diff --git a/dataclasses_json/api.py b/dataclasses_json/api.py
index 3481e93f..29134816 100644
--- a/dataclasses_json/api.py
+++ b/dataclasses_json/api.py
@@ -2,7 +2,7 @@
 import json
 from typing import Any, Callable, Dict, List, Optional, Tuple, Type, TypeVar, Union, overload
 
-from dataclasses_json.cfg import config, LetterCase
+from dataclasses_json.cfg import config, LetterCase, LetterCaseCallable
 from dataclasses_json.core import (Json, _ExtendedEncoder, _asdict,
                                    _decode_dataclass)
 from dataclasses_json.mm import (JsonData, SchemaType, build_schema)
@@ -103,16 +103,16 @@ def schema(cls: Type[A],
 
 
 @overload
-def dataclass_json(_cls: None = ..., *, letter_case: Optional[LetterCase] = ...,
+def dataclass_json(_cls: None = ..., *, letter_case: Union[LetterCaseCallable, LetterCase, None] = ...,
                    undefined: Optional[Union[str, Undefined]] = ...) -> Callable[[Type[T]], Type[T]]: ...
 
 
 @overload
-def dataclass_json(_cls: Type[T], *, letter_case: Optional[LetterCase] = ...,
+def dataclass_json(_cls: Type[T], *, letter_case: Union[LetterCaseCallable, LetterCase, None] = ...,
                    undefined: Optional[Union[str, Undefined]] = ...) -> Type[T]: ...
 
 
-def dataclass_json(_cls: Optional[Type[T]] = None, *, letter_case: Optional[LetterCase] = None,
+def dataclass_json(_cls: Optional[Type[T]] = None, *, letter_case: Union[LetterCaseCallable, LetterCase, None] = None,
                    undefined: Optional[Union[str, Undefined]] = None) -> Union[Callable[[Type[T]], Type[T]], Type[T]]:
     """
     Based on the code in the `dataclasses` module to handle optional-parens
@@ -132,7 +132,7 @@ def wrap(cls: Type[T]) -> Type[T]:
     return wrap(_cls)
 
 
-def _process_class(cls: Type[T], letter_case: Optional[LetterCase],
+def _process_class(cls: Type[T], letter_case: Union[LetterCaseCallable, LetterCase, None],
                    undefined: Optional[Union[str, Undefined]]) -> Type[T]:
     if letter_case is not None or undefined is not None:
         cls.dataclass_json_config = config(letter_case=letter_case,  # type: ignore[attr-defined]
diff --git a/dataclasses_json/cfg.py b/dataclasses_json/cfg.py
index 930e51e0..6c182ec2 100644
--- a/dataclasses_json/cfg.py
+++ b/dataclasses_json/cfg.py
@@ -47,11 +47,13 @@ def __init__(self):
 global_config = _GlobalConfig()
 
 
+LetterCaseCallable = Callable[[str], str]
+
 class LetterCase(Enum):
-    CAMEL = camelcase
-    KEBAB = spinalcase
-    SNAKE = snakecase
-    PASCAL = pascalcase
+    CAMEL: LetterCaseCallable = camelcase
+    KEBAB: LetterCaseCallable = spinalcase
+    SNAKE: LetterCaseCallable = snakecase
+    PASCAL: LetterCaseCallable = pascalcase
 
 
 def config(metadata: Optional[dict] = None, *,
@@ -60,7 +62,7 @@ def config(metadata: Optional[dict] = None, *,
            encoder: Optional[Callable] = None,
            decoder: Optional[Callable] = None,
            mm_field: Optional[MarshmallowField] = None,
-           letter_case: Union[Callable[[str], str], LetterCase, None] = None,
+           letter_case: Union[LetterCaseCallable, LetterCase, None] = None,
            undefined: Optional[Union[str, Undefined]] = None,
            field_name: Optional[str] = None,
            exclude: Optional[Callable[[T], bool]] = None,
diff --git a/dataclasses_json/stringcase.py b/dataclasses_json/stringcase.py
index da0f546f..611e92e4 100644
--- a/dataclasses_json/stringcase.py
+++ b/dataclasses_json/stringcase.py
@@ -24,9 +24,10 @@
 # Copyright © 2020 Louis-Philippe Véronneau <pollo@debian.org>
 
 import re
+from typing import Literal, cast
 
 
-def uplowcase(string, case):
+def uplowcase(string: str, case: Literal["up", "low"]) -> str:
     """Convert string into upper or lower case.
 
     Args:
@@ -42,7 +43,7 @@ def uplowcase(string, case):
         return str(string).lower()
 
 
-def capitalcase(string):
+def capitalcase(string: str) -> str:
     """Convert string into capital case.
     First letters will be uppercase.
 
@@ -60,7 +61,7 @@ def capitalcase(string):
     return uplowcase(string[0], 'up') + string[1:]
 
 
-def camelcase(string):
+def camelcase(string: str) -> str:
     """ Convert string into camel case.
 
     Args:
@@ -76,11 +77,11 @@ def camelcase(string):
         return string
     return (uplowcase(string[0], 'low')
             + re.sub(r"[\-_\.\s]([a-z0-9])",
-                     lambda matched: uplowcase(matched.group(1), 'up'),
+                     lambda matched: uplowcase(cast(str, matched.group(1)), 'up'),
                      string[1:]))
 
 
-def snakecase(string):
+def snakecase(string: str) -> str:
     """Convert string into snake case.
     Join punctuation with underscore
 
@@ -97,11 +98,11 @@ def snakecase(string):
         return string
     return (uplowcase(string[0], 'low')
             + re.sub(r"[A-Z0-9]",
-                     lambda matched: '_' + uplowcase(matched.group(0), 'low'),
+                     lambda matched: '_' + uplowcase(cast(str, matched.group(0)), 'low'),
                      string[1:]))
 
 
-def spinalcase(string):
+def spinalcase(string: str) -> str:
     """Convert string into spinal case.
     Join punctuation with hyphen.
 
@@ -116,7 +117,7 @@ def spinalcase(string):
     return re.sub(r"_", "-", snakecase(string))
 
 
-def pascalcase(string):
+def pascalcase(string: str) -> str:
     """Convert string into pascal case.
 
     Args: