What does the * (star/asterisk) syntax after a yield mean in a recursive generator function? [duplicate]

Question

Let's say I've created an ES6 generator

function *createFibonacciIterator(a = 0, b = 1) {
  yield b;
  yield *createFib(b, b + a); // <== QUESTION IS ABOUT THIS LINE
}

Then I use that generator to get the first 20 results

let fibber = createFibonacciIterator();
for (let ii = 0; ii < 20; ii++) {
    console.log(fibber.next());
}

If I leave the * out of the yield *createFib(b, b + a); line things break, which makes sense because I don't want yield an iterator but an actual value.

What's the technical meaning of * in the generator?

Answer 1

When the * is used in the function declaration this mean that it is a generator function.

But when it is used as yield *myGeneratorFunction(), the definition of the Ecmascript 262 specification, Section 14.4.14, says that the engine tries to resolve the generator function calling the next() method of iterator returned by the generator function.

When the yield is used without the * (for example, yield createFibonacci()), it returns the value of the expression after the yield. In the example is the value returned of the createFibonacci.

Answer 2

For most cases

yield *smth;

makes the same thing as

for (let x of smth) {
  yield x;
}