An array containing optional types Ts and required types Ks

Question

An array containing optional types Ts and required types Ks

Can an array be typed in TypeScript to require at least one or more strings at any index, with any extra elements being numbers?

type NumberArrayWithAtleastOneString = [...(number[] | string)[], string]

const a: NumberArrayWithAtleastOneString = [1,'hello',3] // should work
const b: NumberArrayWithAtleastOneString = ['hello', 3, 'world'] // should work
const d: NumberArrayWithAtleastOneString = ['hello'] // should work
const c: NumberArrayWithAtleastOneString = [1,2,3] // should fail
const e: NumberArrayWithAtleastOneString = [] // should fail

Despite setting the correct types for the variables, all of them raise an error upon compilation. For example, const a gives the following alert:

Type '[number, string, number]' is not assignable to type 'NumberArrayWithAtleastOneString'.
  Type at positions 0 through 1 in source is not compatible with type at position 0 in target.
    Type 'number' is not assignable to type 'string | number[]'.ts(2322)

What would be the correct way to annotate this in TypeScript, or is it not possible?

typescript types

Answer 1

Answer №1

At this moment, there is no direct way to define a specific data type called NumberArrayWithAtLeastOneString in TypeScript that satisfies all your criteria. Check out the documentation on Tuple types for more insights. Tuples can have either a trailing rest element or a leading/middle rest element, but not both simultaneously.

Your best alternative would be to create a generic type like

NumberArrayWithAtLeastOneString<T>

, which acts as a constraint on the type T. This ensures that T is valid only if it extends

NumberArrayWithAtLeastOneString<T>

.

To implement this workaround, you could use the following code snippet:

type NumberArrayWithAtLeastOneString<T extends (string | number)[]> =
    T extends (
        { [I in keyof T]: T[I] extends string ? unknown : never }[number]
    ) ? T : [...T, string]

This code snippet leverages mapped array types and indexed access types to determine whether the given array contains at least one string. If so, it retains the original array; otherwise, it appends a string to the end of the array.

Directly using

NumberArrayWithAtLeastOneString<T>

in a type annotation might be cumbersome due to the need for explicit type arguments. A better approach is to utilize a generic utility function that infers the type argument automatically when called:

const numberArrayWithAtLeastOneString = <T extends (string | number)[]>(
    ...t: NumberArrayWithAtLeastOneString<T>
) => t;

By employing this function, you can streamline type inference and simplify variable assignments without specifying redundant type arguments every time.

Give it a try with some examples:

const a = numberArrayWithAtLeastOneString(1, 'hello', 3); // okay
const b = numberArrayWithAtLeastOneString('hello', 3, 'world'); // okay
const d = numberArrayWithAtLeastOneString('hello'); // okay
const c = numberArrayWithAtLeastOneString(1, 2, 3); // error
const e = numberArrayWithAtLeastOneString(); // error

The last two examples will throw errors indicating insufficient arguments since a string is expected in the array for validation purposes.

Access the Playground link here

Answer 2

At this moment, there is no direct way to define a specific data type called NumberArrayWithAtLeastOneString in TypeScript that satisfies all your criteria. Check out the documentation on Tuple types for more insights. Tuples can have either a trailing rest element or a leading/middle rest element, but not both simultaneously.

Your best alternative would be to create a generic type like

NumberArrayWithAtLeastOneString<T>

, which acts as a constraint on the type T. This ensures that T is valid only if it extends

NumberArrayWithAtLeastOneString<T>

.

To implement this workaround, you could use the following code snippet:

type NumberArrayWithAtLeastOneString<T extends (string | number)[]> =
    T extends (
        { [I in keyof T]: T[I] extends string ? unknown : never }[number]
    ) ? T : [...T, string]

This code snippet leverages mapped array types and indexed access types to determine whether the given array contains at least one string. If so, it retains the original array; otherwise, it appends a string to the end of the array.

Directly using

NumberArrayWithAtLeastOneString<T>

in a type annotation might be cumbersome due to the need for explicit type arguments. A better approach is to utilize a generic utility function that infers the type argument automatically when called:

const numberArrayWithAtLeastOneString = <T extends (string | number)[]>(
    ...t: NumberArrayWithAtLeastOneString<T>
) => t;

By employing this function, you can streamline type inference and simplify variable assignments without specifying redundant type arguments every time.

Give it a try with some examples:

const a = numberArrayWithAtLeastOneString(1, 'hello', 3); // okay
const b = numberArrayWithAtLeastOneString('hello', 3, 'world'); // okay
const d = numberArrayWithAtLeastOneString('hello'); // okay
const c = numberArrayWithAtLeastOneString(1, 2, 3); // error
const e = numberArrayWithAtLeastOneString(); // error

The last two examples will throw errors indicating insufficient arguments since a string is expected in the array for validation purposes.

Access the Playground link here

An array containing optional types Ts and required types Ks

Answer №1

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