union types 聯合類型
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type CanCross = "green" | "orange";
type ShouldStop = "orange" | "red";
let canCross = "orange" as CanCross; // ?
let shouldStop = "orange" as ShouldStop; // ?
unknow 類型，是所有類型的父集
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let something: unknown;
something = "Hello"; // ?
something = 2; // ?
something = { name: "Alice" }; // ?
something = () => "?"; // ?
A | unknown = unknown
A & unknown = A
交集，對象的交集就是包含2個對象中的所有類型
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never類型，string & number = never
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A | never = A
A & never = never
any類型，TS最差實踐，是所有類型的父集，也是所有類型的子集
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A | any = any
A & any = any
