getSorters.ts

interface SorterObj {
  sorts: string[][],
  addSort(by?: string, how?: string): SorterObj;
}

const getSorter = function(): SorterObj {
  const modal: SorterObj = {
    sorts: [],
    addSort: function(this: SorterObj, by: string, how?: string): SorterObj {
      if (by) {
        if (how) {
          this.sorts.push([
            by,
            how === 'asc' ? 'ASC' : 'DESC'
          ])
        } else {
          this.sorts.push([by, 'DESC'])
        }
      }
      return this;
    }
  };
  return modal;
}

const sorter = getSorter()
                  .addSort('id')
                  .addSort('xx', 'asc')