How to bind ‘this’ keyword to resolve classical error message ‘state of undefined’ in React?

The ‘this’ keyword in JavaScript is always being somewhat tough to understand for learners. Basically this keyword inside a function is determined by looking at how the method is actually invoked. Usually in JavaScript, we invoked the method using syntax obj.method(). In this case, the value of ‘this’ keyword inside the method function is the object ‘obj’. How we invoked the method is all root of the unpredictable value of ‘this’ often.

In React when we use event handlers we basically give a reference of a function to that event handler and when that event occurs that function gets invoked, here’s a catch, it’s invoked at some point of time not immediately, and when its invoked, it’s invoked as its own, there is now any components instance object using which it gets invoked and that’s why the value of ‘this’ keyword inside the method is undefined. To resolve this issue we usually bind the value of ‘this’ keyword using the JavaScript bind method.

Error Message:

Example 1: 

index.js :

Javascript

import React from 'react'
import ReactDOM from 'react-dom'
import App from './App'

ReactDOM.render(<App />, document.querySelector('#root'))

App.js :

Javascript

import React, { Component } from 'react'

class App extends Component {
  static defaultProps = {
    courseContent : [
      'JSX', 'React Props', 'React State',
      'React Lifecycle Methods',
      'React Event Handlers', 'React Router',
      'React Hooks', 'Readux',
      'React Context'
    ]
  }

  constructor(props){
    super(props)

    // Set initial state
    this.state = {msg : 'React Course', content:''}

    // Binding this keyword of method handleClick
    this.handleClick = this.handleClick.bind(this)
  }

  renderContent(){
    return (
      <ul>
        {this.props.courseContent.map(content => (
          <li>{content}</li>
        ))}
      </ul>
    )
  }

  handleClick(){

    // Changing state
    this.setState({
      msg : 'Course Content',
      content : this.renderContent()
    })
  }

  render(){

    // Reference of handleClick, called at some
    // point of time on its own gets called like
    // handleClick(), thats why this inside
    // handleClick is undefined
    const button = !this.state.content &&
    <button onClick={this.handleClick}>
       Click here to know contents!
    </button>
    return (
      <div>
        <h2>Message :</h2>



<p>{this.state.msg}</p>






<p>{this.state.content}</p>



        {button}
      </div>
    )
  }
}

export default App

Output :

Example 2:

index.js :

Javascript

import React from 'react'
import ReactDOM from 'react-dom'
import App from './App'

ReactDOM.render(<App />, document.querySelector('#root'))

App.js :

Javascript

import React from 'react'
import TodoList from './TodoList'

const App = () => {
  return <TodoList />
}

export default App

TodoList.js :

Javascript

import React, { Component } from 'react'
import TodoForm from './TodoForm'

class TodoList extends Component{
  constructor(props){
    super(props)
    this.state = { todos : [] }
    // Binding this keyword of method createTodo
    this.createTodo = this.createTodo.bind(this)
    // Binding this keyword of method renderTodo
    this.renderTodos = this.renderTodos.bind(this)
  }

  createTodo(todo){
    this.setState({
      todos : [...this.state.todos, todo]
    })
  }


  renderTodos(){
    const todos = this.state.todos.map(todo => (
      <li key={todo}>
        {todo.task}
      </li>
    ))
    return <ul>{todos}</ul>
  }

  render(){
    return(
      <div>
        <h1>Todo List</h1>
        {this.renderTodos()}
        <TodoForm create={this.createTodo}/>
      </div>
    )
  }
}

export default TodoList

TodoForm.js :

Javascript

import React, { Component } from 'react'

class TodoForm extends Component{
  constructor(props){
    super(props)

    this.state = { task:'' }

    // Binding this keyword of method handleChange
    this.handleChange = this.handleChange.bind(this)

    // Binding this keyword of method handleSubmit
    this.handleSubmit = this.handleSubmit.bind(this)
  }

  handleChange(event){
    this.setState({
      [event.target.name] : event.target.value
    })
  }

  handleSubmit(event){
    event.preventDefault()
    this.props.create(this.state)
    this.setState({ task : '' })
  }

  render(){
    return(
      <form onSubmit={this.handleSubmit}>
        <div>
          <label htmlFor='task'>New Todo</label>
          <input
            name='task'
            value={this.state.task}
            onChange={this.handleChange}
          />
          <button>Add New Todo</button>
        </div>
      </form>
    )
  }
}

export default TodoForm

Output :