40 lines
1.1 KiB
Plaintext
40 lines
1.1 KiB
Plaintext
/**
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Extract all optional keys from the given type.
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This is useful when you want to create a new type that contains different type values for the optional keys only.
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@example
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```
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import type {OptionalKeysOf, Except} from 'type-fest';
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interface User {
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name: string;
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surname: string;
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luckyNumber?: number;
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}
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const REMOVE_FIELD = Symbol('remove field symbol');
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type UpdateOperation<Entity extends object> = Except<Partial<Entity>, OptionalKeysOf<Entity>> & {
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[Key in OptionalKeysOf<Entity>]?: Entity[Key] | typeof REMOVE_FIELD;
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};
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const update1: UpdateOperation<User> = {
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name: 'Alice'
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};
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const update2: UpdateOperation<User> = {
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name: 'Bob',
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luckyNumber: REMOVE_FIELD
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};
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```
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@category Utilities
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*/
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export type OptionalKeysOf<BaseType extends object> =
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BaseType extends unknown // For distributing `BaseType`
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? (keyof {
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[Key in keyof BaseType as BaseType extends Record<Key, BaseType[Key]> ? never : Key]: never
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}) & (keyof BaseType) // Intersect with `keyof BaseType` to ensure result of `OptionalKeysOf<BaseType>` is always assignable to `keyof BaseType`
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: never; // Should never happen
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