TypeScript 泛型与类型体操入门

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function identity<T>(value: T): T {
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  return value
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}
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const a = identity<string>('hello')  // a: string
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const b = identity(42)               // b: number（自动推断）

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function pluck<T, K extends keyof T>(obj: T, key: K): T[K] {
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  return obj[key]
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}
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pluck({ name: 'Cyrene', age: 18 }, 'name')  // string

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type User = { id: number; name: string; email: string }
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type UserPreview = Pick<User, 'id' | 'name'>   // { id; name }

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type ElementType<T> = T extends (infer U)[] ? U : T
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type A = ElementType<number[]>   // number
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type B = ElementType<string>     // string

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type R = ReturnType<() => Date>   // Date

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function get<T, K extends keyof T>(obj: T, key: K): T[K] | undefined {
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  return obj[key]
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}