Is it possible in TypeScript to change a string literal type into a number type?

Question

Is it possible in TypeScript to change a string literal type into a number type?

Would it be feasible to develop a utility type Number<T> that can take a string literal type and convert it into a number? If conversion is not possible, should the utility return a never type?

type Five = Number<'5'> // `Five` is considered as the number 5

In anticipation of addressing questions regarding the purpose of this utility:

The reason for posing this question is my attempt to create an Add utility type for adding numbers.

type createArray<Len, Ele, Arr extends Ele[] = []> =  Arr['length'] extends Len ? Arr : createArray<Len, Ele, [Ele, ...Arr]>

type Add<A extends number, B extends number> = [...createArray<A, 1>, ...createArray<B, 1>]['length']

At present, the utility functions correctly with:

type Answer = Add<3,10> // The answer is 13

However, it currently only works with the number type. It would also be desirable for it to exclude the string type, allowing functionality like this: type Answer = Add<'3','10'>

typescript

Answer 1

Answer №1

With the upcoming TypeScript 4.8 release, it will be possible to convert string literal types into other types such as number, bigint, or boolean. You can find more details in this PR.

By defining a utility type called ParseInt, we are able to "cast" a string literal to a number type.

type ParseInt<T> = T extends `${infer N extends number}` ? N : never

type T0 = ParseInt<"1">    // 1
type T1 = ParseInt<"100">  // 100
type T2 = ParseInt<"-13">  // -13
type T3 = ParseInt<"abc">  // never

Below are examples demonstrating conversions for other types:

type ParseBigint<T> = T extends `${infer N extends bigint}` ? N : never

type T4 = ParseBigint<"1">    // 1n
type T5 = ParseBigint<"100">  // 100n
type T6 = ParseBigint<"abc">  // never


type ParseBoolean<T> = T extends `${infer N extends boolean}` ? N : never

type T7 = ParseBoolean<"true">   // true
type T8 = ParseBoolean<"false">  // false
type T9 = ParseBoolean<"abc">    // never

Take a look at it in action on the TypeScript playground here.

Answer 2

With the upcoming TypeScript 4.8 release, it will be possible to convert string literal types into other types such as number, bigint, or boolean. You can find more details in this PR.

By defining a utility type called ParseInt, we are able to "cast" a string literal to a number type.

type ParseInt<T> = T extends `${infer N extends number}` ? N : never

type T0 = ParseInt<"1">    // 1
type T1 = ParseInt<"100">  // 100
type T2 = ParseInt<"-13">  // -13
type T3 = ParseInt<"abc">  // never

Below are examples demonstrating conversions for other types:

type ParseBigint<T> = T extends `${infer N extends bigint}` ? N : never

type T4 = ParseBigint<"1">    // 1n
type T5 = ParseBigint<"100">  // 100n
type T6 = ParseBigint<"abc">  // never


type ParseBoolean<T> = T extends `${infer N extends boolean}` ? N : never

type T7 = ParseBoolean<"true">   // true
type T8 = ParseBoolean<"false">  // false
type T9 = ParseBoolean<"abc">    // never

Take a look at it in action on the TypeScript playground here.

Answer 3

Answer №2

Turning an arbitrary string literal type into a numeric literal type (referred to as StringToNumber<T>) is not possible. There was a request made at microsoft/TypeScript#47141 for this feature, but it was declined by the TypeScript team. They are not inclined to support it. Another open issue at microsoft/TypeScript#26382 requests support for performing arbitrary mathematics on literal types, including StringToNumber<T>; however, the outcome is uncertain.

If your requirement is limited to non-negative whole numbers below 1000, you can implement this functionality yourself using tuple manipulation and recursion, similar to how Add works:

type StringToNumber<T extends string, A extends any[] = []> =
  T extends keyof [0, ...A] ? A['length'] : StringToNumber<T, [0, ...A]>

You can test it with examples like:

type Thirteen = StringToNumber<"13">;
// type Thirteen = 13

However, keep in mind that this method has limitations similar to Add, and providing unexpected inputs may lead to compiler performance issues or errors.

The input strings should be restricted to valid numeric values:

type Digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "";
type NonZero = Exclude<Digit, "0" | "">
type LessThanAThousand = "0" | `${NonZero}${Digit}${Digit}`

type StringToNumber<T extends LessThanAThousand, A extends any[] = []> =
  T extends LessThanAThousand ? T extends keyof [0, ...A] ?
  A['length'] : StringToNumber<T, [0, ...A]> : never;

type Oops = StringToNumber<"0.4"> // error
// ----------------------> ~~~~~
// Type '"0.4"' does not satisfy the constraint 'LessThanAThousand'.(2344)

This approach may work, but caution is advised unless there is a strong justification for its use. The TypeScript team does not consider the utility of Add type worth supporting, which could explain why the request for StringToNumber<T> was declined.

Playground link to code

Answer 4

Turning an arbitrary string literal type into a numeric literal type (referred to as StringToNumber<T>) is not possible. There was a request made at microsoft/TypeScript#47141 for this feature, but it was declined by the TypeScript team. They are not inclined to support it. Another open issue at microsoft/TypeScript#26382 requests support for performing arbitrary mathematics on literal types, including StringToNumber<T>; however, the outcome is uncertain.

If your requirement is limited to non-negative whole numbers below 1000, you can implement this functionality yourself using tuple manipulation and recursion, similar to how Add works:

type StringToNumber<T extends string, A extends any[] = []> =
  T extends keyof [0, ...A] ? A['length'] : StringToNumber<T, [0, ...A]>

You can test it with examples like:

type Thirteen = StringToNumber<"13">;
// type Thirteen = 13

However, keep in mind that this method has limitations similar to Add, and providing unexpected inputs may lead to compiler performance issues or errors.

The input strings should be restricted to valid numeric values:

type Digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "";
type NonZero = Exclude<Digit, "0" | "">
type LessThanAThousand = "0" | `${NonZero}${Digit}${Digit}`

type StringToNumber<T extends LessThanAThousand, A extends any[] = []> =
  T extends LessThanAThousand ? T extends keyof [0, ...A] ?
  A['length'] : StringToNumber<T, [0, ...A]> : never;

type Oops = StringToNumber<"0.4"> // error
// ----------------------> ~~~~~
// Type '"0.4"' does not satisfy the constraint 'LessThanAThousand'.(2344)

This approach may work, but caution is advised unless there is a strong justification for its use. The TypeScript team does not consider the utility of Add type worth supporting, which could explain why the request for StringToNumber<T> was declined.

Playground link to code

Answer 5

Answer №3

To manually create a custom Map, follow these steps:

type MapStrNum = {
  "apple": 1;
  "banana": 2;
};

Next, implement this:

type One = MapNumStr['apple'] // One will equal 1

Answer 6

To manually create a custom Map, follow these steps:

type MapStrNum = {
  "apple": 1;
  "banana": 2;
};

Next, implement this:

type One = MapNumStr['apple'] // One will equal 1

Is it possible in TypeScript to change a string literal type into a number type?

Answer №1

Answer №2

Answer №3

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