From 2d556cbb2eca7999d40069649cef0d26c797d9be Mon Sep 17 00:00:00 2001
From: Dunqing <29533304+Dunqing@users.noreply.github.com>
Date: Mon, 22 Dec 2025 09:46:14 +0000
Subject: [PATCH] fix(formatter): should treat TSMappedType as a complex type
 in assignment-like formatting (#17257)

Part of #16956
---
 .../src/utils/assignment_like.rs              |  8 +++-
 .../ts/assignments/complex-type-arguments.ts  |  9 ++++
 .../complex-type-arguments.ts.snap            | 42 +++++++++++++++++++
 3 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts
 create mode 100644 crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts.snap

diff --git a/crates/oxc_formatter/src/utils/assignment_like.rs b/crates/oxc_formatter/src/utils/assignment_like.rs
index b9e4ac0befa81..9e43d636d7b76 100644
--- a/crates/oxc_formatter/src/utils/assignment_like.rs
+++ b/crates/oxc_formatter/src/utils/assignment_like.rs
@@ -994,7 +994,13 @@ fn is_complex_type_arguments(type_arguments: &TSTypeParameterInstantiation) -> b
     let is_first_argument_complex = ts_type_argument_list.first().is_some_and(|first_argument| {
         matches!(
             first_argument,
-            TSType::TSUnionType(_) | TSType::TSIntersectionType(_) | TSType::TSTypeLiteral(_)
+            TSType::TSUnionType(_)
+                | TSType::TSIntersectionType(_)
+                | TSType::TSTypeLiteral(_)
+                // TODO: This is not part of Prettier's logic, but it makes sense to consider mapped types as
+                // complex because it is the same as type literals in terms of structure.
+                // NOTE: Once the `will_break` logic is added, this will have to be revisited.
+                | TSType::TSMappedType(_)
         )
     });
 
diff --git a/crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts b/crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts
new file mode 100644
index 0000000000000..6273db8985a3d
--- /dev/null
+++ b/crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts
@@ -0,0 +1,9 @@
+// Type argument is a `TSMappedType`
+const emitter = createGlobalEmitter<{
+  [key in Event["type"]]: Extract<Event, { type: key }>
+}>()
+
+// Type argument is a `TSTypeLiteral`
+const emitter2 = createGlobalEmitter<{
+  longlonglonglongKey: Extract<Event, { type: key }>
+}>()
diff --git a/crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts.snap b/crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts.snap
new file mode 100644
index 0000000000000..0e1a05c7a89c5
--- /dev/null
+++ b/crates/oxc_formatter/tests/fixtures/ts/assignments/complex-type-arguments.ts.snap
@@ -0,0 +1,42 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// Type argument is a `TSMappedType`
+const emitter = createGlobalEmitter<{
+  [key in Event["type"]]: Extract<Event, { type: key }>
+}>()
+
+// Type argument is a `TSTypeLiteral`
+const emitter2 = createGlobalEmitter<{
+  longlonglonglongKey: Extract<Event, { type: key }>
+}>()
+
+==================== Output ====================
+------------------
+{ printWidth: 80 }
+------------------
+// Type argument is a `TSMappedType`
+const emitter = createGlobalEmitter<{
+  [key in Event["type"]]: Extract<Event, { type: key }>;
+}>();
+
+// Type argument is a `TSTypeLiteral`
+const emitter2 = createGlobalEmitter<{
+  longlonglonglongKey: Extract<Event, { type: key }>;
+}>();
+
+-------------------
+{ printWidth: 100 }
+-------------------
+// Type argument is a `TSMappedType`
+const emitter = createGlobalEmitter<{
+  [key in Event["type"]]: Extract<Event, { type: key }>;
+}>();
+
+// Type argument is a `TSTypeLiteral`
+const emitter2 = createGlobalEmitter<{
+  longlonglonglongKey: Extract<Event, { type: key }>;
+}>();
+
+===================== End =====================