Typed uppercase/lowercase/capitalize/uncapitalize function for use with template literal types.

typedStringUtils.ts

/**
 * For Template Literal Types. Don't forget to add 'as const' after `...` .
 */
export function typedUppercase<S extends string>(str: S) {
  return str.toUpperCase() as Uppercase<S>
}

/**
 * For Template Literal Types. Don't forget to add 'as const' after `...` .
 */
export function typedLowercase<S extends string>(str: S) {
  return str.toLowerCase() as Lowercase<S>
}

/**
 * For Template Literal Types. Don't forget to add 'as const' after `...` .
 */
export function typedCapitalize<S extends string>(str: S) {
  return str.replace(/^./, c => c.toUpperCase()) as Capitalize<S>
}

/**
 * For Template Literal Types. Don't forget to add 'as const' after `...` .
 */
export function typedUncapitalize<S extends string>(str: S) {
  return str.replace(/^./, c => c.toLowerCase()) as Uncapitalize<S>
}