> how can you know what happens when you call it?

Documentation, good API design. Real-world example of a function I call pretty often (which I’m also interested in knowing your better alternative to):

  const extend = (dest, from) => {
      for (const x of from) {
          dest.push(x);
      }
  };

It doesn’t return anything, so I’d say it’s pretty clear its entire purpose is to mutate one argument.

(Non-stack-overflowing version of dest.push(...x), by the way.)