Angular /如何将获取的数据从字符串转换为数组

我在对话框中有编辑表单，但是要接收的用于选择的api数据是字符串形式而不是数组形式，所以出现此错误：

错误：值必须是多选模式下的数组。

campaign.ts

  ngOnInit(): void {
    this.fetchdata();
  }

  fetchdata() {
    this.campaignService.sendGetRequest()
    .subscribe((data: any[])=>{
      this.campaigns = data;
    })
  }

  onEdit(item){
    this.campaigns.findIndex(i => i.id == item.id);
    const dialogRef = this.dialog.open(EditComponent,{
          data: item
        });
  }

编辑

this.editForm = this.fb.group({
  Name: this.data.name,
  SmsText: [this.data.smsText,  [Validators.required]],
  SmsSendDate: [this.data.smsSendDate,  [Validators.required]],
  SmsSendHoursRange: this.fb.group({
    start: [this.data.smsSendHoursRange.start,  [Validators.required]],
    end: [this.data.smsSendHoursRange.end,  [Validators.required]],
  }),
  Branch: [this.data.branch],
  InspectionResult: [this.data.inspectionResult],
  Categories: [this.data.categories],
  DateOfIssue: [this.data.dateOfIssue],
  Brand: [this.data.brand],
  VehicleCategory: [this.data.vehicleCategory],
  OwnerStatus: [this.data.ownerStatus],
  IsPhys: [this.data.isPhys],
  IsJRDC: [this.data.isJRDC]
});

// get centers
this.campaignService.getCenters().subscribe((data: any[])=>{
  this.centers = data;
})
// get vehicles
this.campaignService.getCategories().subscribe((data: any[])=>{
  this.categories = data;
})

edit.html

  <div class="column">
    <mat-form-field appearance="fill">
      <mat-label>Car category</mat-label>
      <mat-select multiple formControlName="Categories">
        <mat-option *ngFor="let item of categories" [value]="item.id">{{item.description}}</mat-option>
      </mat-select>
    </mat-form-field>
  </div>
<div class="columns">
  <mat-form-field appearance="fill">
    <mat-label>Centers</mat-label>
    <mat-select multiple formControlName="Branch">
      <mat-option *ngFor="let item of centers" [value]="item.centerCode">{{item.address}}</mat-option>
    </mat-select>
  </mat-form-field>
</div>

这是我想在数组中进行转换以供选择的api响应

branch: "G25,G22"
brand: "OPEL, SUBARU"
inspectionResult: "1"
...............