0018-TupleLength-easy

18 Tuple Length

关键词: TypeScript, TypeChallenge

题目

对于给定的元组，您需要创建一个通用的Length，选择元组的长度

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

答案

type Length<T extends any[]> = T["length"];

分析

Length的类型参数首先要是元组, 约束T extends any[]

对于T, 我们只需要获取"length"属性并返回即可

伪码如下

function Length(T) {
  if(!(T extends Array)) throw new Error()

  return typeOf(T["length"])
}