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