Array that provides specific data types for indexes

Question

Array that provides specific data types for indexes

Could be a tricky question or maybe not feasible, but I've been scratching my head over it for quite some time and just can't seem to crack it. Appreciate any help!

The T generic creates a union type:

const arrayToArray = <T>(values: T[]) => values;

When I call the function with:

const result = arrayToArray([1, 'hi', 3]);

It results in the type:

const result: (string | number)[]

My desired output is:

const result: [number, string, number]

I want 'one' and 'three' to trigger a type error as they are not numbers.

const useResultType: typeof result = ['one', 'two', 'three'];

typescript typescript-generics

Answer 1

Answer №1

To obtain tuples inferred from an array literal, there are a couple of methods to persuade the compiler to accomplish this:

One approach is to utilize tuples in rest parameters. This method eliminates the array literal from the call:

const arrayToArray = <T extends any[]>(...values: T) => values;

const result = arrayToArray(1, 'hi', 3); //[number, string, number]

Another method involves using a union of tuple and array constraint to indicate to the compiler that you desire a union. This approach maintains passing the parameter as an array:

const arrayToArray = <T extends [any] | any[]>(values: T) => values;

const result = arrayToArray([1, 'hi', 3]); // [number, string, number]

If you wish to have a readonly tuple in 3.4 (which is not yet released), you can directly employ an as const assertion:

const result = [1, 'hi', 3]  as const; // readonly [number, string, number]

Answer 2

To obtain tuples inferred from an array literal, there are a couple of methods to persuade the compiler to accomplish this:

One approach is to utilize tuples in rest parameters. This method eliminates the array literal from the call:

const arrayToArray = <T extends any[]>(...values: T) => values;

const result = arrayToArray(1, 'hi', 3); //[number, string, number]

Another method involves using a union of tuple and array constraint to indicate to the compiler that you desire a union. This approach maintains passing the parameter as an array:

const arrayToArray = <T extends [any] | any[]>(values: T) => values;

const result = arrayToArray([1, 'hi', 3]); // [number, string, number]

If you wish to have a readonly tuple in 3.4 (which is not yet released), you can directly employ an as const assertion:

const result = [1, 'hi', 3]  as const; // readonly [number, string, number]

Array that provides specific data types for indexes

Answer №1

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