SimpleClass.js

class MyClass {
  str = "test string"; // not an error in babel + core-js
  static x = 0; // not an error in babel + core-js
  
  constructor() {
    this.str = "test string";
    this.x = 0;
    console.log(this.str);
    console.log(this.x);
  }
}
// obviously outside of class scope.

TOLD YOU SO.