Generator: return() done

function* gen() { yield 1; yield 2; yield 3 }
const it = gen()
console.log(it.next())       // → { value: 1, done: false }
console.log(it.return(99))   // → { value: 99, done: true }
console.log(it.next())       // → { value: undefined, done: true }

generator.return() terminates the generator early and returns the provided value with done set to true.