1 | type IsAny<T> = 0 extends (1 & T) ? true : false; // https://stackoverflow.com/a/49928360/3406963
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2 | type IsNever<T> = [T] extends [never] ? true : false;
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3 | type IsUnknown<T> = IsNever<T> extends false ? T extends unknown ? unknown extends T ? IsAny<T> extends false ? true : false : false : false : false;
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4 |
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5 | /**
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6 | Create a function type with a return type of your choice and the same parameters as the given function type.
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7 |
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8 | Use-case: You want to define a wrapped function that returns something different while receiving the same parameters. For example, you might want to wrap a function that can throw an error into one that will return `undefined` instead.
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9 |
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10 | @example
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11 | ```
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12 | import {SetReturnType} from 'type-fest';
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13 |
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14 | type MyFunctionThatCanThrow = (foo: SomeType, bar: unknown) => SomeOtherType;
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15 |
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16 | type MyWrappedFunction = SetReturnType<MyFunctionThatCanThrow, SomeOtherType | undefined>;
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17 | //=> type MyWrappedFunction = (foo: SomeType, bar: unknown) => SomeOtherType | undefined;
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18 | ```
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19 | */
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20 | export type SetReturnType<Fn extends (...args: any[]) => any, TypeToReturn> =
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21 | // Just using `Parameters<Fn>` isn't ideal because it doesn't handle the `this` fake parameter.
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22 | Fn extends (this: infer ThisArg, ...args: infer Arguments) => any ? (
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23 | // If a function did not specify the `this` fake parameter, it will be inferred to `unknown`.
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24 | // We want to detect this situation just to display a friendlier type upon hovering on an IntelliSense-powered IDE.
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25 | IsUnknown<ThisArg> extends true ? (...args: Arguments) => TypeToReturn : (this: ThisArg, ...args: Arguments) => TypeToReturn
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26 | ) : (
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27 | // This part should be unreachable, but we make it meaningful just in case…
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28 | (...args: Parameters<Fn>) => TypeToReturn
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29 | );
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