Addition And Subtraction Type Proposal

[Dlurak]

Typescript doesn't support arithmetic operators on a type level.
I suggest introducing new types for Addition and Subtraction

I've semi successfully implemented these types already as they can be seen in this Gist.

Especially a type for addition can be very useful to create recursive types.

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[sindresorhus]

What's the real-world use-case for this?

[Dlurak]

When subtracting/adding numbers typescript looses all specific information which number it is, so 1+1 is just number instead of 2.
This can become problematic with indices of arrays.
Here is a code example:

const list = [1, 2, 3,] as const;
const element = list[3 - 1];
// TS doesn't now that 3-1 is two so the type of element is 1 | 2 | 3

type demonstration = typeof list[3 - 1]
// Error: Tuple type 'readonly [1, 2, 3]' of length '3' has no element at index '3'.
// There is no subtraction on a type level


type demonstration2 = typeof[Subtract<3, 1>]
// would be 3

// get an element of a list by index but with an offset
const offset = 2
const index = 0
const offsetedElement = list[offset + index]
// Again of type 1 | 2 | 3

[Dlurak]

And also for recursive types it can be quite useful to have a counter (This type makes it possible to mimic a for loop on a type level). Here is another code example in which i used the Add type to create a type that repeats a string n times.

type AppendString<T extends string, S extends string> = `${T}${S}`;

type RepeatString<
  S extends string,
  N extends number,
  Counter extends number = 0,
  TotalString extends string = "",
> = Counter extends N
  ? TotalString
  : RepeatString<S, N, Add<Counter, 1>, AppendString<TotalString, S>>;

And here is an example of using the Add type to get the length of a string:

type LengthOfString<
  T extends string,
  N extends number = 0,
> = T extends `${infer R}${infer L}` ? LengthOfString<L, Add<N, 1>> : N;

[sindresorhus]

These were added in https://github.com/sindresorhus/type-fest/releases/tag/v4.12.0

Add was named Sum.