JavaScript error "Super Expression must either be null or a function"

I just got this error when working on some TypeScript code.

Uncaught TypeError: Super Expression must either be null or a function

The line for the error pointed to the constructor of a class. What’s happening? Circular dependencies it turns out.

// in foo.ts
class Foo {
  foo() {
    new Bar().doSomething();
  }
}

// in bar.ts
class Bar extends Foo {
  // ...
}

It’s obvious when boiled down like this, but it’s something you’ll want to make sure to avoid. I solved this issue by making Bar not extend Foo. It added little to no duplicated code for me in this case, so just separating the classes was easy enough.

JavaScript error “Super Expression must either be null or a function”