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1 | /**
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2 | Extract the keys from a type where the value type of the key extends the given `Condition`.
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3 |
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4 | Internally this is used for the `ConditionalPick` and `ConditionalExcept` types.
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5 |
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6 | @example
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7 | ```
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8 | import {ConditionalKeys} from 'type-fest';
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9 |
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10 | interface Example {
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11 | a: string;
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12 | b: string | number;
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13 | c?: string;
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14 | d: {};
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15 | }
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16 |
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17 | type StringKeysOnly = ConditionalKeys<Example, string>;
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18 | //=> 'a'
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19 | ```
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20 |
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21 | To support partial types, make sure your `Condition` is a union of undefined (for example, `string | undefined`) as demonstrated below.
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22 |
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23 | @example
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24 | ```
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25 | type StringKeysAndUndefined = ConditionalKeys<Example, string | undefined>;
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26 | //=> 'a' | 'c'
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27 | ```
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28 | */
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29 | export type ConditionalKeys<Base, Condition> = NonNullable<
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30 | // Wrap in `NonNullable` to strip away the `undefined` type from the produced union.
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31 | {
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32 | // Map through all the keys of the given base type.
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33 | [Key in keyof Base]:
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34 | // Pick only keys with types extending the given `Condition` type.
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35 | Base[Key] extends Condition
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36 | // Retain this key since the condition passes.
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37 | ? Key
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38 | // Discard this key since the condition fails.
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39 | : never;
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40 |
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41 | // Convert the produced object into a union type of the keys which passed the conditional test.
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42 | }[keyof Base]
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43 | >;
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