问角度管道未从嵌套的ngFor返回数组

EN

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
 name: 'sortPipe'
})
export class SortPipePipe implements PipeTransform {
 listArr = []  
  transform(value: any[], args?: string): any[] {
  value.forEach((val, index) => {
    if (val.group.indexOf(args) > -1) {
     console.log(val); // this retuning object and not array
     listArr.push(val); // not allowed
    }
  });
  retune val ; //this is object not array
 });
 return null;
 }
}