From 91f8453d099221413c1827e9f13f340617a7b063 Mon Sep 17 00:00:00 2001
From: jamashita <jamashita@arachnid.xyz>
Date: Fri, 4 Oct 2024 20:34:09 +0900
Subject: [PATCH 1/2] =?UTF-8?q?docs:=20=E2=9C=8F=EF=B8=8F=20ReturnType<T>?=
 =?UTF-8?q?=20=E3=81=AE=E5=9F=B7=E7=AD=86?=
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---
 .../type-reuse/utility-types/return-type.md   | 46 +++++++++++++++++++
 sidebars.js                                   |  1 +
 2 files changed, 47 insertions(+)
 create mode 100644 docs/reference/type-reuse/utility-types/return-type.md

diff --git a/docs/reference/type-reuse/utility-types/return-type.md b/docs/reference/type-reuse/utility-types/return-type.md
new file mode 100644
index 00000000..27568d5b
--- /dev/null
+++ b/docs/reference/type-reuse/utility-types/return-type.md
@@ -0,0 +1,46 @@
+---
+description: 関数の戻り値の型を取得する
+title: "ReturnType<T>"
+---
+
+`ReturnType<T>`は、関数型`T`の戻り値を取得するユーティリティ型です。
+
+## ReturnType&lt;T>の型引数
+
+### T
+
+型引数`T`には、関数の型を渡します。関数でない型を渡すと`never`型になります。
+
+## NonNullableの使用例
+
+```ts twoslash
+// @errors: 2344
+type ReturnType1 = ReturnType<() => string>;
+//   ^?
+type ReturnType2 = ReturnType<(arg: string) => string | number>;
+//   ^?
+type ReturnType3 = ReturnType<() => never>;
+//   ^?
+```
+
+多くは`typeof`と併用して実際の関数の戻り値を取得します。
+
+```ts twoslash
+const isEven = (num: number) => {
+  return num / 2 === 0;
+};
+
+type isEvenRetType = ReturnType<typeof isEven>;
+//   ^?
+```
+
+`ReturnType<T>`は内部的には`infer`を使って実装されています。
+
+```ts twoslash
+// @noErrors
+type ReturnType<T extends (...args: any) => any> = T extends (
+  ...args: any
+) => infer R
+  ? R
+  : any;
+```
diff --git a/sidebars.js b/sidebars.js
index 0089859f..47dd47c7 100644
--- a/sidebars.js
+++ b/sidebars.js
@@ -354,6 +354,7 @@ module.exports = {
                 "reference/type-reuse/utility-types/extract",
                 "reference/type-reuse/utility-types/no-infer",
                 "reference/type-reuse/utility-types/nonnullable",
+                "reference/type-reuse/utility-types/return-type",
               ],
             },
             "reference/type-reuse/mapped-types",

From 415a287f96ca3f27ead56d8557fddd5b6172b2cd Mon Sep 17 00:00:00 2001
From: jamashita <jamashita@arachnid.xyz>
Date: Fri, 4 Oct 2024 20:41:56 +0900
Subject: [PATCH 2/2] =?UTF-8?q?docs:=20=E2=9C=8F=EF=B8=8F=20NonNullable<T>?=
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MIME-Version: 1.0
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Content-Transfer-Encoding: 8bit

---
 docs/README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/README.md b/docs/README.md
index 504441d8..13f7c8af 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -1528,7 +1528,7 @@ type T3 = Extract<T1, T2>;
 
 #### NonNullable
 
-- `NonNullable`は、nullまたはundefinedを含む型からいずれも除外するユーティリティ型。
+- [`NonNullable`](reference/type-reuse/utility-types/nonnullable.md)は、nullまたはundefinedを含む型からいずれも除外するユーティリティ型。
 
 ```typescript twoslash
 type T1 = string | null | undefined;