javascript - 无法分配给对象的只读属性
问题描述
#interestingProblem 任何人都可以解释一下,我在更新第一个代码块中的状态时遇到问题,但是当我更新状态时没有问题,如下面的第二个代码块。
我有一个问题:(不能分配给对象数量的只读属性)
const newItem = action.payload
newItem.quantity = 1
state.items = [...state.items, newItem]
当我写这样的代码时我没有问题
const newItem = action.payload
state.items = [...state.items, { ...newItem, quantity: 1 }]
解决方案
第一种方法是action.payload
直接变异,因为您不是创建副本newItem
而是传递相同的引用。鉴于action.payload
是只读的,您将面临错误:
// passing the same reference, 'newItem' points to 'action.payload'
// hence newItem is not copy
const newItem = action.payload
// here you mutate 'action.payload' since 'newItem' points to same reference
newItem.quantity = 1
state.items = [...state.items, newItem]
第二种方法有效,因为您正在创建一个action.payload
不变异的副本:
// here 'newItem' still points to same reference 'action.payload'
const newItem = action.payload
// but here you are spreading the values into a new object, not mutating directly
state.items = [...state.items, { ...newItem, quantity: 1 }]
相反,您应该首先为您的工作方法创建一个副本:
// here you create a new object from 'action.payload''action.payload'
// hence newItem contains the same values but it's a different object
const newItem = { ...action.payload }
// now you are not mutating 'action.payload', only 'newItem' that's a new object
newItem.quantity = 1
state.items = [...state.items, newItem]