This specific function:
const proxy = () => Math.random() < 0.5 ? numbersArray : stringsArray;
defines a return type of:
number[] | string[]
Indicating that the resulting array will contain either all numbers or all strings, but never a mix of both.
Now, what about the type of each item in this returned array? Initially it may seem like it's number | string, however, upon closer inspection, it appears to be more accurately depicted as (number | string)[]. This signifies that the array consists of strings or numbers, with each element having the potential to be either data type. Therefore, you now have the freedom to intermingle both types within the array. It is important to note that this distinction impacts how the member types are inferred and utilized in constructing the original array type.
Consider the following generic function:
function func<T>(array: T[]): T[][] {
return [[array[0], array[1]], [array[2], array[3]]];
}
Here, the function attempts to identify the member type T for the input array T[]. However, as demonstrated earlier, defining an appropriate member type for string[] | number[] can be challenging. Consequently, TypeScript faces difficulty inferring the correct T value, leading to errors.
In order to rectify this situation and ensure functionality, the member type T must be ascertainable. One possible solution involves explicitly declaring the return type of the proxy function as a compatible array type where only the elements of the array constitute a union.
const proxy: () => (string | number)[] =
() => Math.random() < 0.5 ? numbersArray : stringsArray;
By making this adjustment, T now assumes the value of string | number, facilitating proper execution as initially anticipated.
const resultUnion = func(proxy()); // type: (string | number)[][]
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