Mean Stack Interview Questions & Answers for 2025

You can configure properties in the application using the app.set method. This method takes a key and a value as arguments and sets the value for the specified key in the application's settings. 

Here is an example of how you might use the app.set method to configure properties in an Express application: 

const express = require('express'); 
const app = express(); 
// Set the 'view engine' property to 'ejs' 
app.set('view engine', 'ejs'); 
// Set the 'jsonp callback name' property to 'callback' 
app.set('jsonp callback name', 'callback'); 

One of the most common MEAN stack developer interview questions for experienced, don't miss this one.

In Angular, every component has a lifecycle. Angular creates and renders these components and also destroys them before removing them from the DOM. This is achieved with the help of lifecycle hooks. Here is the list of them -  

• ngOnChanges() - Responds when Angular sets/resets data-bound input properties. 
• ngOnInit() - Initialize the directive/component after Angular first displays the data-bound properties and sets the directive/component's input properties 
• ngDoCheck() - Detect and act upon changes that Angular cannot or will not detect on its own. 
• ngAfterContentInit() - Responds after Angular projects external content into the component's view. 
• ngAfterContentChecked() - Respond after Angular checks the content projected into the component. 
• ngAfterViewInit() - Respond after Angular initializes the component's views and child views. 
• ngAfterViewChecked() - This hook responds after Angular checks the component's views and child views. 
• ngOnDestroy() - This hook helps clean up just before Angular destroys the directive/component. 

String Interpolation is a one-way data-binding technique that outputs the data from TypeScript code to HTML view. It is denoted using double curly braces. This template expression helps display the data from the component to the view.  

E.g.: 

{{ data }} 

The ngFor directive in Angular is used to generate lists and tables in HTML templates. It allows you to loop over an array or an object and create a template for each element. The syntax for using ngFor includes a "let" keyword, which creates a local variable that is available within the template, and an "of" keyword, which indicates that we are iterating over an iterable.  

The * symbol before ngFor creates a parent template. For example, the following code uses ngFor to loop over an array of items and create a list element for each item: 

<ul>  
<li *ngFor = "let items in itemlist"> {{ item }} </li> 
</ul> 

The ngFor directive iterates over the itemlist array and creates a list element for each item in the array. The item variable is a local variable that is available within the template and represents the current item being iterated over. The ngFor directive is a powerful tool for generating lists and tables in Angular templates and can greatly simplify the process of rendering data in a web application. 

There are two approaches, namely, Template and Reactive forms when working with Angular. They both have their advantages and appropriate scenarios where they should be used, check them out below: 

Template-driven approach 

One way to create forms in Angular is by using the conventional form tag and adding controls to the form using the NGModel directive. The Angular framework automatically interprets and creates a form object representation for the form tag. Multiple controls can be grouped using the NGControlGroup module. To generate a form value, you can use the "form.value" object, and form data can be exported as JSON values when the submit method is called. Basic HTML validations can be used to validate form fields, or custom validations can be implemented using directives. This method of creating forms in Angular is considered to be straightforward.  

Reactive Form Approach 

Reactive forms in Angular are a programming paradigm that is oriented around data flows and the propagation of change. With reactive forms, the component directly manages the data flows between form controls and data models. Unlike template-driven forms, reactive forms are code-driven and break from the traditional declarative approach. They eliminate the need for two-way data binding, which is considered an anti-pattern. Reactive form control creation is typically synchronous, which allows for unit testing with synchronous programming techniques. Overall, reactive forms offer a code-driven approach to managing data flows and propagating change in an Angular application. 

This question is a regular feature in any MEAN Stack Interview, be ready to tackle it with the explanation below.

Sharing data between components in Angular is a common task that can be accomplished by creating a service and injecting it into the components that need to share data. To generate a new service, you can use the Angular CLI's ng generate service command, which will create a new service file in the src/app folder.

For example, to create a new service called MyDataService, you can run the following command:

ng generate service my-data-service 

Once the service is created, you can inject it into any component that needs to share data by importing the service and adding it to the component's constructor.

import { MyDataService } from './my-data.service'; 
constructor(private myDataService: MyDataService) { } 

Once the service is injected into the component, you can use it to share data between components using the setData() and getData() methods.

this.myDataService.setData('some data'); 
const data = this.myDataService.getData(); 

Overall, using a service to share data between components in Angular is a straightforward process that can help to facilitate communication and data sharing in your application.

The event loop is a mechanism in JavaScript and Node.js that allows the runtime to execute asynchronous code in a non-blocking manner. It works by continuously checking a queue of tasks and executing them when they are ready to be run. The event loop helps to ensure that the main thread of execution is not blocked by long-running tasks, allowing the program to remain responsive to new events and inputs. The event loop is a key feature of JavaScript's asynchronous programming model and is what allows Node.js to handle large numbers of concurrent connections efficiently.

The Buffer class in Node.js is a global class that allows you to create, manipulate, and work with binary data in the form of Buffer objects in JavaScript.

Buffers are useful for working with binary data, such as reading and writing data to files, sending and receiving data over a network, and working with data stored in a database. They are also often used when working with streams, as they allow you to buffer data before processing it or sending it to the next stage of the pipeline.

You can also use the Buffer class to perform various operations on buffers, such as comparing them, searching for substrings, and concatenating them.

Overall, the Buffer class is an important part of the Node.js ecosystem and is widely used for working with binary data in JavaScript.

streams are a way to read and write data in a continuous, asynchronous manner. They provide a way to process data incrementally, rather than reading or writing it all at once, which makes them useful for handling large amounts of data or for working with data that is generated or consumed over time. 

There are four types of streams in Node.js: readable, writable, duplex, and transform. 

1. Readable streams allow you to read data from a source, such as a file or a network socket. 
2. Writable streams allow you to write data to a destination, such as a file or a network socket.  
3. Duplex streams are streams that are both readable and writable, allowing you to read from and write to the same stream. 
4. Transform streams are a type of duplex stream that transform the data as it is written and read. They are often used to modify the data in some way, such as compressing or encrypting it.  

Streams are widely used for working with data flexibly and efficiently. 

chaining refers to the practice of calling multiple methods on an object or value in a single statement. This can be done by returning the object or value from each method, allowing the next method to be called on it. 

Chaining is often used to simplify code and make it more concise by reducing the need to create intermediate variables. It is particularly useful when working with streams, as it allows you to perform multiple operations on the stream in a single statement. 

Here is an example of chaining in Node.js: 

const fs = require('fs'); 
fs.createReadStream('input.txt') 
.pipe(process.stdout) 
.on('error', err => { 
console.error(err); 
}); 

In this example, we use chaining to create a readable stream from the input.txt file, pipe the stream to the standard output, and attach an error handler to the stream.  

In Angular, pipes are a way to transform and format data in templates. They are a declarative way to apply transformations to data within the template, without having to write imperative code in the component class. 

Pipes are denoted by the | character, followed by the name of the pipe and any optional parameters. For example, to format a number as a currency using the built-in currency pipe, you might use the following syntax in a template: 

<p>Total: {{ total | currency }}</p> 

In this example, the currency pipe will transform the total value into a formatted currency string, such as $1,234.56.

Pure pipes are pipes that are only executed when the input value to the pipe changes. This means that if the input value is the same as the previous value, the pipe will not be re-executed. Pure pipes are efficient because they are only called when the input value changes, and they do not need to track any internal state. 

Eg: 

@Pipe({ 
name: 'uppercase', 
pure: true 
}) 
export class UppercasePipe implements PipeTransform { 
transform(value: string): string { 
return value.toUpperCase(); 
} 
} 

Impure pipes, on the other hand, are pipes that are executed on every change detection cycle, regardless of whether the input value has changed. This means that impure pipes can be called multiple times even if the input value has not changed. Impure pipes are useful when the transformation performed by the pipe is expensive or when the pipe needs to track its internal state. 

Eg: 

@Pipe({ 
name: 'random', 
pure: false 
}) 
export class RandomPipe implements PipeTransform { 
transform(value: any[]): any { 
return value[Math.floor(Math.random() * value.length)]; 
} 
} 

In general, it is recommended to use pure pipes whenever possible, as they are more efficient and easier to understand. 

A route guard is a feature that allows you to control access to routes based on certain conditions. Route guards are implemented as services that can be used to protect routes from unauthorized access or to perform certain actions before allowing access to a route.

Route guards are typically implemented using the `canActivate` or `canActivateChild` interfaces, which define methods that are called by the router to determine whether a route can be activated. These methods can return a boolean value indicating whether the route can be activated, or a Promise or Observable that resolves to a boolean value.

The Timers module in Node.js contains functions that execute code after a set period of time. 

• setTimeout/clearTimeout – Is used to schedule code execution after a designated amount of time in milliseconds. 
• setInterval/clearInterval – can be used to execute a block of code multiple times, i.e. after a set interval. 
• setImmediate/clearImmediate – will execute code at the end of the current event loop cycle. 
• process.nextTick – used to schedule a callback function to be invoked in the next iteration of the Event Loop. 

It will print 0 1 2 3 4, because we use let instead of var here. The variable i is only seen in the for loop's block scope.

One of the key features of Node.js is its use of the event loop, which is a mechanism that continually listens for and processes events. The event loop listens for events from various sources, such as user input, network requests, and timer events, and executes the appropriate event handler when an event occurs.

In Node.js, event-driven programming is often used to build scalable, high-performance applications that can handle a large number of concurrent connections. For example, a Node.js server might use event-driven programming to handle incoming HTTP requests by listening for events and executing the appropriate event handler when a request is received.

Overall, event-driven programming is a fundamental concept in Node.js and is an important part of its design and performance characteristics.

The Node.js process model refers to the way that Node.js handles processes and threads. Node.js is built on top of the Chrome V8 JavaScript engine, which is designed to run JavaScript code in a single-threaded, non-blocking manner. This means that Node.js uses a single thread to execute JavaScript code, and it uses non-blocking I/O operations to prevent blocking the thread while waiting for I/O operations to complete.

One of the key benefits of the Node.js process model is that it allows Node.js applications to handle a large number of concurrent connections without the need for threading. This is because the single-threaded, non-blocking nature of Node.js allows it to efficiently handle multiple connections and requests without the overhead of creating and managing multiple threads.

When it comes to error handling on the backend side, specifically for the APIs a combination of error handling, input validation, and good API documentation can help to ensure that your API is robust, reliable, and easy to use. 

There are a few different strategies that you can use to handle errors and validate input in an API: 

Error handling: To handle errors that occur during the execution of an API, you can use try-catch blocks to catch and handle exceptions that are thrown. You can also use HTTP status codes to indicate the type of error that occurred, such as a 4xx status code for a client error or a 5xx status code for a server error. 

Input validation: To validate input in an API, you can use a variety of techniques, such as: 

1. Validating the format and type of the input using regular expressions or type checking 
2. Ensuring that required fields are present and have a valid value 
3. Checking that the input falls within a certain range or meets certain criteria 
4. Sanitizing the input to remove any potentially malicious or unwanted characters 

API documentation: Providing clear and comprehensive documentation for your API can help to ensure that users understand the format and requirements for the input that your API expects. This can help to reduce the likelihood of errors and ensure that your API is used correctly. 

Angular implements a "real DOM" (also known as a "live DOM"), which means that it uses a full, mutable version of the Document Object Model (DOM).

In a real DOM, changes to the DOM are made directly to the actual DOM tree, and these changes are immediately reflected in the browser. This can be more efficient in certain situations, as it allows for fine-grained control over the DOM and can avoid the need for expensive DOM manipulations.

However, the real DOM can also be slower than other types of DOMs, as it requires more resources and can be more complex to work with. As a result, some frameworks and libraries, such as React, implement a "virtual DOM" instead, which is a lightweight, in-memory representation of the DOM that can be used to efficiently update the actual DOM without incurring the overhead of direct DOM manipulation.

One of the most frequently posed interview questions for MEAN stack developer, be ready for it. You can have your answer in this format -

MVVM (Model-View-ViewModel) is a software design pattern that is used to develop software applications. It is similar to the MVC (Model-View-Controller) pattern, but it separates the user interface logic from the business logic, while MVC separates the data access logic from the business logic. Here the separation of concerns facilitates the easier development, testing, and maintenance of software applications.

In the MVVM pattern, the Model layer is responsible for storing and managing data, which can be a database, a web service, or a local data source. The View layer is responsible for displaying data to the user, such as through a GUI, a CLI, or a web page. The ViewModel layer handles user input and updates the View layer accordingly, containing the business logic of the application.

The ViewModel layer acts as a bridge between the Model and View layers, and it is responsible for converting the data from the Model layer into a format that is suitable for display in the View layer. It also handles user input and passes it to the Model layer for processing.

MVVM architecture is often used in conjunction with other design patterns, such as MVP (Model-View-Presenter) and MVC, to create complete software applications.

The abbreviation 'AOT' is defined as "Ahead Of Time" compilation, which is the compilation of high-level programming language into a native machine code to execute the resulting binary file natively, here the code is generated at application build time, instead of run-time.

It is used as it is one way of improving the performance of programs on run time and in particular, the startup time so to improve the warming-up period when compiling code, the compilation happens before the program is run, therefore it is usually added as a build step.

In software development, "eager loading" and "lazy loading" refer to two different strategies for loading data or resources on demand. 

Eager loading refers to the practice of loading all of the data or resources that are required for a particular feature or module at once, typically when the feature or module is first initialized. This can help to improve the performance of the feature or module by reducing the number of additional requests that are needed to load additional data or resources. However, it can also increase the initial load time of the feature or module, as it requires all of the data or resources to be loaded at once. 

Lazy loading refers to the practice of loading data or resources only when they are needed, rather than loading them all at once. This can help to reduce the initial load time of a feature or module, as it only loads the data or resources that are needed. However, it can also result in slower performance if the data or resources are needed frequently, as it requires additional requests to be made each time they are needed. 

Both eager loading and lazy loading have their trade-offs and can be useful in different situations. It is important to carefully consider the requirements and performance needs of a particular feature or module when deciding which loading strategy to use.

Although you might think that they are alike, they are not, and it is expected for an experienced developer to know the difference between them.

Authentication is the process of verifying the identity of a user or system. It involves presenting a set of credentials, such as a username and password, and verifying that the credentials are valid. Authentication is typically the first step in a security process and is used to determine whether a user or system is whom they claim to be.

Authorization, on the other hand, is the process of granting or denying access to specific resources or actions based on the authenticated identity. It involves determining what a user or system is allowed to do based on their permissions or privileges. For example, a user might be authenticated as a member of a certain group, but they might not be authorized to perform certain actions or access certain resources unless they have the appropriate permissions.

In summary, authentication is the process of verifying identity, while authorization is the process of granting or denying access based on the authenticated identity. Both are important for securing systems and ensuring that users and systems are only able to perform the actions and access the resources that they are permitted to.

A must-know for anyone heading into a MEAN Stack interview, this question is frequently asked in MEAN Stack Interviews.

In MongoDB, indexes are data structures that allow the database to quickly locate specific documents within a collection. Indexes can be created on one or more fields in a collection, and they are used to improve the performance of read operations by allowing the database to quickly locate the desired documents without having to scan the entire collection. 

There are several types of indexes available in MongoDB, including single-field indexes, multi-field indexes, compound indexes, and text indexes. Each type of index is optimized for different types of queries and can be used to improve the performance of specific types of operations. 

Indexes are an important tool for improving the performance of a MongoDB database, as they allow the database to locate documents more efficiently and can greatly reduce the time it takes to execute queries.  

However, it is important to carefully consider the trade-offs of using indexes, as they can also increase the overhead of write operations and require additional storage space.

A table scan is a type of database operation that involves scanning through all of the rows in a table to locate specific data. Table scans are often used when a database query does not have a suitable index available to use, or when the query requires data from all rows in the table. 

Table scans can be inefficient, as they require the database to read and process every row in the table, which can take a significant amount of time for large tables. As a result, it is generally best to avoid table scans whenever possible, as they can harm the performance of a database. 

To improve the performance of queries that may require a table scan, it is often helpful to create indexes on the relevant fields in the table. This allows the database to locate the desired data more quickly and efficiently, without having to scan the entire table. 

Overall, table scans are a useful tool in a database, but they should be used sparingly to avoid impacting the performance of the database. 

The Aggregation Framework is a set of tools in MongoDB that allows developers to perform complex data processing and analysis on their data. It provides a powerful set of operators and pipeline stages that can be used to transform, filter, and group data in a variety of ways. 

The Aggregation Framework is often used to perform tasks such as: 

• Grouping data by a specific field or set of fields 
• Calculating statistics and summaries of data 
• Projecting data to a new shape or format 
• Filtering data based on specific criteria 
• Sorting data in a specific order 

To perform aggregation in MongoDB, you can use the aggregate() method, which takes an array of pipeline stages as its argument. Each stage in the pipeline performs a specific operation on the data, and the stages can be combined in a variety of ways to achieve the desired result. 

Eg: 

db.sales.aggregate([ 
{ 
$group: { 
_id: "$category", 
totalSales: { $sum: "$amount" } 
} 
} 
])