属性“productId”没有初始值设定项，并且未在构造函数中明确分配

Question

我正在使用 Angular 和节点，但收到此错误“属性 'productId' 没有初始值设定项并且未明确分配 在构造函数中。” for productForm: FormGroup,model: Product,title: string,productId: number 这是我创建产品的代码-

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup, Validators} from '@angular/forms';
import { ProductService } from '../services/product.service';
import { Product } from '../models/product';
import { ActivatedRoute, Router } from '@angular/router';

@Component({
  selector: 'app-create-product',
  templateUrl: './create-product.component.html',
  styleUrls: ['./create-product.component.css']
})
export class CreateProductComponent implements OnInit {

  productForm: FormGroup;
  model: Product;
  title: string;
  productId: number;

  constructor(
    private fb: FormBuilder,
    private productService: ProductService,
    private route: ActivatedRoute,
    private router: Router
  ) { }

  ngOnInit() {
    this.title = "Create Product";
    this.createForm();

    // edit product
    this.productId = +this.route.snapshot.paramMap.get('id');
    if(this.productId) {
      this.getProduct();
    }
  }