If using ES6 `extends`, call `super()` before accessing `this`

I am working on rewriting some code that used an ES5 “Class” helper, to use actual ES6 classes.

I soon stumbled upon a weird error in which apparently valid code would be throwing an |this| used uninitialized in A class constructor error:

class A extends B {
  constructor() {
    this.someVariable = 'some value'; // fails
  }
}

I was absolutely baffled as to why this was happening… until I found the answer in a stackoverflow post: I had to call super() before accessing this.

With that, the following works perfectly:

class A extends B {
  constructor() {
    super(); // ☜☜☜ ❗️❗️❗️
    this.someVariable = 'some value'; // works!
  }
}

Edit: filed a bug in Firefox to at least get a better error message!