My question involves combining different types to form a union, and I am seeking to generate a specialized utility type that produces an array with distinct types, for example:
type MyType = string | number
type ArrayOfMyType = ToArray<MyType> // -> string[] | number[], but not (string | number)[]
Upon investigating the solution proposed by Lishugupta652, I found it intriguing. Despite being clearly AI-generated, it functions effectively. Understanding the reasoning behind its success is more crucial than simply copy-pasting code snippets and expecting acceptance.
The functionality hinges on type distribution, a concept elaborated in this referenced article:
Type distribution [...] involves a compiler feature that applies conditionals individually to elements within generic union types, rather than as a whole.
If we directly utilize a generic union type to create a new type, such as demonstrated here:
type ToArray<T> = T[];
type MyArray = ToArray<string | number>
T[] will yield (string | number)[] since the entire union type is utilized, irrespective of its constituent items.
Enter the extends property. By employing extends, the conditional is applied to each element, with T representing the current iteration. This results in distributed typing, enabling us to achieve the desired outcome.
type ToArray<T> = T extends unknown ? T[] : never;
type MyArray = ToArray<string | number> // string[] | number[]
An exception arises with boolean types (after my investigation revealed no other exceptions), as booleans function akin to literal types encompassing two values: true and false. Consequently, distributing the generic union type produces one iteration each for true and false. Thus, an exception is necessary in this specific scenario.
type ToArray<T> = T extends boolean ? boolean[] : T extends any ? T[] : never;
type ArrayOfMyType = ToArray<string | number| boolean>; // string[] | number[] | boolean
For both true and false, the resulting type will be boolean[], as extends boolean ? evaluates true for both cases.
interface MyType {
type: string | number;
}
type ConvertToArray<T> = T extends boolean ? boolean[] : T extends any ? T[] : never;
type ArrayOfMyTypes = ConvertToArray<MyType>; // string[] | number[]
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