JavaScript provides generator functions expressed as function*, where we can pause and later resume the function execution at the middle points. At these points we use the yield keyword. In other words, it makes no sense at all to create generator functions that contain no yield keyword, since such functions could be written as normal functions.

Invalid:

function* f1() {
  return "f1";
}

Valid:

function* f1() {
  yield "f1";
}

// generator function with empty body is allowed
function* f2() {}

function f3() {
  return "f3";
}