Can I have a class like this?

class Human {
  constructor(name) {
    this.name = name;
  }
  someMethod() {
    this.hasBrain = true;
    console.warn(this.hasBrain);
  }
}

This is working and not throwing any error. My question is, now, is hasBrain a class property or it is a class field? if it is a class field, Does it have to be declared?