Fix camel-casing keys in nested objects when using union with null (#119)

Author: yutak23

index.d.ts

@@ -3,7 +3,8 @@ import type {CamelCase, PascalCase} from 'type-fest';
 // eslint-disable-next-line @typescript-eslint/ban-types
 type EmptyTuple = [];
 
-type ObjectOptional = Record<string, unknown> | undefined;
+// Allow union with, for example, `undefined` and `null`.
+type ObjectUnion = Record<string, unknown> | unknown;
 
 /**
 Return a default type if input type is nil.
@@ -38,7 +39,7 @@ type AppendPath<S extends string, Last extends string> = S extends ''
 Convert keys of an object to camelcase strings.
 */
 export type CamelCaseKeys<
-	T extends ObjectOptional | readonly any[],
+	T extends ObjectUnion | readonly any[],
 	Deep extends boolean = false,
 	IsPascalCase extends boolean = false,
 	PreserveConsecutiveUppercase extends boolean = false,
@@ -71,7 +72,7 @@ export type CamelCaseKeys<
 			]
 				? T[P]
 				: [Deep] extends [true]
-					? T[P] extends ObjectOptional | readonly any[]
+					? T[P] extends ObjectUnion | readonly any[]
 						? CamelCaseKeys<
 						T[P],
 						Deep,

index.test-d.ts

@@ -471,3 +471,16 @@ function camelcaseKeysPascalCase<
 
 expectType<{fooBar: {hogeHoge: string}}>(camelcaseKeysDeep({foo_bar: {hoge_hoge: 'hoge_hoge'}}));
 expectType<{FooBar: string}>(camelcaseKeysPascalCase({foo_bar: 'foo_bar'}));
+
+// Test for union type
+// eslint-disable-next-line @typescript-eslint/ban-types
+const objectCamelcased: CamelCaseKeys<{foo_bar: {foo_prop: string} | null}, true>
+	= camelcaseKeys({foo_bar: {foo_prop: 'foo_props'}}, {deep: true});
+// eslint-disable-next-line @typescript-eslint/ban-types
+const nullCamelcased: CamelCaseKeys<{foo_bar: {foo_prop: string} | null}, true>
+	= camelcaseKeys({foo_bar: null}, {deep: true});
+
+// eslint-disable-next-line @typescript-eslint/ban-types
+expectType<{fooBar: {fooProp: string} | null}>(objectCamelcased);
+// eslint-disable-next-line @typescript-eslint/ban-types
+expectType<{fooBar: {fooProp: string} | null}>(nullCamelcased);