Generators: function* and yield

What a generator gives you

A function* (note the asterisk) returns a generator — an object that's BOTH an iterator AND iterable. Each yield produces the next value; execution pauses there and resumes on the next .next() call.

function* range(from, to) {
  for (let n = from; n <= to; n++) yield n;
}

for (const n of range(1, 3)) console.log(n); // 1, 2, 3
[...range(1, 3)];                             // [1, 2, 3]

Same result as the Symbol.iterator object in C6-03 — five lines instead of fifteen.

Yielding sequences

yield can be called any number of times, in any pattern:

function* greet() {
  yield "hi";
  yield "there";
  yield "friend";
}

Each yield is a checkpoint. The generator's runtime saves the function's local state between calls — that's why you can have an explicit for loop with yield inside it.

return inside a generator

return value finishes the generator immediately. Subsequent .next() calls return { value: undefined, done: true }.