Key remapping & template literal types

type Original = { first_name: string; last_name: string }

type Camelize<T> = {
  [K in keyof T as K extends `${infer A}_${infer B}`
    ? `${A}${Capitalize<B>}`
    : K]: T[K]
}

type UserCamel = Camelize<Original>  // { firstName: string; lastName: string }

type Props = { onClick: () => void; onSubmit: () => void; title: string }

type WithoutHandlers<T> = {
  [K in keyof T as K extends `on${string}` ? never : K]: T[K]
}

type Clean = WithoutHandlers<Props>  // { title: string }

type Fields = "name" | "email"

type Getters<T extends string> = {
  [K in T as `get${Capitalize<K>}`]: () => string
}

type ClientAPI = Getters<Fields>
// { getName: () => string; getEmail: () => string }

type Route = `/${"" | `${string}`}`

type HttpMethod = "get" | "post"

type Endpoints<T extends string, M extends string> = {
  [K in T as `${M}_${K}`]: { path: `/${K}`; method: Uppercase<M> }
}

type API = Endpoints<"users" | "login", HttpMethod>
// { get_users: { path: "/users"; method: "GET" }, post_login: { path: "/login"; method: "POST" } }