Set State

demo.js

import React, { Component } from 'react'

export default class Test extends Component {
    // if you're using flow or any sort of type checker, you can declar proptypes up here
    state: {
      value: string
    }

    // "The constructor method is a special method for creating and initializing an object created with a class." - MDN
    constructor() {
      super()

      // set your initial state in the constructor
      this.state = {
        value: ''
      }
    }

    onChange = (e) => {
      e.preventDefault()

      // a different way of setting state
      const value = {
        ...this.state.value, 
        value: e.target.value
      };
      this.setState({ value });
    }

    render() {
      // I like to extract props and state values before the return, but that's up to you
      const { value } = this.state

      return <input value={ value } onChange={ this.onChange } />
    }
}