chunk large object arrays to string

arr2chunks

# chunk array to string
private *arr2chunks(arr: any[], n: number) {
  for (let i = 0; i < arr.length; i += n) {
    const str = JSON.stringify(arr.slice(i, i + n))
    const open = i === 0 ? '[':'';
    const close = n*(i+1) < arr.length ? ',' : ']';
    yield `${open}${str.slice(1, str.length-1)}${close}`;
  }
  yield null;
}