From 94cbd2a6be90417feb71687532243458e93f0e00 Mon Sep 17 00:00:00 2001
From: Sunny Hirai <thesunny@gmail.com>
Date: Sat, 20 Jun 2020 11:52:37 -0700
Subject: [PATCH] Optional properties (#390)

---
 src/types.ts           | 33 ++++++++++++++++++++++++++++++++-
 test/types/object.ts   |  7 ++++++-
 test/types/optional.ts |  2 +-
 3 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/src/types.ts b/src/types.ts
index 3d19e3d6..4c8d83b7 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -249,6 +249,37 @@ export function number(): Struct<number> {
   })
 }
 
+/**
+ * Type helper to Flatten the Union of optional and required properties.
+ */
+
+type Flatten<T> = T extends infer U ? { [K in keyof U]: U[K] } : never
+
+/**
+ * Type helper to extract the optional keys of an object
+ */
+
+type OptionalKeys<T> = {
+  [K in keyof T]: undefined extends T[K] ? K : never
+}[keyof T]
+
+/**
+ * Type helper to extract the required keys of an object
+ */
+
+type RequiredKeys<T> = {
+  [K in keyof T]: undefined extends T[K] ? never : K
+}[keyof T]
+
+/**
+ * Type helper to create optional properties when the property value can be
+ * undefined (ie. when `optional()` is used to define a type)
+ */
+
+type OptionalizeObject<T> = Flatten<
+  { [K in RequiredKeys<T>]: T[K] } & { [K in OptionalKeys<T>]?: T[K] }
+>
+
 /**
  * Validate that an object with specific entry values.
  */
@@ -258,7 +289,7 @@ export function object<V extends StructRecord<any>>(): Struct<
 >
 export function object<V extends StructRecord<any>>(
   Structs: V
-): Struct<{ [K in keyof V]: StructType<V[K]> }, V>
+): Struct<OptionalizeObject<{ [K in keyof V]: StructType<V[K]> }>, V>
 export function object<V extends StructRecord<any>>(
   Structs?: V
 ): Struct<any, any> {
diff --git a/test/types/object.ts b/test/types/object.ts
index 7f3616ba..16ac9d3d 100644
--- a/test/types/object.ts
+++ b/test/types/object.ts
@@ -1,4 +1,4 @@
-import { assert, object, number } from '../..'
+import { assert, object, optional, number, string } from '../..'
 import { test } from '..'
 
 test<Record<string, unknown>>((x) => {
@@ -10,3 +10,8 @@ test<{ a: number }>((x) => {
   assert(x, object({ a: number() }))
   return x
 })
+
+test<{ a?: number }>((x) => {
+  assert(x, object({ a: optional(number()) }))
+  return x
+})
diff --git a/test/types/optional.ts b/test/types/optional.ts
index 532fafac..d04a77f2 100644
--- a/test/types/optional.ts
+++ b/test/types/optional.ts
@@ -6,7 +6,7 @@ test<string | undefined>((x) => {
   return x
 })
 
-test<{ a: string | undefined }>((x) => {
+test<{ a?: string | undefined }>((x) => {
   assert(x, object({ a: optional(string()) }))
   return x
 })