无法在角度6中设置未定义的属性“title”

您需要初始化 typeOfProofObj 首先是变量,然后尝试访问它的属性。

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  name = 'Angular';
  typeOfProofObj: DropdownModelWithtitle = { title: null, dropDownOptions: null };

  constructor() {
     this.typeOfProofObj.title = "ss";
     this.typeOfProofObj.dropDownOptions = [ {displayName: 'ss', value: 'sss'} ];    
  }

}

您可以将DropDownModelWithTitle从接口更改为类, 并将typeofProofObj设置为新的DropDownModelWithTitle()。

interface DropDownModel {
  displayName: string;
  value: string;
 }

 class DropdownModelWithtitle {
  title: string;
  dropDownOptions: DropDownModel[];
 }

 @Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
  })
 export class AppComponent  {
   name = 'Angular';
   typeOfProofObj = new DropdownModelWithtitle();
   constructor(){
     this.typeOfProofObj.title = "ss";
     this.typeOfProofObj.dropDownOptions = [{displayName:'ss',value:'sss'}];

 }

}