With this.test(), you reference the instance. No test property exists directly on the instance, so as with standard prototypal inheritance, the prototype object is examined for the property. This prototype is A.prototype, which does have a test property, so that gets called.

With super.test(), you instead start looking for the test property on the superclass - A.prototype, which is found immediately and called.

If you had a test method on B as well, you'd see the difference.

class A {
  data1 = 1;
  constructor(){
    this.data2 = 2;
  }
  test() { console.log('a') }
}
class B extends A {
  constructor() {
    super();
  }
  test() {
    console.log('b');
  }
  funcA() { this.test() }
  funcB() { super.test() }
}
const foo = new B()
foo.funcA();
foo.funcB();

BTW, what's the difference between data1 and data2 declared in and out the constructor, which is preferred

It's up to you. Though, class fields (assigning outside of the constructor) are slightly more concise (especially if your constructor doesn't contain anything else) but less supported - if you want to use them, make sure to transpile your code.