/**
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Extract the keys from a type where the value type of the key extends the given `Condition`.
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Internally this is used for the `ConditionalPick` and `ConditionalExcept` types.
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@example
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```
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import {ConditionalKeys} from 'type-fest';
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interface Example {
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a: string;
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b: string | number;
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c?: string;
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d: {};
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}
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type StringKeysOnly = ConditionalKeys<Example, string>;
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//=> 'a'
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```
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To support partial types, make sure your `Condition` is a union of undefined (for example, `string | undefined`) as demonstrated below.
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@example
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```
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type StringKeysAndUndefined = ConditionalKeys<Example, string | undefined>;
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//=> 'a' | 'c'
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```
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*/
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export type ConditionalKeys<Base, Condition> = NonNullable<
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// Wrap in `NonNullable` to strip away the `undefined` type from the produced union.
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{
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// Map through all the keys of the given base type.
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[Key in keyof Base]:
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// Pick only keys with types extending the given `Condition` type.
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Base[Key] extends Condition
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// Retain this key since the condition passes.
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? Key
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// Discard this key since the condition fails.
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: never;
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// Convert the produced object into a union type of the keys which passed the conditional test.
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}[keyof Base]
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>;
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