获取元组长度

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题目

创建一个Length泛型，这个泛型接受一个只读的元组，返回这个元组的长度。

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

解答

type Length<T extends readonly any[]> = T['length']

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type Length<T extends readonly any[]> = T extends { length: infer L }
  ? L
  : never;