union types 联合类型
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type CanCross = "green" | "orange";
type ShouldStop = "orange" | "red";
let canCross = "orange" as CanCross; // ✅
let shouldStop = "orange" as ShouldStop; // ✅
unknow 类型，是所有类型的父集
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let something: unknown;
something = "Hello"; // ✅
something = 2; // ✅
something = { name: "Alice" }; // ✅
something = () => "?"; // ✅
A | unknown = unknown
A & unknown = A
交集，对象的交集就是包含2个对象中的所有类型
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never类型，string & number = never
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A | never = A
A & never = never
any类型，TS最差实践，是所有类型的父集，也是所有类型的子集
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A | any = any
A & any = any
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