Im new to jamsine karma. I want to unit test case for below function, but it never covering whole function at all.after pipe its not covering

I have tried multiple ways . but not got any luck

getDetails() {
    this.spinner.show();
    const myTimer = timer(1000,120000);
    this.sub = myTimer
    .pipe(
      mergeMap( (): any => this.commonService.details())
      ).subscribe(
      (res: any) => {
        if (!res?.items) {
          this.noRecord = true;
        } else {
          this.tableData = res.items;
          res.items.forEach((element: any) => {
            if (element.startTime && element.endTime) {
              element['duration'] = this.calculateDuration(element.startTime, element.endTime);
            } else {
              element['duration'] = 'NA';
            }
            if(element.creationTime){
              element['elaspedTime'] = this.getElapsedTime(element.creationTime);
            }
          });
          this.noRecord = false;
          this.spinner.hide();
        }
      },
      (error: Error) => {
        this.spinner.hide();
        this.noRecord = true;
      }
    )
  }