Typescript Count Substring of a String Literal Type

Ever wonder how can you count certain characters of a string literal type?

here is how:

type GetCountOfSubString<
    String_ extends string,
    SubString extends string,
    Count extends unknown[] = []
> = String_ extends `${string}${SubString}${infer Tail}`
    ? GetCountOfSubString<Tail, SubString, [1, ...Count]>
    : Count['length']

type NumberOfA = GetCountOfSubString<"a--a--aa--a","a"> // 5

playground

limitation: the count cannot exceed 999 because the max depth of TS recursion is only 1000