Removing optional modifier in a mapped type behaves inconsistently b/w array vs object

[juhort]

🔎 Search Terms

This doesn't seem related to #31025. This is about the inconsistent behavior and specifically when exactOptionalPropertyTypes option is enabled.

🕗 Version & Regression Information

Fails with v5.9.3.

⏯ Playground Link

https://www.typescriptlang.org/play/?exactOptionalPropertyTypes=true&ts=5.3.3#code/PTAEAEFMA8EMGMAuB5ADoglgewHawDYAKATlqpMYgJ4AqV5AzgFyiLECukAUF9eaACVIARwA8NAHygAvKADeAbUKgMOUAGtIVLADNQNALoBaAPwsaSgwF9QIUAGVYAW0ihYDVgAtXAI3YZ8RCNVUAADIWF-YkgAE1DWem4uOxJIBgoANzSvV1CAH1B2HBjIHVVY0IAaUBisNJwAckRQBgB3AnwsVpVmwFByXkTQAHUMRE9kHwArSCQZQREo2NE5WDMWtlUAc1ACopKynFirCWTgaSk5NxYGDZxt3eLS8piAblArHjt7dvxO1o8xrkHvtnvF+nxXCMxgBBYjEWBUOYRRYxUQKAAUN2IWx2hUeB1iAEoTAYTiBzqAFFitgYgA

💻 Code

// @exactOptionalPropertyTypes: true

type Req<T> = {[P in keyof T]-?: T[P]} // Same as the built-in `Required` type

// Preserves the `| undefined`, doesn't swallow it ✅
type WithObject = Required<{a?: string | undefined}>
//=> { a: string | undefined; }

// Swallows the `| undefined` ✅
type WithArray = Required<[(string | undefined)?]>
//=> [string]

🙁 Actual behavior

With exactOptionalPropertyTypes enabled, ?: string | undefined is different than ?: string. And mapped types respect this differentiation by not swallowing the | undefined with the -? mapping modifier.

// @exactOptionalPropertyTypes: true

type Req<T> = {[P in keyof T]-?: T[P]} // Same as the built-in `Required` type

// Preserves the `| undefined`, doesn't swallow it ✅
type WithObject = Req<{a?: string | undefined}> 
//=> { a: string | undefined; }

However, when instantiated with arrays, the | undefined is swallowed during mapping. This is the issue!

// @exactOptionalPropertyTypes: true

type Req<T> = {[P in keyof T]-?: T[P]} // Same as the built-in `Required` type

// Swallows the `| undefined` ❌
type WithArray = Req<[(string | undefined)?]> 
//=> [string]

And this is not just a display issue:

const withObject: WithObject = {a: undefined}; // ✅ Fine
 
const withArray: WithArray = [undefined]; // ❌ Not fine

---

When exactOptionalPropertyTypes is disabled, the | undefined gets swallowed, and that is fine and unrelated to this particular issue.

// @exactOptionalPropertyTypes: false

type Req<T> = {[P in keyof T]-?: T[P]} // Same as the built-in `Required` type

// Swallows the `| undefined` ✅
type WithObject = Req<{a?: string | undefined}> 
//=> { a: string; }

🙂 Expected behavior

The behavior should be consistent. The | undefined bit should not be swallowed when mapping even for arrays.

Additional information about the issue

Here's another example:

// @exactOptionalPropertyTypes: true

type ToStringOrUnd<T> = {[P in keyof T]-?: string | undefined}

// Preserves the `| undefined` ✅
type WithObject = ToStringOrUnd<{a: 1; b?: 2}> 
//=> { a: string | undefined; b: string | undefined; }

// Preserves the `| undefined` for the required element ✅
// But, doesn't preserve it for the optional element ❌
type WithArray = ToStringOrUnd<[1, 2?]> 
//=> [string | undefined, string]

Playground Link

The -? mapping modifier seems to swallow the | undefined when operating on arrays, and specifically when operating with optional array elements.

[Andarist]

This is a trivial fix - but I guess I'm not supposed to open behavior-changing PRs right now 😅 anyway, here is the patch: https://github.com/microsoft/typescript-go/compare/main...Andarist:fix/strip-optionality-mapped-over-tuples-eopt?expand=1

And a TS-TS patch:

Strada patch

From 485fbcc549aad4020e968e2dfc49773a0583b139 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Mon, 30 Mar 2026 09:03:33 +0200
Subject: [PATCH] Fix optionality stripping when mapping over tuples under EOPT

---
 src/compiler/checker.ts                       |  2 +-
 ...actoptionalpropertytypes=false).errors.txt | 17 ++++++++++
 ...(exactoptionalpropertytypes=false).symbols | 25 ++++++++++++++
 ...y2(exactoptionalpropertytypes=false).types | 33 +++++++++++++++++++
 ...2(exactoptionalpropertytypes=true).symbols | 25 ++++++++++++++
 ...ty2(exactoptionalpropertytypes=true).types | 33 +++++++++++++++++++
 .../compiler/stripMembersOptionality2.ts      | 10 ++++++
 7 files changed, 144 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).errors.txt
 create mode 100644 tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).symbols
 create mode 100644 tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).types
 create mode 100644 tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).symbols
 create mode 100644 tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).types
 create mode 100644 tests/cases/compiler/stripMembersOptionality2.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0567712f11..98da5e0046 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -20958,7 +20958,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const propType = instantiateType(getTemplateTypeFromMappedType(type.target as MappedType || type), templateMapper);
         const modifiers = getMappedTypeModifiers(type);
         return strictNullChecks && modifiers & MappedTypeModifiers.IncludeOptional && !maybeTypeOfKind(propType, TypeFlags.Undefined | TypeFlags.Void) ? getOptionalType(propType, /*isProperty*/ true) :
-            strictNullChecks && modifiers & MappedTypeModifiers.ExcludeOptional && isOptional ? getTypeWithFacts(propType, TypeFacts.NEUndefined) :
+            strictNullChecks && modifiers & MappedTypeModifiers.ExcludeOptional && isOptional ? removeMissingOrUndefinedType(propType) :
             propType;
     }
 
diff --git a/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).errors.txt b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).errors.txt
new file mode 100644
index 0000000000..1a5feaa71f
--- /dev/null
+++ b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).errors.txt
@@ -0,0 +1,17 @@
+stripMembersOptionality2.ts(4,29): error TS2322: Type 'undefined' is not assignable to type 'string'.
+stripMembersOptionality2.ts(6,27): error TS2322: Type 'undefined' is not assignable to type 'string'.
+
+
+==== stripMembersOptionality2.ts (2 errors) ====
+    // https://github.com/microsoft/TypeScript/issues/63291
+    
+    type WithObject1 = Required<{ a?: string | undefined }>;
+    const obj1: WithObject1 = { a: undefined };
+                                ~
+!!! error TS2322: Type 'undefined' is not assignable to type 'string'.
+!!! related TS6500 stripMembersOptionality2.ts:3:31: The expected type comes from property 'a' which is declared here on type 'Required<{ a?: string | undefined; }>'
+    type WithArray1 = Required<[(string | undefined)?]>;
+    const tup1: WithArray1 = [undefined];
+                              ~~~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'string'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).symbols b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).symbols
new file mode 100644
index 0000000000..664779a819
--- /dev/null
+++ b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).symbols
@@ -0,0 +1,25 @@
+//// [tests/cases/compiler/stripMembersOptionality2.ts] ////
+
+=== stripMembersOptionality2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63291
+
+type WithObject1 = Required<{ a?: string | undefined }>;
+>WithObject1 : Symbol(WithObject1, Decl(stripMembersOptionality2.ts, 0, 0))
+>Required : Symbol(Required, Decl(lib.es5.d.ts, --, --))
+>a : Symbol(a, Decl(stripMembersOptionality2.ts, 2, 29))
+
+const obj1: WithObject1 = { a: undefined };
+>obj1 : Symbol(obj1, Decl(stripMembersOptionality2.ts, 3, 5))
+>WithObject1 : Symbol(WithObject1, Decl(stripMembersOptionality2.ts, 0, 0))
+>a : Symbol(a, Decl(stripMembersOptionality2.ts, 3, 27))
+>undefined : Symbol(undefined)
+
+type WithArray1 = Required<[(string | undefined)?]>;
+>WithArray1 : Symbol(WithArray1, Decl(stripMembersOptionality2.ts, 3, 43))
+>Required : Symbol(Required, Decl(lib.es5.d.ts, --, --))
+
+const tup1: WithArray1 = [undefined];
+>tup1 : Symbol(tup1, Decl(stripMembersOptionality2.ts, 5, 5))
+>WithArray1 : Symbol(WithArray1, Decl(stripMembersOptionality2.ts, 3, 43))
+>undefined : Symbol(undefined)
+
diff --git a/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).types b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).types
new file mode 100644
index 0000000000..a41845308f
--- /dev/null
+++ b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=false).types
@@ -0,0 +1,33 @@
+//// [tests/cases/compiler/stripMembersOptionality2.ts] ////
+
+=== stripMembersOptionality2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63291
+
+type WithObject1 = Required<{ a?: string | undefined }>;
+>WithObject1 : Required<{ a?: string | undefined; }>
+>            : ^^^^^^^^^^^^^^^                  ^^^^
+>a : string | undefined
+>  : ^^^^^^^^^^^^^^^^^^
+
+const obj1: WithObject1 = { a: undefined };
+>obj1 : Required<{ a?: string | undefined; }>
+>     : ^^^^^^^^^^^^^^^                  ^^^^
+>{ a: undefined } : { a: undefined; }
+>                 : ^^^^^^^^^^^^^^^^^
+>a : undefined
+>  : ^^^^^^^^^
+>undefined : undefined
+>          : ^^^^^^^^^
+
+type WithArray1 = Required<[(string | undefined)?]>;
+>WithArray1 : [string]
+>           : ^^^^^^^^
+
+const tup1: WithArray1 = [undefined];
+>tup1 : [string]
+>     : ^^^^^^^^
+>[undefined] : [undefined]
+>            : ^^^^^^^^^^^
+>undefined : undefined
+>          : ^^^^^^^^^
+
diff --git a/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).symbols b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).symbols
new file mode 100644
index 0000000000..664779a819
--- /dev/null
+++ b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).symbols
@@ -0,0 +1,25 @@
+//// [tests/cases/compiler/stripMembersOptionality2.ts] ////
+
+=== stripMembersOptionality2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63291
+
+type WithObject1 = Required<{ a?: string | undefined }>;
+>WithObject1 : Symbol(WithObject1, Decl(stripMembersOptionality2.ts, 0, 0))
+>Required : Symbol(Required, Decl(lib.es5.d.ts, --, --))
+>a : Symbol(a, Decl(stripMembersOptionality2.ts, 2, 29))
+
+const obj1: WithObject1 = { a: undefined };
+>obj1 : Symbol(obj1, Decl(stripMembersOptionality2.ts, 3, 5))
+>WithObject1 : Symbol(WithObject1, Decl(stripMembersOptionality2.ts, 0, 0))
+>a : Symbol(a, Decl(stripMembersOptionality2.ts, 3, 27))
+>undefined : Symbol(undefined)
+
+type WithArray1 = Required<[(string | undefined)?]>;
+>WithArray1 : Symbol(WithArray1, Decl(stripMembersOptionality2.ts, 3, 43))
+>Required : Symbol(Required, Decl(lib.es5.d.ts, --, --))
+
+const tup1: WithArray1 = [undefined];
+>tup1 : Symbol(tup1, Decl(stripMembersOptionality2.ts, 5, 5))
+>WithArray1 : Symbol(WithArray1, Decl(stripMembersOptionality2.ts, 3, 43))
+>undefined : Symbol(undefined)
+
diff --git a/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).types b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).types
new file mode 100644
index 0000000000..28dc49ba23
--- /dev/null
+++ b/tests/baselines/reference/stripMembersOptionality2(exactoptionalpropertytypes=true).types
@@ -0,0 +1,33 @@
+//// [tests/cases/compiler/stripMembersOptionality2.ts] ////
+
+=== stripMembersOptionality2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63291
+
+type WithObject1 = Required<{ a?: string | undefined }>;
+>WithObject1 : Required<{ a?: string | undefined; }>
+>            : ^^^^^^^^^^^^^^^                  ^^^^
+>a : string | undefined
+>  : ^^^^^^^^^^^^^^^^^^
+
+const obj1: WithObject1 = { a: undefined };
+>obj1 : Required<{ a?: string | undefined; }>
+>     : ^^^^^^^^^^^^^^^                  ^^^^
+>{ a: undefined } : { a: undefined; }
+>                 : ^^^^^^^^^^^^^^^^^
+>a : undefined
+>  : ^^^^^^^^^
+>undefined : undefined
+>          : ^^^^^^^^^
+
+type WithArray1 = Required<[(string | undefined)?]>;
+>WithArray1 : [string | undefined]
+>           : ^^^^^^^^^^^^^^^^^^^^
+
+const tup1: WithArray1 = [undefined];
+>tup1 : [string | undefined]
+>     : ^^^^^^^^^^^^^^^^^^^^
+>[undefined] : [undefined]
+>            : ^^^^^^^^^^^
+>undefined : undefined
+>          : ^^^^^^^^^
+
diff --git a/tests/cases/compiler/stripMembersOptionality2.ts b/tests/cases/compiler/stripMembersOptionality2.ts
new file mode 100644
index 0000000000..ad344c321e
--- /dev/null
+++ b/tests/cases/compiler/stripMembersOptionality2.ts
@@ -0,0 +1,10 @@
+// @strict: true
+// @exactOptionalPropertyTypes: true, false
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/63291
+
+type WithObject1 = Required<{ a?: string | undefined }>;
+const obj1: WithObject1 = { a: undefined };
+type WithArray1 = Required<[(string | undefined)?]>;
+const tup1: WithArray1 = [undefined];
-- 
2.53.0