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1 | // Helper type. Not useful on its own.
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2 | type Without<FirstType, SecondType> = {[KeyType in Exclude<keyof FirstType, keyof SecondType>]?: never};
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3 |
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4 | /**
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5 | Create a type that has mutually exclusive keys.
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6 |
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7 | This type was inspired by [this comment](https://github.com/Microsoft/TypeScript/issues/14094#issuecomment-373782604).
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8 |
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9 | This type works with a helper type, called `Without`. `Without<FirstType, SecondType>` produces a type that has only keys from `FirstType` which are not present on `SecondType` and sets the value type for these keys to `never`. This helper type is then used in `MergeExclusive` to remove keys from either `FirstType` or `SecondType`.
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10 |
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11 | @example
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12 | ```
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13 | import {MergeExclusive} from 'type-fest';
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14 |
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15 | interface ExclusiveVariation1 {
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16 | exclusive1: boolean;
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17 | }
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18 |
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19 | interface ExclusiveVariation2 {
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20 | exclusive2: string;
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21 | }
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22 |
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23 | type ExclusiveOptions = MergeExclusive<ExclusiveVariation1, ExclusiveVariation2>;
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24 |
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25 | let exclusiveOptions: ExclusiveOptions;
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26 |
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27 | exclusiveOptions = {exclusive1: true};
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28 | //=> Works
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29 | exclusiveOptions = {exclusive2: 'hi'};
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30 | //=> Works
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31 | exclusiveOptions = {exclusive1: true, exclusive2: 'hi'};
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32 | //=> Error
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33 | ```
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34 | */
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35 | export type MergeExclusive<FirstType, SecondType> =
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36 | (FirstType | SecondType) extends object ?
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37 | (Without<FirstType, SecondType> & SecondType) | (Without<SecondType, FirstType> & FirstType) :
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38 | FirstType | SecondType;
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39 |
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