创建使用两个"layers"枚举的强类型嵌套对象的最佳方法，其中不需要所有枚举值？

Question

例如

enum Size {
Small: 'small'
Medium: 'medium'
Large: 'large'
}
enum Weekday {
Monday: 'Monday'
Tuesday: 'Tuesday'
...
}

// Only need Size.Medium and Size.Large, and anywhere from 1 to 7 Weekday entries
const mapping: { [size in Size]?: { [weekday in Weekday]? : MenuType } = {
[Size.Medium] : { Weekday.Tuesday : <menuObject> }
[Size.Large] : { 
Weekday.Monday : <menuObject>
Weekday.Friday : <menuObject>
}
}

当我尝试使用const menu = mapping[Size.Large][Weekday.Monday]获取菜单时，我得到一个错误说Object is possibly undefined。

创建这个映射的最好方法是什么，使它仍然是强类型的?

Answer 1

正如@Linda Paiste所说，这是正常的行为。

当您使用?:操作符时，它表示以下键方式是否被定义(未定义)。

所以当TypeScript遍历你的mapping对象时，不确定Size.Large后面的数据是否会被定义。这就是为什么它会给你一个错误。

---

您的选择是:

---

或者你让每个键都是强制性的，比如:

游乐场

enum Size {
Small = 'small',
Medium = 'medium',
Large = 'large',
};
enum Weekday {
Monday = 'Monday',
Tuesday = 'Tuesday',
Friday = 'Friday',
};
type MenuType = {};
const mapping: {
[size in Size]: { 
[weekday in Weekday] : MenuType;
};
} = {
[Size.Small]: { 
[Weekday.Monday]: {},
[Weekday.Tuesday]: {},
[Weekday.Friday]: {},
},
[Size.Medium]: { 
[Weekday.Monday]: {},
[Weekday.Tuesday]: {},
[Weekday.Friday]: {},
},
[Size.Large]: { 
[Weekday.Monday]: {},
[Weekday.Tuesday]: {},
[Weekday.Friday]: {},
},
};
const menu = mapping[Size.Large]?.[Weekday.Monday];

---

处理潜在的未定义的值，例如:

游乐场

enum Size {
Small = 'small',
Medium = 'medium',
Large = 'large',
};
enum Weekday {
Monday = 'Monday',
Tuesday = 'Tuesday',
Friday = 'Friday',
};
type MenuType = {};
const mapping: {
[size in Size]?: { 
[weekday in Weekday]? : MenuType;
};
} = {
[Size.Medium]: { 
[Weekday.Tuesday]: {},
},
[Size.Large]: { 
[Weekday.Monday]: {},
[Weekday.Friday]: {},
},
};
const menu = mapping[Size.Large]?.[Weekday.Monday];