Use class fields

[cjolowicz]

This requires a Babel plugin:

• https://babeljs.io/docs/en/babel-plugin-proposal-class-properties

yarn add --dev @babel/plugin-proposal-class-properties

.babelrc

{
  "plugins": ["@babel/plugin-proposal-class-properties"]
}

Class fields are a TC39 proposal:

• https://github.com/tc39/proposal-class-fields
• https://tc39.github.io/proposal-class-fields/

Additional documentation:

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes#Field_declarations