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Iterator.concat()

Iterator.concat(...iterables), added in ECMAScript 2026, lazily sequences any number of iterables one after another into a single Iterator (which also has every helper method from 17): the first iterable is fully consumed before the second one is even touched, and so on, without ever materializing an intermediate array the way [...a,...b] would.

function* naturals() {
  let n = 1;
  while (true) yield n++;
}

const seq = Iterator.concat(['a', 'b'], new Set([1, 2]), naturals());
console.log(seq.take(6).toArray());

function* naturals() {
  let n = 1;
  while (true) yield n++;
}

const seq = Iterator.concat(['a', 'b'], new Set([1, 2]), naturals());
console.log(seq.take(6).toArray());