题目
中文
创建一个通用的Length，接受一个readonly的数组，返回这个数组的长度。
例如：
type tesla = ['tesla', 'model 3', 'model X', 'model Y']
type spaceX = ['FALCON 9', 'FALCON HEAVY', 'DRAGON', 'STARSHIP', 'HUMAN SPACEFLIGHT']
type teslaLength = Length<tesla> // expected 4
type spaceXLength = Length<spaceX> // expected 5
English
For given a tuple, you need create a generic Length, pick the length of the tuple
For example:
type tesla = ['tesla', 'model 3', 'model X', 'model Y']
type spaceX = ['FALCON 9', 'FALCON HEAVY', 'DRAGON', 'STARSHIP', 'HUMAN SPACEFLIGHT']
type teslaLength = Length<tesla> // expected 4
type spaceXLength = Length<spaceX> // expected 5
答案
type Length<T extends readonly any[]> = T extends { length: infer U } ? U : never;