Mern Stack Development Training by Experts

;

Our Training Process

Mern Stack Development - Syllabus, Fees & Duration

Module 1: Introduction to React

  • What is React?
  • Why React?
  • React version history
  • React 16 vs React 15
  • Just React – Hello World
  • Using create-react-app
  • Anatomy of react project
  • Running the app
  • Debugging first react app

Module 2: Templating using JSX

  • Working with React. create Element
  • Expressions
  • Using logical operators
  • Specifying attributes
  • Specifying children
  • Fragments

Module 3: About Components

  • Significance of component architecture
  • Types of components
  • Functional
  • Class based
  • Pure
  • Component Composition

Module 4: Working with state and props

  • What is state and it significance
  • Read state and set state
  • Passing data to component using props
  • Validating props using prop Types
  • Supplying default values to props using default Props

Module 5: Rendering lists

  • Using react key prop
  • Using map function to iterate on arrays to generate elements

Module 6: Event handling in React

  • Understanding React event system
  • Understanding Synthetic event
  • Passing arguments to event handlers

Module 7: Understanding component lifecycle and  handling errors

  • Understand the lifecycle methods
  • Handle errors using error boundaries

Module 8: Working with forms

  • Controlled components
  • Uncontrolled components
  • Understand the significance to default Value prop
  • Using react ref prop to get access to DOM element

Module 9: Context

  • What is context
  • When to use context
  • Create Context
  • Context Provider
  • Context Consumer
  • Reading context in class

Module 10: Code-Splitting

  • What is code splitting
  • Why do you need code splitting
  • React lazy
  • Suspense
  • Route-based code splitting

Module 11: hooks

  • What are hooks
  • Why do you need hooks
  • Different types of hooks
  • Using state and effect hooks
  • Rules of hooks

Module 12: Routing with react router

  • Setting up react router
  • Understand routing in single page applications
  • Working with Browser Router and Hash Router components
  • Configuring route with Route component
  • Using Switch component to define routing rules
  • Making routes dynamic using route params
  • Working with nested routes
  • Navigating to pages using Link and NavLink component
  • Redirect routes using Redirect Component
  • Using Prompt component to get consent of user for navigation
  • Path less Route to handle failed matches

Module 14: Immutable.js

  • What is Immutable.js?
  • Immutable collections
  • Lists
  • Maps
  • Sets

Module 17: Unit Testing

  • Understand the significance of unit testing
  • Understand unit testing jargon and tools
  • Unit testing react components with Jest
  • Unit testing react components with enzyme

Module 18: Web pack Primer

  • What is web pack
  • Why web pack
  • Install and setup web pack
  • Working with web pack configuration file
  • Working with loaders
  • Working with plugins
  • Setting up Hot Module Replacement

Module 19: Isomorphic React

  • What is server-side rendering (SSR)?
  • Why SSR
  • Working with render To String and render To Static Markup methods

Syllabus -> React JS
This syllabus is not final and can be customized as per needs/updates
IT Courses / Internship

Module 1: Introduction to Node.js

  • What is Node.js
  • Features of Node.js
  • Concept
  • Where to fit and not fit
  • Event-Driven programming style
  • What is Asynchronous

Module 2: Installation / Setup

  • Local environment setup
  • Node.js runtime
  • Download source code
  • Installation on OS
  • Verify

Module 3: Node Package Manager

  • Install module by NPM
  • Global vs Local setup
  • Update Module
  • CRUD Module

Module 4: Express Framework

  • What is Express
  • Setup Express
  • Request And Response
  • Handling Routes
  • Route Middleware
  • Objects
  • Cookies Management
  • File upload
  • HTTP methods

Module 5: Buffers and Streams

  • What is buffers and Streams
  • Benefit of Streams over buffers
  • Create / Write / Read operation on Buffers
  • Process on Buffers
  • Read / Write data by Streams
  • Pipeline
  • Chaining Stream

Module 7: REST API

  • RESTful Architecture
  • HTTP URI and Methods
  • RESTful web services
  • Expose solution as API
  • Best practice for REST API solution

Module 8: Callback

  • What is Callback
  • Benefit of Callback
  • Asynchronous communications
  • Block and Non-Blocking
  • Standard Callback pattern
  • Async Flow control Library
  • Executing in parallel

Module 9: Events

  • What is Events
  • Events types
  • Event Emitter API
  • Multiple event listeners
  • Event emitter pattern
  • Class methods
  • Event Loop
  • Blocking Event Loop
  • Escaping Event Loop

Module 10: Connecting with Database

  • Introduction of MySQL
  • Connect with MySQL
  • Introduction of MongoDB
  • Features of MongoDB
  • Connect with MongoDB
  • Defining a schema
  • Defining a Model

Module 11: External Processes and Services

  • What is processes
  • Spawning Child process
  • Create and kill processes
  • Building with HTTP Severs
  • HTTP requests
  • Secure HTTP Server

Module 12: External Processes and Services

  • Using Test Runner
  • Using Assertion Testing Module
  • Built-in Debugger
  • Console log
  • Node Inspector

Syllabus -> Node JS
This syllabus is not final and can be customized as per needs/updates
IT Courses / Internship

Module 1 : INTRODUCTION EXPRESS JS

  • The model-view-controller pattern
  • Defining Jade templates
  • Configuring Express
  • Postman configuration
  • Using REST
  • JSON Data
  • Reading POST data
  • CRUD operations
  • Adding middleware

Module 2 : EXPRESS JS WITH MONGODB AND SQLITE

  • Connecting to RDBMS and Mongodb databases
  • Performing CRUD operations
  • Module 3 : SOCKET.IO, THE FRONT-END, AND A CHAT APP

    • Getting Started
    • Adding Socket.io To Your App
    • Exploring The Front-end
    • Sending Live Data Back & Forth
    • Creating The Front-end UI
    • Showing Messages In App
    • Working With Time
    • Timestamps
    • Show Message Time In Chat App
    • Chat application Project

    Module 4 : INTRODUCTION TO TASK MANAGERS WITH UNIT TESTING

    • Working with gulp
    • Working with grunt
    • Working with unit and E2E testing

    Module 5: LEARNING EXPRESS ROUTES

    • starting a server
    • Writing your first route
    • Sending a response
    • Macthing route paths
    • Getting a single expression
    • Setting status codes
    • Macthing longer paths
    • Other HTTP methods

    Syllabus -> Express JS
    This syllabus is not final and can be customized as per needs/updates
    IT Courses / Internship

    Module 1: Introduction to NoSQL

    • What Is NoSQL?
    • Why NoSQL databases are required
    • Types of NoSQL Database
    • NoSQL vs SQL Comparison
    • ACID & BASE Property
    • CAP Theorem
    • Benefits of NoSQL databases
    • Installation
    • Start and Stop the MongoDB process

    Module 2: MongoDB Architecture

    • Document, Collection, Databases
    • JSON and BSON
    • Storage Engines
    • Read Path
    • Journaling
    • Write Path
    • Working Set
    • Capped Collection
    • Oplog collection
    • TTL Index
    • GridFS

    Module 3: CRUD Operations

    • MongoDB Data Types
    • Inserting, Update, Deleting the documents
    • Querying the documents
    • Bulk insert operation
    • Updating multiple document
    • Limiting documents
    • Filtering documents

    Module 4: Schema Design and Data modeling

    • Dynamic Schema
    • What is Data modeling?
    • RDBMS and MongoDB Data modeling difference
    • Embedding Document
    • Reference Document

    Module 5: Indexes

    • Index concepts in MongoDB
    • Types of indexes
    • Indexes and its use cases
    • Creating Indexes
    • Managing Indexes
    • Index strategies

    Module 6: Database Administration in MongoDB

    • Database status
    • Troubleshooting issues
    • Current Operations
    • Rotating log files
    • Users and Roles
    • Copy and Clone database
    • DB and Collection Stats
    • Explain plan
    • Profiling
    • Changing configuration files
    • Upgrading the database

    Module 7: MongoDB: Backup and Security

    • Concept of backups
    • mongoexport/mongoimport
    • mongodump/mongorestore
    • Oplog backups
    • LVM Backups
    • Backups using MMS/Ops Manager
    • Purpose of security
    • Authentication and authorization
    • Role based access control

    Module 8: Replication in MongoDB

    • Concept of replication
    • ReplicaSet member roles
    • Voting and Electing primary
    • Role of Oplog in replication
    • Read and Write Concern
    • Arbiter,Hidden and Delayed replica node
    • Priority settings
    • Replicaset nodes health check
    • Concept of resyncing the nodes
    • Rollbacks during failover
    • Keyfile authentication

    Module 9: MongoDB Scalability

    • Concept of Scalability
    • Sharding concept
    • Shardkey and Chunks
    • Choosing shardkey
    • Sharding components
    • Types of Sharding
    • Balanced data distribution
    • Sharded and Non-sharded collection
    • Sharded Replicaset
    • Tag aware sharding

    Module 10: MongoDB Monitoring and Other Tools

    • MMS Manager
    • Ops Manager
    • Mongo utility commands
    • Mongo developer tools
    • MongoDB Atlas
    • MongoDB client drivers

    Syllabus -> MongoDB
    This syllabus is not final and can be customized as per needs/updates
    IT Courses / Internship
     
    10000+
    20+
    50+
    25+

    Mern Stack Development Jobs in Surat

    Enjoy the demand

    Find jobs related to Mern Stack Development in search engines (Google, Bing, Yahoo) and recruitment websites (monsterindia, placementindia, naukri, jobsNEAR.in, indeed.co.in, shine.com etc.) based in Surat, chennai and europe countries. You can find many jobs for freshers related to the job positions in Surat.

    • mern stack developer
    • mern stack
    • mern stack programmer
    • reach js developer
    • node js developer
    • express js developer
    • mern stack expert
    • mern stack fresher
    • full stack developer
    • mongo db

    Mern Stack Development Internship/Course Details

    Mern Stack Development internship jobs in Surat
    Mern Stack Development Node. React has lots of advantages, the main reason for people to choose react is its faster performance and good interface. Learn from expert trainers with the best skills through the best sources. Thus the basic syntax and specifications are of JavaScript only. Working with MongoDB NoSQL information is much easier than operating with any relational database. Node Package Manager (NPM) is one of all the world’s prime package registries. js is a straightforward programming language. js are generally preferred. The training costs for developers are usually high, but when it comes to Node. Complete, updated list.

    Meet a Few of our Industry Experts 🚀 Your Pathway to IT Career

    Jayanti

    Mobile: +91 98474 90866
    Location: Karnataka, Online (Surat)
    Qualification: MA lit

    Experience: Classroom management curriculum development proficiency in English as a teacher languages   more..

    parvathy

    Mobile: +91 8301010866
    Location: Kerala, Online (Surat)
    Qualification: bca

    Experience: i have 1 year and 8 months of experience in python along with django and have worked in webscrapping with  more..

    Roshan

    Mobile: +91 91884 77559
    Location: Nagpur, Maharashtra, Online (Surat)
    Qualification: BE Computer Science And Engineering

    Experience: Java Selenium Webdriver TestNg Bdd Cucumber Api testing and RestAssured MERN Stack Android Dev   more..

    Kirti

    Mobile: +91 91884 77559
    Location: Maharashtra, Online (Surat)
    Qualification: MCOM

    Experience: Manual testing aslo have 4 month mobile application testing experience   more..

    Monika

    Mobile: +91 91884 77559
    Location: Odisha, Online (Surat)
    Qualification: Graduation

    Experience: Internship and training - ⁠•completed internship programme at odisha farm machinery research and development centre ⁠ •completed intersection programme at  more..

    Gurpreet

    Mobile: +91 89210 61945
    Location: Jammu and Kashmir, Online (Surat)
    Qualification: BE in electrical branch

    Experience: I am a electrical engineering graduate with over 1 year of experience as a biomedical engineer and auditor in a  more..

    Lukhman

    Mobile: +91 98474 90866
    Location: Karnataka, Online (Surat)
    Qualification: Be computer science

    Experience: Java sql html css networking  more..

    Kartik

    Mobile: +91 91884 77559
    Location: Punjab, Online (Surat)
    Qualification: B tech Mechanical

    Experience: I have worked in tata motors as an intern in production department Now it's been 1 5 years I am  more..

    Nikhil

    Mobile: +91 9895490866
    Location: Pune, Online (Surat)
    Qualification: MSC computer science

    Experience: I have knowledge about manual testing Jira tool | Resume for Application for Software Testing  more..

    Anjali

    Mobile: +91 9446600368
    Location: Banaras, Uttarpradesh, Online (Surat)
    Qualification: Btech

    Experience: Dear mam I hope this message finds you doing well I am reaching out to you because I am interested  more..

    Jagdish

    Mobile: +91 8301010866
    Location: Maharashtra, Online (Surat)
    Qualification: Bsc IT

    Experience: Manual testing functional testing API testing UI UX testing black box and Unit testing create test case and execute test  more..

    Sreerag

    Mobile: +91 98474 90866
    Location: Ernakulam, Online (Surat)
    Qualification: BTech in computer science and engineering

    Experience: 2 Months hands on experience in manual Automation API and Functional testing | Resume for   more..

    sarang

    Mobile: +91 9446600368
    Location: Kerala, Online (Surat)
    Qualification: BCA

    Experience: completed BCA in 2023 and followed it up with a 6 month development course from SDC(Ernakulam) I have the ability  more..

    Suman

    Mobile: +91 9895490866
    Location: West Bengal, Online (Surat)
    Qualification: MFA

    Experience: "I recently completed my Master's in Applied Art and have also been doing freelance work for several companies and clients  more..

    Mothi

    Mobile: +91 91884 77559
    Location: Tamil Nadu, Online (Surat)
    Qualification: Master degree CS

    Experience: I am finished course in python full stack developer In Front end I know HTML CSS and Bootstrap In backend  more..

    ATHUL

    Mobile: +91 98474 90866
    Location: Thrissur , Online (Surat)
    Qualification: Bsc electronics

    Experience: Software Testing: Manual testing and Automation testing ❖ Knowledge of test management tools ❖ Defect tracking tool abilities ❖  more..

    Neethu

    Mobile: +91 98474 90866
    Location: Kerala, Online (Surat)
    Qualification: B.tech

    Experience: Provide technical support to 30+ customers per day for Apple devices and products translated complex technology concepts into accessible language  more..

    Shubhra

    Mobile: +91 91884 77559
    Location: Mumbai, Online (Surat)
    Qualification: Bsc Computer Science

    Experience: I have done software testing manual and automation testing Experience is fresher Good command on writing defect reports with 3  more..

    Himanshu

    Mobile: +91 91884 77559
    Location: Uttar Pradesh, Online (Surat)
    Qualification: MCA

    Experience: I've done three internship and all of them is done by using python  more..

    Prathamesh

    Mobile: +91 9446600368
    Location: Dahisar Mumbai, Online (Surat)
    Qualification: 12 + IT Computer engineering Diploma

    Experience: I have experience in python developing app software | Resume for   more..

    Disha

    Mobile: +91 89210 61945
    Location: Maharashtra, Online (Surat)
    Qualification: B.des

    Experience: As a graphic designer I have strong hands-on experience with industry-standard software such as Adobe Creative Suite (Photoshop Illustrator InDesign)  more..

    ankush

    Mobile: +91 89210 61945
    Location: Chandigarh, Online (Surat)
    Qualification: BE

    Experience: I have extensive experience in graphics design website brand experience design uiux  more..

    Chinjumol

    Mobile: +91 9446600368
    Location: Kerala, Online (Surat)
    Qualification: MSC computer science

    Experience: Test scenario Test case document Defect Bug Bug report Bug life cycle SDLC STLC Selenium with Java Junit and TestNG  more..

    Madhuri

    Mobile: +91 9895490866
    Location: Maharashtra, Online (Surat)
    Qualification: MBA

    Experience: Experience in Software Quality Analyst (QA) & Manual Testing and API Testing  Knowledge in API Testing Tools: Postman   more..

    Ahmad

    Mobile: +91 98474 90866
    Location: Banglore , Online (Surat)
    Qualification: B.tech

    Experience: Html css javascript mongodb express js React js node js python django java  more..

    Eram

    Mobile: +91 91884 77559
    Location: Jharkhand, Online (Surat)
    Qualification: BTech in computer science and engineering

    Experience: C Java language python Oops DBMS computer networks operating system MS- excel MS- word excellent written and verbal communication skills  more..

    Abhay

    Mobile: +91 91884 77559
    Location: Uttar Pradesh, Online (Surat)
    Qualification: MCA

    Experience: 6 months of experience in Java & Web development Experience in Spring+React js development Command in Java Sql PHP JavaScript  more..

    karthik

    Mobile: +91 91884 77559
    Location: Kerala, Online (Surat)
    Qualification: Bsc Computer Science

    Experience: EXPERIENCE: Recently completed a six-month internship as a Quality Analyst at Aventus Informatics Private Limited Infopark Thrissur During this time  more..

    Pavith

    Mobile: +91 98474 90866
    Location: Tamil Nadu, Online (Surat)
    Qualification: Bachelor of Technology

    Experience: With a solid background in frontend and full stack development coupled with proficiency in technologies such as MERN stack and  more..

    Rahana

    Mobile: +91 8301010866
    Location: Ernakulam , Online (Surat)
    Qualification: Mca

    Experience: I have 1 year experience in ETL testing   more..

    Sreedeep

    Mobile: +91 9895490866
    Location: Manjeri Malappuram, Online (Surat)
    Qualification: Mca

    Experience: Working as a software developer in bicoders solution kinfra (angular and flutter)Application for Angular JS  more..

    Anandhu

    Mobile: +91 9446600368
    Location: Idukki, Online (Surat)
    Qualification: BCA

    Experience: Skilled in both frontend and backend technologies And worked as a python django intern in Camerinfolks Pvt Ltd at Kochi  more..

    Tejas

    Mobile: +91 9895490866
    Location: Maharashtra, Online (Surat)
    Qualification: Bcca

    Experience: WordPress html css javascript bootstrap and php mysql  more..

    Kaushal

    Mobile: +91 91884 77559
    Location: Uttar Pradesh, Online (Surat)
    Qualification: Cyber security Ethical hacker web pentesting

    Experience: Cyber security Ethical hacker and website penetration testing Networks security nmap burpsuite Linux kali Linux Google Dorking bug bounty bug  more..

    Vanshaj

    Mobile: +91 91884 77559
    Location: Delhi, Online (Surat)
    Qualification: Btech CSE

    Experience: Hi I am Vanshaj Sharma I have been working with Flutter Firebase Postman GitHub Jira Notion AWS etc for more  more..

    Kirubavathy

    Mobile: +91 9446600368
    Location: Tamil Nadu, Online (Surat)
    Qualification: M.A., B.Ed.,

    Experience: o Student development o Communication in English o Extracurricular activities o Classroom management o Extracurricular activities o Lesson plan development  more..

    Dinesh

    Mobile: +91 91884 77559
    Location: Tamil Nadu, Online (Surat)
    Qualification: Be

    Experience: Soc cybersecurity pentesting ethical hacking vulnerability assessment  more..

    Isha

    Mobile: +91 9895490866
    Location: Delhi, Online (Surat)
    Qualification: PhD

    Experience: Statistics (Hypothesis testing CI EDA) Supervised and Unsupervised Machine Learning (Linear Regression Logistic Regression Naive Bayes Decision Trees Random Forest  more..

    Nitha

    Mobile: +91 9895490866
    Location: Tamil Nadu, Online (Surat)
    Qualification: Currently Pursuing my B.tech(2yr))

    Experience: I have already done my 1st internship in a company called Fraction IT as a web developer and i have  more..

    Najila.k

    Mobile: +91 8301010866
    Location: Kerala, Online (Surat)
    Qualification: Bsc computer science

    Experience: Iam complter Bsc computer science Then currently iam done the course python django and Mttc course   more..

    Yagesh

    Mobile: +91 9446600368
    Location: Rajasthan, Online (Surat)
    Qualification: btech

    Experience: c++ python html css rescently graduated  more..

    Abhishek

    Mobile: +91 9895490866
    Location: Kolkata, Online (Surat)
    Qualification: B.tech

    Experience: Php wordpress   more..

    Ankita

    Mobile: +91 9895490866
    Location: Maharashtra, Online (Surat)
    Qualification: BE computer

    Experience: I know the computer knowledge And social media knowledge 1 year experience in web development  more..

    Neethu

    Mobile: +91 9446600368
    Location: Kerala, Online (Surat)
    Qualification: Mphil in History

    Experience: I have more than two years of experience in teaching both offline and freelance Iam more passionate in teaching and  more..

    SOUMYA

    Mobile: +91 8301010866
    Location: Odisha, Online (Surat)
    Qualification: MCA

    Experience: C Java python html css js  more..

    Esha

    Mobile: +91 98474 90866
    Location: Uttar Pradesh, Online (Surat)
    Qualification: Btech

    Experience: Mathematics python machine learning 1 year experience in FIITJee  more..

    Kb

    Mobile: +91 91884 77559
    Location: Tamil Nadu, Online (Surat)
    Qualification: Dfa

    Experience: Know very well Adobe Illustrator Adobe Photoshop Adobe indesign Skills: poster design Logo design Layout Graphic in Adobe Illustrator page  more..

    Muddssirhusain

    Mobile: +91 8301010866
    Location: Maharashtra, Online (Surat)
    Qualification: Bsc

    Experience: I have been doing API penetration testing and web application penetration testing for 3 years and am doing full time  more..

    Akaamsh

    Mobile: +91 89210 61945
    Location: New Delhi , Online (Surat)
    Qualification: MA - Advertising and Marketing Management

    Experience: 4 years experience in digital art and design Also experienced in film making and video editing   more..

    sangeetha

    Mobile: +91 89210 61945
    Location: Tamil Nadu, Online (Surat)
    Qualification: B.E CSE

    Experience: sql java html selenium agile process manual testing  more..

    Monisha

    Mobile: +91 9446600368
    Location: Mumbai, Online (Surat)
    Qualification: Diploma in architecture

    Experience: I have 2 3 years experience in architecture and interior design I worked in architecture layouts sections elevations and interiors  more..

    Avadhut

    Mobile: +91 91884 77559
    Location: Maharashtra, Online (Surat)
    Qualification: B.com

    Experience: With over 2 years of experience I specialize in WordPress and Shopify development proficient in HTML PHP CSS and Bootstrap  more..

    Nikita

    Mobile: +91 98474 90866
    Location: Maharashtra, Online (Surat)
    Qualification: Graduate

    Experience: Dedicated and detail-oriented Manual Tester with 2 years of hands-on experience in testing software applications Proficient in designing and executing  more..

    Vaghasiya

    Mobile: +91 91884 77559
    Location: Gujarat, Online (Surat)
    Qualification: Bca

    Experience: I have 2 6 years of experience in flutter development and created more than 10 apps using flutter I have  more..

    Ishwar

    Mobile: +91 89210 61945
    Location: Nalasopara,Mumbai,Maharashtra, Online (Surat)
    Qualification: BSC CS Graduated

    Experience: I'm Ishwar Vishwakarma a proficient web developer with expertise in HTML CSS JavaScript and Bootstrap I hold a Bachelor's degree  more..

    Shalu

    Mobile: +91 89210 61945
    Location: Kerala, Online (Surat)
    Qualification: MCA

    Experience: Enough knowledge in QA 2 5year Experience in testing   more..

    Jaafer

    Mobile: +91 91884 77559
    Location: Coimbatore, Online (Surat)
    Qualification: B com e-commerce

    Experience: Having knowledge with some seo off page works like blog commenting directory submission classified ads Quora and handling social media  more..

    Prashant

    Mobile: +91 89210 61945
    Location: Maharashtra, Online (Surat)
    Qualification: Bachelor degree and IT certification

    Experience: Knowledge in Linux system admin and hardware & networking hands on experience   more..

    Samiksha

    Mobile: +91 91884 77559
    Location: Maharashtra, Online (Surat)
    Qualification: bachelor's

    Experience: i'm a fresher looking for an internship with a proper skillset for frontend development and problem solving skills   more..

    Adwaitha

    Mobile: +91 98474 90866
    Location: Kerala, Online (Surat)
    Qualification: Btech in electronics and communication

    Experience: Speaking skills like hindi English Malayalam tamil languages also have strong background skills in software testing specialist  more..

    Success Stories

    The enviable salary packages and track record of our previous students are the proof of our excellence. Please go through our students' reviews about our training methods and faculty and compare it to the recorded video classes that most of the other institutes offer. See for yourself how TechnoMaster is truly unique.

    Photos of Training / Internships

    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat
    Internship/projects in surat

    Trained more than 10000+ students who trust Nestsoft TechnoMaster

    Get Your Personal Trainer