Error Unable to access React instance this inside event handler

Question

I am writing a simple component in ES6 (with BabelJS), and functions this.setState is not working.

Typical errors include something like

Cannot read property 'setState' of undefined

or

this.setState is not a function

Here is the code:

import React from 'react'

class SomeClass extends React.Component {
  constructor(props) {
    super(props)
    this.state = {inputContent: 'startValue'}
  }

  sendContent(e) {
    console.log('sending input content '+React.findDOMNode(React.refs.someref).value)
  }

  changeContent(e) {
    this.setState({inputContent: e.target.value})
  } 

  render() {
    return (
      <div>
        <h4>The input form is here:</h4>
        Title: 
        <input type="text" ref="someref" value={this.inputContent} 
          onChange={this.changeContent} /> 
        <button onClick={this.sendContent}>Submit</button>
      </div>
    )
  }
}
export default SomeClass

How to solve this error?

Answer 1

Hello @kartik,

You can use an arrow function together with the class properties proposal:

class SomeClass extends React.Component {
  changeContent = (e) => {
    this.setState({inputContent: e.target.value})
  } 

  render() {
    return <input type="text" onChange={this.changeContent} />;
  }
}

Because the arrow function is declared in the scope of the constructor, and because arrow functions maintain this from their declaring scope, it all works. The downside here is that these wont be functions on the prototype, they will all be recreated with each component. However, this isn't much of a downside since bind results in the same thing.

Hope this works!!

If you need to know more about React, Its recommended to join React JS Course today.

Thank You!!