javascript - 无法分配给对象的只读属性

Question

#interestingProblem 任何人都可以解释一下吗🤔 我在第一个代码块中更新状态时遇到问题，但是当我在第二个代码块中更新状态时没有问题，如下所示。

我有一个问题:(无法分配给对象数量的只读属性)

const newItem = action.payload
newItem.quantity = 1
state.items = [...state.items, newItem]

我写这样的代码没问题

const newItem = action.payload
state.items = [...state.items, { ...newItem, quantity: 1 }]

Answer 1

第一种方法是直接改变 action.payload，因为您没有创建 newItem 的副本，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，因为您是从 action.payload 创建副本而不是对其进行变异:

// 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]