Typescript type camelCase / snake_case conversion

camelCase-snake_case-types.ts

type CamelCase<S extends string> = S extends `${infer P1}_${infer P2}${infer P3}`
? `${Lowercase<P1>}${Uppercase<P2>}${CamelCase<P3>}`
: Lowercase<S>

type KeysToCamelCase<T> = {
  [K in keyof T as CamelCase<string & K>]: T[K]
}


type CamelToSnakeCase<S extends string> = S extends `${infer T}${infer U}` ?
  `${T extends Capitalize<T> ? "_" : ""}${Lowercase<T>}${CamelToSnakeCase<U>}` :
  S

type KeysToSnakeCase<T> = {
[K in keyof T as CamelToSnakeCase<string & K>]: T[K]
}

I was using type KeysToSnakeCase, passing it an object that already has snake case results in it having 2 underscores:

You can define a type that removes the first character if it's an underscore and utilize that

type RemoveUnderscoreFirstLetter<S extends string> =
  S extends `${infer FirstLetter}${infer U}`
    ? `${FirstLetter extends "_" ? U : `${FirstLetter}${U}`}`
    : S;

type CamelToSnakeCase<S extends string> =
  S extends `${infer T}${infer U}`
    ? `${T extends Capitalize<T> ? "_" : ""}${RemoveUnderscoreFirstLetter<
        Lowercase<T>
      >}${CamelToSnakeCase<U>}`
    : S;

type KeysToSnakeCase<T extends object> = {
  [K in keyof T as CamelToSnakeCase<K & string>]: T[K];
};

Parsing error: Type expected.eslint

This doesn't work for objects keys in arrays like this:

const data = {
  name: 'Rocket',
  parts: [ { brandName: 'starx' }]
}

Thanks for sharing!