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 Cochin

    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 Cochin, chennai and europe countries. You can find many jobs for freshers related to the job positions in Cochin.

    • 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 Cochin
    Mern Stack Development Students will be able to: Store data in NoSQL, develop a document-oriented MongoDB database that provides performance and scalability,. js developers is less compared to other technologies as it has come to common in recent years. js for developing web applications. By the end of this hands-on intensive training course. js you are ready to execute multiple actions at the same time. Node Package Manager (NPM) is one of all the world’s prime package registries. js are generally preferred. React has lots of advantages, the main reason for people to choose react is its faster performance and good interface. Work with streams as well as events to improve I / O non-blocking, rest Application creation, etc. key-value pairs.

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

    Vinay

    Mobile: +91 9446600368
    Location: Uttar Pradesh, Online (Cochin)
    Qualification: B.tech

    Experience: "Proficient in Java C++ Python Skilled in React js front-end development and web scraping Extensive experience as a Subject Matter  more..

    Shelke

    Mobile: +91 91884 77559
    Location: Maharashtra, Online (Cochin)
    Qualification: Diploma in mechanical engineering

    Experience: I have experience in Auto cad catia for product design and also I'm working on product base company i have  more..

    Anusree

    Mobile: +91 9895490866
    Location: Kochi, Online (Cochin)
    Qualification: MCA

    Experience: Certificate in software testing   more..

    Vidatri

    Mobile: +91 91884 77559
    Location: Delhi, Online (Cochin)
    Qualification: MCA

    Experience: I have knowledge on HTML CSS wordpress Bootstrap etc I have approx 1 5 yrs of experience as wordpress developer  more..

    Mumtaz

    Mobile: +91 98474 90866
    Location: Karnataka, Online (Cochin)
    Qualification: B arch

    Experience: I have 2 years of experience as a architect and interior design   more..

    Jeevan

    Mobile: +91 9446600368
    Location: Karnataka, Online (Cochin)
    Qualification: Bachelor of Engineering

    Experience: Autocad matlab Keli-microversion core java MySQL automation testing manual testing  more..

    Sonika

    Mobile: +91 98474 90866
    Location: Dehradun, Online (Cochin)
    Qualification: B.Tech CSE

    Experience: 2 5 years of experience in manual and automation testing Skills include Selenium java functional regression smoke retesting defect management  more..

    Alen

    Mobile: +91 9446600368
    Location: Kochi, Online (Cochin)
    Qualification: Bsc computer science

    Experience: Python django angular html css restAPI MySQL I have 6 months internship training program on python django angular Full stack  more..

    Jerlin

    Mobile: +91 89210 61945
    Location: Kerala, Online (Cochin)
    Qualification: Bca

    Experience: Manual and automation testing  more..

    Parvathy

    Mobile: +91 89210 61945
    Location: Kerala, Online (Cochin)
    Qualification: Bsc computer science

    Experience: Hi I'm Parvathy Rajesh My resume are attached to this email for your consideration For the past one year I  more..

    Praseeesh

    Mobile: +91 9446600368
    Location: Kerala, Online (Cochin)
    Qualification: plus two

    Experience: Results-oriented Python Django Developer with extensive experience in building scalable web applications and optimizing backend performance Proficient in leveraging Django's  more..

    Tejas

    Mobile: +91 9895490866
    Location: Maharashtra, Online (Cochin)
    Qualification: BE/BTECH

    Experience: I am experienced with python html css and been a part of college cultural group i am excited to work  more..

    Sudhanshu

    Mobile: +91 9895490866
    Location: Karnataka, Online (Cochin)
    Qualification: B.Tech

    Experience: I am Sudhanshu Ranjan a Software Developer at Tech Martin Pvt Ltd with 2 months of internship experience I have  more..

    Aniket

    Mobile: +91 9446600368
    Location: Maharashtra, Online (Cochin)
    Qualification: BE civil

    Experience: I am a structural designer at a firm Perform design and gives working drawings for site execution And i have  more..

    Khushboo

    Mobile: +91 91884 77559
    Location: Mumbai, Online (Cochin)
    Qualification: Master’s in Science

    Experience: 1 year 3 months of total experience in Banking Domain with IDFC First Bank with below skill set Manual testing  more..

    Atul

    Mobile: +91 9895490866
    Location: Uttar Pradesh, Online (Cochin)
    Qualification: B Tech

    Experience: Fresher with skills ready for interview Java Html Css JavaScript Mysql Software testing  more..

    Bharath

    Mobile: +91 98474 90866
    Location: Haidarabad, Online (Cochin)
    Qualification: Graduation

    Experience: Python Full Stack Developer in R Square Media I have 2 6 years of experience on python full developer |  more..

    Vishnu

    Mobile: +91 91884 77559
    Location: Hyderabad, Online (Cochin)
    Qualification: MCA

    Experience: I had done software testing course I have knowledge on manual corejava selenium sql | Resume for   more..

    pradeep

    Mobile: +91 8301010866
    Location: Bihar, Online (Cochin)
    Qualification: Diploma

    Experience: I have 2 years experience in WordPress php development and 8 years experience in web designing  more..

    Bobby

    Mobile: +91 89210 61945
    Location: BHOPAL, Online (Cochin)
    Qualification: B.tech(cse)

    Experience: python Sap Abap Django mysql css html 1 5 year experience in sap abapApplication for Python Django  more..

    Soumodeep

    Mobile: +91 9895490866
    Location: West Bengal, Online (Cochin)
    Qualification: bsc in cyber security and netwirking

    Experience: ethical hacking vapt pen-testing linex Bharat os parrot ubuntu web security  more..

    Akshara

    Mobile: +91 9446600368
    Location: Kerala, Online (Cochin)
    Qualification: MCA

    Experience: Programming in c c++ Java php etc 6month experience in web development  more..

    Srishti

    Mobile: +91 91884 77559
    Location: Delhi, Online (Cochin)
    Qualification: French graduate

    Experience: I'm applying for this role because I strongly believe I have the necessary skills qualities and experience to carry out  more..

    Rakshitha

    Mobile: +91 91884 77559
    Location: Karnataka, Online (Cochin)
    Qualification: Bsc

    Experience: Adobe photoshop Illustrator In design |   more..

    BHUSHAN

    Mobile: +91 8301010866
    Location: Nagpur, Online (Cochin)
    Qualification: B.E

    Experience: i have good knowledge of python oops django mysql postgresql and currently i am looking a opportunity in development in  more..

    Sohan

    Mobile: +91 91884 77559
    Location: Rajasthan, Online (Cochin)
    Qualification: B tech CSE

    Experience: I have 5 years in experience in android development I am finding the organization where I can work for a  more..

    Salma

    Mobile: +91 89210 61945
    Location: Thrissur , Online (Cochin)
    Qualification: B. Tech

    Experience: I am currently pursing my python full stack internship in inmakes infotech and acquires deep knowledge in machine learning too  more..

    Sonali

    Mobile: +91 8301010866
    Location: Maharashtra, Online (Cochin)
    Qualification: M. Sc. Computer science

    Experience: Manual testing java Sql  more..

    Nutan

    Mobile: +91 9446600368
    Location: Maharashtra, Online (Cochin)
    Qualification: Btech

    Experience: Using Jira and Confluence for testing and wireshark and postman for network tresses Sql used in between   more..

    Mridul

    Mobile: +91 91884 77559
    Location: Gujarat, Online (Cochin)
    Qualification: Bachelor of Engineering in Computer Engineering

    Experience: SKILLS Flutter Firebase State Manangement - Provider GetX Google Maps API TFLite ReactJS NodeJs HTML CSS Javascript SQL MongoDB GIT  more..

    Nandamuri

    Mobile: +91 9895490866
    Location: Visakhapatnam, Online (Cochin)
    Qualification: M.Tech

    Experience: python data science artificial intelligence machine learning| Application for Python Data Science Machine Learning Artificial Intelligence  more..

    Shirisha

    Mobile: +91 98474 90866
    Location: Hyderabad, Online (Cochin)
    Qualification: Bsc computer science

    Experience: I have 2 years of experience in manual testing and knowledge on SQL queries and for test management tool we  more..

    Apurv

    Mobile: +91 89210 61945
    Location: Maharashtra, Online (Cochin)
    Qualification: Btech in Computer Engineering

    Experience: python web development aws cloud django framework cpp  more..

    Sohan

    Mobile: +91 91884 77559
    Location: Haryana, Online (Cochin)
    Qualification: Mca

    Experience: I am basically from haryana I am currently staying in Mohali I have 6 month experience in php   more..

    Kanchan

    Mobile: +91 98474 90866
    Location: Maharashtra, Online (Cochin)
    Qualification: Bachelor's of fine arts

    Experience: With four years of experience as a graphic designer I have honed my skills in creating visually compelling designs using  more..

    Vineeth

    Mobile: +91 98474 90866
    Location: Alappuzha, Online (Cochin)
    Qualification: B.Tech, CS

    Experience: Python Django HTML5 CSS3 Javascript Booistrap sqlite Web developer at Boilpot Technologies Web app creation and mentoring in web app  more..

    sirajudheeen

    Mobile: +91 9895490866
    Location: tirur, Online (Cochin)
    Qualification: Bca

    Experience: python angular ionic C# Asp Net Django Flask Mysql SqlServer 6 month python internshp 4 month fullstack developerApplication for Meanstack  more..

    Vinnya

    Mobile: +91 98474 90866
    Location: Tamil Nadu, Online (Cochin)
    Qualification: under graduate degree

    Experience: cybersecurity wireshark pentesting firewall network security  more..

    Sandhana

    Mobile: +91 9895490866
    Location: Kerala, Online (Cochin)
    Qualification: Msc cs

    Experience: Work as E learning developer for 1 year   more..

    Sreeramadas

    Mobile: +91 98474 90866
    Location: Telangana, Online (Cochin)
    Qualification: BSC computers

    Experience: Ms office SQL and python   more..

    Sourodip

    Mobile: +91 89210 61945
    Location: West Bengal, Online (Cochin)
    Qualification: B.Tech

    Experience: I have knowledge about c C++ Java SQl python   more..

    Nandini

    Mobile: +91 91884 77559
    Location: Delhi, Online (Cochin)
    Qualification: 12 pass, currently in second year of my bachelor's

    Experience: Teaching experience for 2 yrs as an offline tutor C++ python ms PowerPoint Excel canva team work quality proficiency in  more..

    Jyoti

    Mobile: +91 8301010866
    Location: Palampur Kangra Himachal Pradesh, Online (Cochin)
    Qualification: Diploma in computer science wngg

    Experience: Software testing skills ISTQB foundation level certified tester 9 years software testing experienceApplication for Software Testing  more..

    Rohit

    Mobile: +91 9895490866
    Location: Rajasthan, Online (Cochin)
    Qualification: B-Tech

    Experience: I am very eager to work in software testing or quality analyst as I have a good knowledge in manual  more..

    suraj

    Mobile: +91 98474 90866
    Location: Maharashtra, Online (Cochin)
    Qualification: BCA

    Experience: As Fullstack python developer i am technically capable to developing websites using django framework as well as Rest APIs  more..

    Abhineet

    Mobile: +91 9895490866
    Location: Haryana, Online (Cochin)
    Qualification: Bsc computer science

    Experience: Web developer Microsoft Office c C+ python  more..

    Kirti

    Mobile: +91 9895490866
    Location: Punjab, Online (Cochin)
    Qualification: Graduation

    Experience: I have done digital marketing diploma I have a knowledge of seo blogs n content writing  more..

    Lavanya.D

    Mobile: +91 89210 61945
    Location: Tamil Nadu, Online (Cochin)
    Qualification: BCA

    Experience: Java python photo shop tally ms office Corel draw typing speed ms powerpoint ms xl ms word HTML   more..

    Prathamesh

    Mobile: +91 91884 77559
    Location: Maharashtra, Online (Cochin)
    Qualification: Master's

    Experience: Experienced Flutter and Android Developer with a proven record of launching 5+ successful projects totaling over 500 000 users Skilled  more..

    Shalini

    Mobile: +91 8301010866
    Location: Raipur Chhattisgarh , Online (Cochin)
    Qualification: Bs.c+interior design

    Experience: I have 4 years experience in interior field I know about modular furniture & carpentry work both I know sketchup  more..

    Soumik

    Mobile: +91 89210 61945
    Location: West Bengal, Online (Cochin)
    Qualification: B.tech

    Experience: aws azure devops cicd ansible terraform python shell docker kubernetes git prometheus grafana  more..

    Karthick

    Mobile: +91 91884 77559
    Location: Tamil Nadu, Online (Cochin)
    Qualification: B.Tech(Information technology)

    Experience: My skills are core Java mysql Android app development Linux commands and now I'm currently learning aws cloud partioner certificates  more..

    Laxmi

    Mobile: +91 89210 61945
    Location: Gujarat, Online (Cochin)
    Qualification: B.A

    Experience: Manual testing Basic automation testing Functional testing Non-functional testing Database testing API testing Regression testing Integration testing Unit testing Mobile  more..

    Vaibhav

    Mobile: +91 89210 61945
    Location: Maharashtra, Online (Cochin)
    Qualification: MASTERS IN COMPUTER SCIENCE

    Experience: 3+ years of experience in Flutter development also have a good understanding in Rest Api development using sails js |  more..

    MohanGaneshMunagala

    Mobile: +91 91884 77559
    Location: Bangalore, Online (Cochin)
    Qualification: Bachelors of computer applications

    Experience: Manual testing API Testing database testing automation testing | Resume for   more..

    Anirudh

    Mobile: +91 9895490866
    Location: Uttar Pradesh, Online (Cochin)
    Qualification: B.tech

    Experience: I have good understanding about MEAN Stack development   more..

    Karthiga

    Mobile: +91 98474 90866
    Location: Tamil Nadu, Online (Cochin)
    Qualification: M.sc cs&IT

    Experience: Hai I'm Karthiga I'm a self self motivator and a knowledge seeker ihave a computer knowledge and I'm very much  more..

    M.

    Mobile: +91 9446600368
    Location: Tamil Nadu, Online (Cochin)
    Qualification: BSC cs

    Experience: I had completed Java testing course and now I am working junior HR and business analyst well-known skill Java testing  more..

    Arya

    Mobile: +91 91884 77559
    Location: Idukki, Online (Cochin)
    Qualification: Diploma in computer science

    Experience: I completed a certified course on full stack python in december 2021 Application for Python Django  more..

    Archana

    Mobile: +91 91884 77559
    Location: Kerala, Online (Cochin)
    Qualification: MCA

    Experience: I'm Archana M K and I have 2 years of experience as a Python developer Also have knowledge in languages  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 cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin
    Internship/projects in cochin

    Trained more than 10000+ students who trust Nestsoft TechnoMaster

    Get Your Personal Trainer