问未定义的属性，该属性在所有警报点都已填充

EN

login(surName: string, passWord){    
    const name = this.members.find( x => x.surname === surName)
    const pass = this.members.find( x => x.password === passWord)
    if(name && pass){
      for (const element of this.members) {
        if(element.surname === surName){
            alert(element.surname + surName);
            this.persist.setPersistence(surName, element.color);
            alert("You are Logged in" + element.surname + element.color);
            this.router.navigate(['/dashbaord']);
            return;
        }
    }
    }else{
      alert("User Details are incorrect please make sure your username and password is correct");
  }
}

import { Injectable } from '@angular/core';
import { Persistance} from '../Persistance/persistance.model'

@Injectable({
  providedIn: 'root'
})
export class PersistService {
  persist: Persistance;
  constructor() { }

  setPersistence(surname : string, color: string){
    alert(surname);
    this.persist.surname = surname;
    this.persist.color = color;
  }
}

core.js:4197 ERROR TypeError: Cannot set property 'surname' of undefined