MapTypes
题目
实现 MapTypes<T, R>，它将对象 T 中的类型转换为由类型 R 定义的不同类型，R 的结构如下：
interface StringToNumber {
mapFrom: string // 值为字符串的键的值
mapTo: number // 将被转换为数字
}示例：
interface StringToNumber { mapFrom: string, mapTo: number }
MapTypes<{ iWillBeANumberOneDay: string }, StringToNumber>
// gives { iWillBeANumberOneDay: number; }请注意，用户可以提供一个联合类型：
interface StringToNumber { mapFrom: string, mapTo: number }
interface StringToDate { mapFrom: string, mapTo: Date }
MapTypes<{ iWillBeNumberOrDate: string }, StringToDate | StringToNumber>
// gives { iWillBeNumberOrDate: number | Date; }如果我们在映射中不存在这种类型，就让它保持原样：
interface StringToNumber { mapFrom: string, mapTo: number }
MapTypes<{ iWillBeANumberOneDay: string, iWillStayTheSame: Function }, StringToNumber>
// // gives { iWillBeANumberOneDay: number, iWillStayTheSame: Function }解题思路
答案
type MapTypes<T, R extends { mapFrom: unknown, mapTo: unknown }> = {
[P in keyof T]: T[P] extends R['mapFrom']
? R extends { mapFrom: T[P] }
? R['mapTo']
: never
: T[P]
}验证
type cases = [
Expect<Equal<MapTypes<{ stringToArray: string }, { mapFrom: string, mapTo: [] }>, { stringToArray: [] }>>,
Expect<Equal<MapTypes<{ stringToNumber: string }, { mapFrom: string, mapTo: number }>, { stringToNumber: number }>>,
Expect<Equal<MapTypes<{ stringToNumber: string, skipParsingMe: boolean }, { mapFrom: string, mapTo: number }>, { stringToNumber: number, skipParsingMe: boolean }>>,
Expect<Equal<MapTypes<{ date: string }, { mapFrom: string, mapTo: Date } | { mapFrom: string, mapTo: null }>, { date: null | Date }>>,
Expect<Equal<MapTypes<{ date: string }, { mapFrom: string, mapTo: Date | null }>, { date: null | Date }>>,
Expect<Equal<MapTypes<{ fields: Record<string, boolean> }, { mapFrom: Record<string, boolean>, mapTo: string[] }>, { fields: string[] }>>,
Expect<Equal<MapTypes<{ name: string }, { mapFrom: boolean, mapTo: never }>, { name: string }>>,
Expect<Equal<MapTypes<{ name: string, date: Date }, { mapFrom: string, mapTo: boolean } | { mapFrom: Date, mapTo: string }>, { name: boolean, date: string }>>,
]