We will look at how we can use mapped types, conditional types, self-referencing types and the “infer” keyword to create a reusable generic type that traverses down the properties of an object and marks of all them as read-only. This is especially useful when used with Redux, to ensure our whole state tree is marked as read-only and immutable.

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For example we have complex interface:

interface IRootState {userId: string;showCompletedOnly: boolean;todoTypes: string[];todos: ITodo[];iconGrid: string[][]; }interface IEmail {from: string;to: string[];body: string; }interface ITodo {isCompleted: boolean;text: string;linkedEmail: IEmail; }

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If we want to add readonly to all the props of IRootState. We want to do it automaticlly.

type DeepReadonlyObject<T> = { readonly [K in keyof T]: DeepReadonly<T[K]> };type DeepReadonly<T> = T extends (infer E)[] ?ReadonlyArray<DeepReadonlyObject<E>> :T extends object ? DeepReadonlyObject<T> :T; type IReadonlyRootState = DeepReadonly<IRootState>;

轉載于:https://www.cnblogs.com/Answer1215/p/10321465.html

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