Jsonify: Handle undefined in array (#310)

source/jsonify.d.ts

@@ -3,6 +3,13 @@ import {JsonPrimitive, JsonValue} from './basic';
 // Note: The return value has to be `any` and not `unknown` so it can match `void`.
 type NotJsonable = ((...args: any[]) => any) | undefined;
 
+// Note: Handles special case where Arrays with `undefined` are transformed to `'null'` by `JSON.stringify()`
+// Only use with array members
+type JsonifyArrayMember<T> =
+	T extends undefined ?
+		null | Exclude<T, undefined> :
+		Jsonify<T>;
+
 /**
 Transform a type to one that is assignable to the `JsonValue` type.
 
@@ -64,7 +71,7 @@ type Jsonify<T> =
 		? T extends JsonPrimitive
 			? T // Primitive is acceptable
 			: T extends Array<infer U>
-				? Array<Jsonify<U>> // It's an array: recursive call for its children
+				? Array<JsonifyArrayMember<U>> // It's an array: recursive call for its children
 				: T extends object
 					? T extends {toJSON(): infer J}
 						? (() => J) extends (() => JsonValue) // Is J assignable to JsonValue?

test-d/jsonify.ts

@@ -123,6 +123,18 @@ const nonJsonWithInvalidToJSON = new NonJsonWithInvalidToJSON();
 expectNotAssignable<JsonValue>(nonJsonWithInvalidToJSON);
 expectNotAssignable<JsonValue>(nonJsonWithInvalidToJSON.toJSON());
 
+// Special cases of Array with `undefined` member
+// `[undefined]` is not JSON because it contains non JSON value `undefined`
+// However `JSON.parse(JSON.stringify())` transforms array members of `undefined` to `null`
+expectNotAssignable<JsonValue>([undefined]);
+declare const parsedStringifiedArrayWithUndefined1: Jsonify<undefined[]>; // = JSON.parse(JSON.stringify([undefined]));
+expectType<null[]>(parsedStringifiedArrayWithUndefined1);
+expectAssignable<JsonValue>(parsedStringifiedArrayWithUndefined1);
+expectNotAssignable<JsonValue>([undefined, 1]);
+declare const parsedStringifiedArrayWithUndefined2: Jsonify<[undefined, number]>;
+expectType<Array<number | null>>(parsedStringifiedArrayWithUndefined2);
+expectAssignable<JsonValue>(parsedStringifiedArrayWithUndefined2);
+
 // Test that optional type members are not discarded wholesale.
 interface OptionalPrimitive {
 	a?: string;