From 3632d5d56b652ce9708097ac1c9d7105cb3a8006 Mon Sep 17 00:00:00 2001
From: skarab42 <skarab@bluewin.ch>
Date: Fri, 26 Aug 2022 09:31:27 +0200
Subject: [PATCH] Add `Jsonify` support for optional keys #424

---
 source/jsonify.d.ts | 34 ++++++++++++++++++++++++++++------
 test-d/jsonify.ts   | 40 ++++++++++++++++++++++++++++++++++++++--
 2 files changed, 66 insertions(+), 8 deletions(-)

diff --git a/source/jsonify.d.ts b/source/jsonify.d.ts
index 8daa93b79..3b5570c5b 100644
--- a/source/jsonify.d.ts
+++ b/source/jsonify.d.ts
@@ -1,10 +1,32 @@
 import type {JsonPrimitive, JsonValue} from './basic';
+import type {Merge} from './merge';
 import type {NegativeInfinity, PositiveInfinity} from './numeric';
 import type {TypedArray} from './typed-array';
 
 // Note: The return value has to be `any` and not `unknown` so it can match `void`.
 type NotJsonable = ((...args: any[]) => any) | undefined | symbol;
 
+// Returns never if the key or property is not jsonable without testing whether the property is required or optional otherwise return the key.
+type BaseKeyFilter<Type, Key extends keyof Type> = Key extends symbol
+	? never
+	: Type[Key] extends symbol
+	? never
+	: [(...args: any[]) => any] extends [Type[Key]]
+	? never
+	: Key;
+
+// Returns never if the key or property is not jsonable or optional otherwise return the key.
+type RequiredKeyFilter<Type, Key extends keyof Type> = undefined extends Type[Key]
+	? never
+	: BaseKeyFilter<Type, Key>;
+
+// Returns never if the key or property is not jsonable or required otherwise return the key.
+type OptionalKeyFilter<Type, Key extends keyof Type> = undefined extends Type[Key]
+	? Type[Key] extends undefined
+		? never
+		: BaseKeyFilter<Type, Key>
+	: never;
+
 /**
 Transform a type to one that is assignable to the `JsonValue` type.
 
@@ -82,9 +104,9 @@ export type Jsonify<T> =
 				: T extends TypedArray ? Record<string, number>
 				: T extends any[]
 					? {[I in keyof T]: T[I] extends NotJsonable ? null : Jsonify<T[I]>}
-				: {[P in keyof T as P extends symbol ? never
-					: T[P] extends NotJsonable ? never
-					: P
-				]: Jsonify<Required<T>[P]>} // Recursive call for its children
-			: never // Otherwise any other non-object is removed
-		: never; // Otherwise non-JSONable type union was found not empty
+				: Merge<
+					{[Key in keyof T as RequiredKeyFilter<T, Key>]: Jsonify<T[Key]>},
+					{[Key in keyof T as OptionalKeyFilter<T, Key>]?: Jsonify<Exclude<T[Key], undefined>>}
+				> // Recursive call for its children
+				: never // Otherwise any other non-object is removed
+			: never; // Otherwise non-JSONable type union was found not empty
diff --git a/test-d/jsonify.ts b/test-d/jsonify.ts
index e972a8325..0a8aa183c 100644
--- a/test-d/jsonify.ts
+++ b/test-d/jsonify.ts
@@ -235,5 +235,41 @@ declare const jsonifiedOptionalTypeUnion: Jsonify<OptionalTypeUnion>;
 declare const jsonifiedNonOptionalTypeUnion: Jsonify<NonOptionalTypeUnion>;
 
 expectType<{a?: string}>(jsonifiedOptionalPrimitive);
-expectType<{a?: never}>(jsonifiedOptionalTypeUnion);
-expectType<{a: never}>(jsonifiedNonOptionalTypeUnion);
+expectType<{}>(jsonifiedOptionalTypeUnion);
+expectType<{a?: string}>(jsonifiedNonOptionalTypeUnion);
+
+// Test for 'Jsonify support for optional object keys, unserializable object values' #424
+// See https://github.com/sindresorhus/type-fest/issues/424
+type AppData = {
+	// Should be kept
+	requiredString: string;
+	requiredUnion: number | boolean;
+
+	// Should be kept and set to optional
+	optionalString?: string;
+	optionalUnion?: number | string;
+	optionalStringUndefined: string | undefined;
+	optionalUnionUndefined: number | string | undefined;
+
+	// Should be omitted
+	requiredFunction: () => any;
+	optionalFunction?: () => any;
+	requiredFunctionUnion: string | (() => any);
+	optionalFunctionUnion?: string | (() => any);
+	optionalFunctionUndefined: (() => any) | undefined;
+	optionalFunctionUnionUndefined: string | (() => any) | undefined;
+};
+
+type ExpectedAppDataJson = {
+	requiredString: string;
+	requiredUnion: number | boolean;
+
+	optionalString?: string;
+	optionalUnion?: string | number;
+	optionalStringUndefined?: string;
+	optionalUnionUndefined?: string | number;
+};
+
+declare const response: Jsonify<AppData>;
+
+expectType<ExpectedAppDataJson>(response);