Whether or not the return is required depends upon how the arrow function is declared. The right-hand side of an arrow function can be an expression or a brace-enclosed block of statements. When an expression is used, it’s evaluated and implicitly returned — no return is required:

const source = Observable.defer(() => Observable.of(42));

When a block is used, an explicit return is required:

const source = Observable.defer(() => {
  return Observable.of(42);
});

For more information on arrow functions, see the MDN documentation.