orderBy pipe in Angular2

orderBy.ts

@Pipe({
  name: 'orderBy'
})
export class OrderByPipe {
  transform(value: Array<any>, args: any[]): any {
    let field: string = args[0];
    if(value==null) {
      return null;
    }
    if (field.startsWith("-")) {
      field = field.substring(1);
      if (typeof value[field] === 'string' || value[field] instanceof String) {
        return [...value].sort((a, b) => b[field].localeCompare(a[field]));
      }
      return [...value].sort((a, b) => b[field] - a[field]);
    }
    else {
      if (typeof value[field] === 'string' || value[field] instanceof String) {
        return [...value].sort((a, b) => -b[field].localeCompare(a[field]));
      }
      return [...value].sort((a, b) => a[field] - b[field]);
    }
  }
}

The pipe works fine with the last change. Thanks!!