transformKeys.ts

const data = {
  NaMe: "name",
  aGe: 12,
  a: "mask",
};

export type TLiteral = "UPPER" | "LOWER" | "CAPITALIZE" | "UNCAPITALIZE";

export type TransformKeys<T, L extends TLiteral> = {
  [K in keyof T as L extends "UPPER"
    ? Uppercase<string & K>
    : L extends "LOWER"
    ? Lowercase<string & K>
    : L extends "CAPITALIZE"
    ? Capitalize<string & K>
    : L extends "UNCAPITALIZE"
    ? Uncapitalize<string & K>
    : never]: T[K];
};

type UpperCaseKeys = TransformKeys<typeof data, "CAPITALIZE">; // ? { NaMe: string; aGe: number; a: string; }
  
type LowerCaseKeys = TransformKeys<typeof data, "LOWER">; // ? { name: string; age: number; a: string; }
  
type CapitalizeKeys = TransformKeys<typeof data, "UPPER">; // ? { NAME: string; AGE: number; A: string; } 
  
type UncapitalizeKeys = TransformKeys<typeof data, "UNCAPITALIZE">; // ? { nAme: string; aGe: number; a: string; }