How to sequentially chain two sync iterators in JavaScript?

Let's say we have two async iterators,

const asyncIterable1 = {
  [Symbol.asyncIterator]() {
    return {
      i: 0,
      next() {
      if (this.i < 3) {
        return Promise.resolve({ value: this.i++, done: false });
      }

      return Promise.resolve({ done: true });
    }
  };
 }
};

const asyncIterable2 = {
  [Symbol.asyncIterator]() {
    return {
      i: 3,
      next() {
      if (this.i < 5) {
        return Promise.resolve({ value: this.i++, done: false });
      }

      return Promise.resolve({ done: true });
    }
  };
 }
};

Now, is there a way to combine these two iterators into one iterator that would return a sequence of 0,1,2 and then 3,4?

Yeah, I'd use yield* for that:

    const combine = (a, b) => (function* () { yield* a; yield* b; })();
    
    const iterator = combine(
      (function* () { yield 1; yield 2; })(),
      (function* () { yield 3; yield 4; })()
    );
    
    console.log(iterator.next(), iterator.next(), iterator.next(), iterator.next(), iterator.next());

This works analogously for async iterators. You'll loose the return value ("the done yield") of the first iterator though. You could capture it however (the value yield* evaluates to).

For sure if you're among the people that like to reinvent wheels, you can also implement such functionality "by hand" without generator functions:

function combine(...iterators) {
  let pos = 0, iterator = iterators[pos];
  return {
    next() {
      let result = { done: true };
      do {
        result = iterator.next();
        if(!result.done) break;
        iterator = iterators[ pos++ ];
      } while(iterator)
          
      return result;
    }
  };
}

 const iterator = combine(
          (function* () { yield 1; yield 2; })(),
          (function* () { yield 3; yield 4; })()
 );
        
 console.log(iterator.next(), iterator.next(), iterator.next(), iterator.next(), iterator.next());