TypeError：无法读取 null 的属性（读取“数量”）

Question

shopping-cart.service.ts

async addToCart(product:Product){
    this.updateItem(product,1)
}

async removeFromCart(product:Product){
    this.updateItem(product, -1)
}
private getItem(cartId: string, productId: string) {
    return this.db.object('/shopping-carts/' + cartId + '/items/' + productId);
}
private async getOrCreateCartId():Promise<string>{
    let cartId = localStorage.getItem('cartId')
    if(cartId) return cartId;

    let result = await this.create();
    localStorage.setItem('cartId',result.key!);
    return result.key!;
}

private async updateItem(product:Product,change:number){
    let cartId = await this.getOrCreateCartId();
    let item$ = this.getItem(cartId,product.key);
    item$.valueChanges().take(1).subscribe((item:any)=>{
    let quantity = (item.quantity || 0) + change;  // error
    if (quantity === 0) item$.remove();
    else  item$.update({
     title:product.title, 
     imageUrl:product.imageUrl, 
     price: product.price,
     quantity:quantity
   })
 })
}

错误：-
在控制台中出现此错误时，它会在数量为零时从购物车中删除商品（购物车中已经存在的产品），但是当我尝试将商品添加到购物车时，它不允许我将产品添加到购物车并在控制台中向我显示此错误

ERROR TypeError: Cannot read properties of null (reading 'quantity')
    at SafeSubscriber._next (shopping-cart.service.ts:61)
    at SafeSubscriber.__tryOrUnsub (Subscriber.js:183)
    at SafeSubscriber.next (Subscriber.js:122)
    at Subscriber._next (Subscriber.js:72)
    at Subscriber.next (Subscriber.js:49)
    at TakeSubscriber._next (take.js:35)
    at TakeSubscriber.next (Subscriber.js:49)
    at MapSubscriber._next (map.js:35)
    at MapSubscriber.next (Subscriber.js:49)
    at Notification.observe (Notification.js:20)

Answer 1

我认为这是一个异步等待问题

let item$ = this.getItem(cartId,product.key);

所以项目未定义，这就是为什么你有properties of **null**