Things to clear up before jumping into the world of React

public-class-fields.md

5. Public Class Fields in JS

I have found a collection of blogs that will help you understand it. The links are as follows:

1. Why is it important? (the use case)
2. Public Class Fields in React
3. Implementation example

Also, here is a simple explanation:

class User extends React.Component {
  state = {
    username: 'Tyler'
  } 
}

// rather than

class User extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      username: 'Tyler'
    }
  }
}

This is slightly different from Facebook's Setting the Initial State docs.

Having state outside the constructor() means it is a class field, which is a proposal for a new change to the language. It currently isn't supported by JavaScript, but thanks to Babel's fantastic powers of transpiling, we can use it!