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TypeScript - ReturnType<> with generic function
6 points
In this short aritcle we would like to show you how to get result type from generic function in TypeScript.
Problem description:
typeofoperator was created to work with variables in runtime mode
e. g.typeof myFunction,- when
myFunctionis generic it is not posible to dotypeof myFunction<Type>operation, becausemyFunction<Type>doesnt exists in the application memory - check below example to see some trick.
Quick solution:
xxxxxxxxxx1
// ----------------------------------------------------------------------------2
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// Genric function definition:4
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const createCollection = <T extends unknown> (users: T[]) => {6
return {7
getUser: (index: number): T | null => {8
return users[index] ?? null;9
}10
};11
};12
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// ----------------------------------------------------------------------------14
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// Return type extracting:16
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class Wrapper<T extends unknown> {18
//HINT: do not forget to match arguments to your function19
mediate = (args: any[]) => createCollection<T>(args);20
}21
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type GenericCollectionReturnType<T extends unknown> = ReturnType<Wrapper<T>['mediate']>;23
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// ----------------------------------------------------------------------------25
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// Type usage example:27
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const users: GenericCollectionReturnType<string> = createCollection('John', 'Kate');29
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console.log(users.getUser(0)); // John31
console.log(users.getUser(1)); // KateScreenshot:
