow-Origin: *

By using Cross Site Scripting (XSS) technique, users executed malicious scripts (also called payloads) unintentionally by clicking on untrusted links and hence, these scripts pass cookies information to attackers.

BSON is a binary serialization format used to store documents and make remote procedure calls in MongoDB. BSON extends the JSON model to provide additional data types, and ordered fields, and to be efficient for encoding and decoding within different languages.

BSON encodes type and length information, too, making it easier for machines to parse. Consider:

{"hello": "world"} →

\x16\x00\x00\x00           // total document size
\x02                       // 0x02 = type String
hello\x00                  // field name
\x06\x00\x00\x00world\x00  // field value
\x00                       // 0x00 = type EOO ('end of object')

A Decorator is a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter. Decorators are functions that take their target as the argument. With decorators we can run arbitrary code around the target execution or even entirely replace the target with a new definition.

There are 4 things we can decorate in ECMAScript2016 (and Typescript): constructors, methods, properties and parameters.

Angular services are singleton objects which get instantiated only once during the lifetime of an application. They contain methods that maintain data throughout the life of an application, i.e. data does not get refreshed and is available all the time. The main objective of a service is to organize and share business logic, models, or data and functions with different components of an Angular application.

The separation of concerns is the main reason why Angular services came into existence. An Angular service is a stateless object and provides some very useful functions.

We use classes as object factories. A class defines a blueprint of what an object should look like and act like and then implements that blueprint by initialising class properties and defining methods. Classes are present throughout all the phases of our code.

Unlike classes, an interface is a virtual structure that only exists within the context of TypeScript. The TypeScript compiler uses interfaces solely for type-checking purposes. Once code is transpiled to its target language, it will be stripped from interfaces.

A class may define a factory or a singleton by providing initialisation to its properties and implementation to its methods, an interface is simply a structural contract that defines what the properties of an object should have as a name and as a type.

The most popular DevOps tools are:

• Git: Version Control System tool
• Jenkins: Continuous Integration tool
• Selenium: Continuous Testing tool
• Puppet, Chef, Ansible: Configuration Management and Deployment tools
• Nagios: Continuous Monitoring tool
• Docker: Containerization tool

Gitflow workflow employs two parallel long-running branches to record the history of the project, master and develop:

• Master - is always ready to be released on LIVE, with everything fully tested and approved (production-ready).
• Hotfix - Maintenance or “hotfix” branches are used to quickly patch production releases. Hotfix branches are a lot like release branches and feature branches except they're based on master instead of develop.

• Develop - is the branch to which all feature branches are merged and where all tests are performed. Only when everything’s been thoroughly checked and fixed it can be merged to the master.
• Feature - Each new feature should reside in its own branch, which can be pushed to the develop branch as their parent one.

• ECMAScript 5 (ES5): The 5th edition of ECMAScript, standardized in 2009. This standard has been implemented fairly completely in all modern browsers

• ECMAScript 6 (ES6)/ ECMAScript 2015 (ES2015): The 6th edition of ECMAScript, standardized in 2015. This standard has been partially implemented in most modern browsers.

Here are some key differences between ES5 and ES6:

• Arrow functions & string interpolation:
  Consider:

const greetings = (name) => {
      return `hello ${name}`;
}

and even:

const greetings = name => `hello ${name}`;

• Const.
  Const works like a constant in other languages in many ways but there are some caveats. Const stands for ‘constant reference’ to a value. So with const, you can actually mutate the properties of an object being referenced by the variable. You just can’t change the reference itself.

const NAMES = [];
NAMES.push("Jim");
console.log(NAMES.length === 1); // true
NAMES = ["Steve", "John"]; // error

• Block-scoped variables.
  The new ES6 keyword let allows developers to scope variables at the block level. Let doesn’t hoist in the same way var does.
• Default parameter values Default parameters allow us to initialize functions with default values. A default is used when an argument is either omitted or undefined — meaning null is a valid value.

// Basic syntax
function multiply (a, b = 2) {
     return a * b;
}
multiply(5); // 10

• Class Definition and Inheritance
  ES6 introduces language support for classes (class keyword), constructors (constructor keyword), and the extend keyword for inheritance.

• for-of operator
  The for...of statement creates a loop iterating over iterable objects.

• Spread Operator For objects merging

const obj1 = { a: 1, b: 2 }
const obj2 = { a: 2, c: 3, d: 4}
const obj3 = {...obj1, ...obj2}

• Promises
  Promises provide a mechanism to handle the results and errors from asynchronous operations. You can accomplish the same thing with callbacks, but promises provide improved readability via method chaining and succinct error handling.

const isGreater = (a, b) => {
  return new Promise ((resolve, reject) => {
    if(a > b) {
      resolve(true)
    } else {
      reject(false)
    }
    })
}
isGreater(1, 2)
  .then(result => {
    console.log('greater')
  })
 .catch(result => {
    console.log('smaller')
 })

• Modules exporting & importing Consider module exporting:

const myModule = { x: 1, y: () => { console.log('This is ES5') }}
export default myModule;

and importing:

import myModule from './myModule';

All the above-mentioned technologies are key-value storage mechanisms on the client side. They are only able to store values as strings.

The main and most important difference between REST and GraphQL is that GraphQL is not dealing with dedicated resources, instead everything is regarded as a graph and therefore is connected and can be queried to app exact needs.

var firstWord = "Mary";
var secondWord = "Army";

isAnagram(firstWord, secondWord); // true

function isAnagram(first, second) {
  // For case insensitivity, change both words to lowercase.
  var a = first.toLowerCase();
  var b = second.toLowerCase();

  // Sort the strings, and join the resulting array to a string. Compare the results
  a = a.split("").sort().join("");
  b = b.split("").sort().join("");

  return a === b;
}

TypeScript is referred as optionally statically typed, which means you can ask the compiler to ignore the type of a variable. Using any data type, we can assign any type of value to the variable. TypeScript will not give any error checking during compilation.

Consider:

var unknownType: any = 4;
unknownType = "Okay, I am a string";
unknownType = false; // A boolean.

MongoDB 3.2 allows one to combine data from multiple collections into one through the $lookup aggregation stage.

db.books.aggregate([{
    $lookup: {
            from: "books_selling_data",
            localField: "isbn",
            foreignField: "isbn",
            as: "copies_sold"
        }
}])

Mongo is not a relational database, and the devs are being careful to recommend specific use cases for $lookup, but at least as of 3.2 doing join is now possible with MongoDB. The new $lookup operator added to the aggregation pipeline is essentially identical to a left outer join:

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

Node.js, in its essence, is a single thread process. It does not expose child threads and thread management methods to the developer. js does spawn child threads for certain tasks such as asynchronous I/O, but these run behind the scenes and do not execute any application JavaScript code, nor block the main event loop.

If threading support is desired in a Node.js application, there are tools available to enable it, such as the ChildProcess module.

Both responsive and adaptive design attempt to optimize the user experience across different devices, adjusting for different viewport sizes, resolutions, usage contexts, control mechanisms, and so on.

Responsive design works on the principle of flexibility — a single fluid website that can look good on any device. Responsive websites use media queries, flexible grids, and responsive images to create a user experience that flexes and changes based on a multitude of factors. Like a single ball growing or shrinking to fit through several different hoops.

Adaptive design is more like the modern definition of progressive enhancement. Instead of one flexible design, adaptive design detects the device and other features, and then provides the appropriate feature and layout based on a predefined set of viewport sizes and other characteristics. The site detects the type of device used, and delivers the pre-set layout for that device. Instead of a single ball going through several different-sized hoops, you’d have several different balls to use depending on the hoop size.

OneTime Setup

• npm install -g @angular/cli
• ng new projectFolder creates a new application

Bundling Step

• ng build --prod (run in command line when directory is projectFolder)
• flag prod _bundle for production

bundles are generated by default to projectFolder/dist/

Deployment

You can get a preview of your application using the ng serve --prod command that starts a local HTTP server such that the application with production files is accessible using http://localhost:4200.

For a production usage, you have to deploy all the files from the dist folder in the HTTP server of your choice.

db.users.find({name: /a/})  //like '%a%'
db.users.find({name: /^pa/}) //like 'pa%'
db.users.find({name: /ro$/}) //like '%ro'

Or using Mongoose:

db.users.find({'name': {'$regex': 'sometext'}})

I want to query something as SQL's like query:

select * 
from users 
where name like '%m%'

How to do the same in MongoDB?

Answer

db.users.find({"name": /.*m.*/})
// or
db.users.find({"name": /m/})

You're looking for something that contains "m" somewhere (SQL's '%' operator is equivalent to Regexp's '.*'), not something that has "m" anchored to the beginning of the string.

You can create a function within an outer function (a closure) that allows you to update a private variable but the variable wouldn't be accessible from outside the function without the use of a helper function.

function counter() {
  var _counter = 0;
  // return an object with several functions that allow you
  // to modify the private _counter variable
  return {
    add: function(increment) { _counter += increment; },
    retrieve: function() { return 'The counter is currently at: ' + _counter; }
  }
}

// error if we try to access the private variable like below
// _counter;

// usage of our counter function
var c = counter();
c.add(5); 
c.add(9); 

// now we can access the private variable in the following way
c.retrieve(); // => The counter is currently at: 14

It's actually not correct that MongoDB is not ACID-compliant. On the contrary, MongoDB is ACID-compilant at the document level.

Any update to a single document is

• Atomic: it either fully completes or it does not
• Consistent: no reader will see a "partially applied" update
• Isolated: again, no reader will see a "dirty" read
• Durable: (with the appropriate write concern)

What MongoDB doesn't have is transactions - that is, multiple-document updates that can be rolled back and are ACID-compliant.

Multi-document transactions, scheduled for MongoDB 4.0 in Summer 2018*, will feel just like the transactions developers are familiar with from relational databases – multi-statement, similar syntax, and easy to add to any application.

The Angular Ahead-of-Time compiler pre-compiles application components and their templates during the build process. Apps compiled with AOT launch faster for several reasons.

• Application components execute immediately, without client-side compilation.
• Templates are embedded as code within their components so there is no client-side request for template files.
• You don't download the Angular compiler, which is pretty big on its own.
• The compiler discards unused Angular directives that a tree-shaking tool can then exclude.

ClickJacking is an attack that fools users into thinking they are clicking on one thing when they are actually clicking on another. The attack is possible thanks to HTML frames (iframes).

Its other name, user interface (UI) redressing, better describes what is going on. Users think they are using a web page’s normal UI, but in fact there is a hidden UI in control; in other words, the UI has been redressed. When users click something they think is safe, the hidden UI performs a different action.

Cross-Site Request Forgery (CSRF) is an attack that forces authenticated users to submit a request to a Web application against which they are currently authenticated.

A CSRF attack tricks the victim into clicking a URL that contains a maliciously crafted, unauthorized request for a particular Web application. The user’s browser then sends this maliciously crafted request to a targeted Web application. The request also includes any credentials related to the particular website (e.g., user session cookies). If the user is in an active session with a targeted Web application, the application treats this new request as an authorized request submitted by the user.

How to prevent

To defeat a CSRF attack, applications need a way to determine if the HTTP request is legitimately generated via the application’s user interface. The best way to achieve this is through a CSRF token. A CSRF token is a secure random token (e.g., synchronizer token or challenge token) that is used to prevent CSRF attacks. The token needs to be unique per user session and should be of large random value to make it difficult to guess.

Redux is a way to manage application state and improve maintainability of asynchronicity in your application by providing a single source of truth for the application state, and a unidirectional flow of data change in the application. ngrx/store is one implementation of Redux principles.

A blocking call causes results to be returned synchronously.

Performing a blocking system call causes the process to enter the blocked state. Control is let back to the process only after the I/O event that is being waited upon occurs.

const fs = require("fs");
const contents = fs.readFileSync("file.txt", "utf8");
// this line is not reached until the read results are in
console.log(contents);

A grid system is a structure that allows for content to be stacked both vertically and horizontally in a consistent and easily manageable fashion. Grid systems include two key components: rows and columns.

Some Grid Systems:

• Simple Grid
• Pure
• Flexbox Grid
• Bootstrap
• Foundation

Aggregation operations process multiple documents and return computed results. You can use aggregation operations to:

• Group values from multiple documents together.
• Perform operations on the grouped data to return a single result.
• Analyze data changes over time.

MongoDB provides aggregation operations through aggregation pipelines — a series of operations that process data documents sequentially. An aggregation pipeline consists of one or more stages that process documents:

• Each stage performs an operation on the input documents. For example, a stage can filter documents, group documents, and calculate values.
• The documents that are output from a stage are passed to the next stage.
• An aggregation pipeline can return results for groups of documents. For example, return the total, average, maximum, and minimum values.

Consider:

db.orders.aggregate([
   // Stage 1: Filter pizza order documents by pizza size
   {
      $match: { size: "medium" }
   },
   // Stage 2: Group remaining documents by pizza name and calculate total quantity
   {
      $group: { _id: "$name", totalQuantity: { $sum: "$quantity" } }
   }
])

• Angular Elements - Angular Elements is a project that lets you wrap your Angular components as Web Components and embed them in a non-Angular application.
• New Rendering Engine: Ivy - increases in speed and decreases in application size.
• Tree-shakeable providers - a new, recommended, way to register a provider, directly inside the @Injectable() decorator, using the new providedIn attribute
• RxJS 6 - Angular 6 now uses RxJS 6 internally, and requires you to update your application also. RxJS released a library called rxjs-compat, that allows you to bump RxJS to version 6.0 even if you, or one of the libraries you’re using, is still using one of the “old” syntaxes.
• ElementRef<T> - in Angular 5.0 or older, is that the said ElementRef had its nativeElement property typed as any. In Angular 6.0, you can now type ElementRef more strictly.
• Animations - The polyfill web-animations-js is not necessary anymore for animations in Angular 6.0, except if you are using the AnimationBuilder.
• i18n - possibility to have “runtime i18n”, without having to build the application once per locale.

The event loop is what allows Node.js to perform non-blocking I/O operations — despite the fact that JavaScript is single-threaded — by offloading operations to the system kernel whenever possible.

Every I/O requires a callback - once they are done they are pushed onto the event loop for execution. Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background. When one of these operations completes, the kernel tells Node.js so that the appropriate callback may be added to the poll queue to eventually be executed.

• In Blue Green Deployment, you have TWO complete environments. One is Blue environment which is running and the Green environment to which you want to upgrade. Once you swap the environment from blue to green, the traffic is directed to your new green environment. You can delete or save your old blue environment for backup until the green environment is stable.

• In Rolling Deployment, you have only ONE complete environment. The code is deployed in the subset of instances of the same environment and moves to another subset after completion.

Use it whenever it makes sense for code to subscribe to something rather than get a callback from something. The typical use case would be that there's multiple blocks of code in your application that may need to do something when an event happens.

Consider:

// get the reference of EventEmitter class of events module
var events = require('events');

//create an object of EventEmitter class by using above reference
var em = new events.EventEmitter();

//Subscribe for FirstEvent
em.on('FirstEvent', function (data) {
    console.log('First subscriber: ' + data);
});

// Raising FirstEvent
em.emit('FirstEvent', 'This is my first Node.js event emitter example.');