Mapped Type
Index Signatures Type
type Student = {
// key 只是一個變數名稱，可以自己命名
[key: string]: string
}
// 可以是任意
const s1: Student = {
firstName: 'Sam'
}
const s2: Student = {
lastName: 'Wang'
}
Mapped Type
[property in ...]可以想成for ... of
// 兩者效果相同
type Student1 = {
firstName: string
lastName: string
}
type Student2 = {
// key 只是一個變數名稱，可以自己命名
// key 會被限定在定義範圍
[key in 'firstName' | 'lastName']: string
}
const s2: Student2 = {
firstName: 'Sam',
lastName: 'Wang'
}
keyof就是把物件的 key 取出來
type Student = {
age: number
name: string
}
type StudentMethod = {
[key in keyof Student]: () => void
// 等同此寫法，優點是哪天 Student 屬性修改了，這邊如果沒跟著改，就會 error 提醒
// age: () => void
// name: () => void
}
const studentMethod: StudentMethod = {
age: () => {},
name: () => {}
}
- 也可以把 key 當 value
type Student = {
age: number
name: string
}
type StudentKeyMap = {
[key in keyof Student]: key
}
// 等同此寫法
type StudentKeyMap2 = {
age: 'age'
name: 'name'
}
- 搭配
as來自定義 key name
type Student = {
age: number
name: string
}
type StudentMethod = {
// Capitalize 轉成大寫
[key in keyof Student as `set${Capitalize<key>}`]: () => void
}
const studentMethod: StudentMethod = {
setAge: () => {},
setName: () => {}
}
// 改寫法的話
// [key in keyof Student as `set${key}`]: () => void
// key 會變成這樣 setage, setname