mapped types - How to define properties in a Typescript interface with dynamic elements in the key name?

Question

I have an object that can have an n number of properties, each one the same but with their n value in the name.

Example:

const obj = {
  'data-element-0': 'something',
  'data-element-1': 'something else',
  'data-element-2': 'something as well',
  'data-element-3': 'something to feel included',
};

Is there any way to define this interface with more specificity than just using

interface Obj {
  [key: string]: string;
}

Answer 1

You can do smth like that:

type Key = `data-element-${1|2|3|4|5|6|7|8|9|0}`

const obj:Record<Key, string> = {
    'data-element-0': 'something',
    'data-element-1': 'something else',
    'data-element-2': 'something as well',
    'data-element-3': 'something to feel included',
    'data-element-yu': 'something to feel included', // error
};

UPDATE I have also make helpers fot double numbers, from: 0-99:

type NonZeroDigit = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'

type NumberHelper = {
  [P in NonZeroDigit]: {
    [Z in NonZeroDigit]: `${P}${Z}`
  }
}

type NestedValues<T extends Record<string, Record<string, string>>> = {
  [P in keyof T]: P extends string ? Values<T[P]> : never
}
type Values<T> = T[keyof T]

type RemoveTrailingZero<T extends string> = T extends `${infer Fst}${infer Snd}` ? Fst extends `0` ? `${Snd}` : `${Fst}${Snd}` : never;

type Numbers_99 = RemoveTrailingZero<Values<NestedValues<NumberHelper>>>

UPDATE

Here you have an util for generating number range from 0 to 99999

type Values<T> = T[keyof T]

type LiteralDigits = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
type NumberString<T extends number> = `${T}`

type AppendDigit<T extends number | string> = `${T}${LiteralDigits}`

type MakeSet<T extends number> = {
    [P in T]: AppendDigit<P>
}

type RemoveTrailingZero<T extends string> = T extends `${infer Fst}${infer Rest}` ? Fst extends `0` ? RemoveTrailingZero<`${Rest}`> : `${Fst}${Rest}` : never;

type From_1_to_999 = RemoveTrailingZero<Values<{
    [P in Values<MakeSet<LiteralDigits>>]: AppendDigit<P>
}>>

type By<V extends NumberString<number>> = RemoveTrailingZero<Values<{
    [P in V]: AppendDigit<P>
}>>

type From_1_to_99999 =
    | From_1_to_999
    | By<From_1_to_999>
    | By<From_1_to_999
        | By<From_1_to_999>>

Demo

UPDATE 3

If you still want to generate literal numbers, not string numbers, you can use this code, which has been shamelessly stolen from here

type PrependNextNum<A extends Array<unknown>> = A['length'] extends infer T ? ((t: T, ...a: A) => void) extends ((...x: infer X) => void) ? X : never : never;

type EnumerateInternal<A extends Array<unknown>, N extends number> = { 0: A, 1: EnumerateInternal<PrependNextNum<A>, N> }[N extends A['length'] ? 0 : 1];

type Enumerate<N extends number> = EnumerateInternal<[], N> extends (infer E)[] ? E : never;

type Result = Enumerate<43> // 0 | 1 | 2 | ... | 42

*UPDATE 8 September 2021

Since TS 4.5, see Tail recursion PR, it is possible to generate much longer number range.

See example:

type MAXIMUM_ALLOWED_BOUNDARY = 999

type ComputeRange<
    N extends number,
    Result extends Array<unknown> = [],
    > =
    (Result['length'] extends N
        ? Result
        : ComputeRange<N, [...Result, Result['length']]>
    )

const ComputeRange = (N: number, Result: number[] = []): number[] => {
    if (Result.length === N) {
        return Result
    }
    return ComputeRange(N, [...Result, Result.length])
}
// 0 , 1, 2 ... 998
type NumberRange = ComputeRange<MAXIMUM_ALLOWED_BOUNDARY>[number]

Related question

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