Why does babel swear at this class syntax?

Question

JavaScript

Why babel swears at a line test = null;
In the browser, such an entry works and does not give errors, but babel does not approve.
Recording static test = null;is also not acceptable.

class Test {
  test = null;
  showTest() {
    console.log(this.test);
  }
}

What is the correct way to write a property?

Answer 1

Try adding to babel.config.js

module.exports = {
  plugins:  [
    '@babel/plugin-proposal-class-properties'
  ]
}

Well, install, of course, this package

Answer 2

Write properties through a methodconstructor()