title: React this.state is undefined?
body: <p>I am following a beginner tutorial from Pluralsight, on form submit a value is passed to <code>addUser</code> component method and I need to push userName to <code>this.state.users</code> but I get error </p>
  
  <pre><code> App.jsx:14 Uncaught TypeError: Cannot read property 'users' of undefined
  </code></pre>
  
  <p>Component</p>
  
  <pre><code>import React from 'react'
  import User from 'user'
  import Form from 'form'
  
  class Component extends React.Component {
      constructor() {
          super()
          this.state = {
              users: null
          }
      }
      // This is triggered on form submit in different component
      addUser(userName) { 
          console.log(userName) // correctly gives String
          console.log(this.state) // this is undefined
          console.log(this.state.users) // this is the error
          // and so this code doesn't work
          /*this.setState({
              users: this.state.users.concat(userName)
          })*/
      }
      render() {
          return (
              &lt;div&gt;
              &lt;Form addUser={this.addUser}/&gt;
              &lt;/div&gt;
              )
      }
  }
  
  export default Component
  </code></pre>