From fd8b8797dfc9481a2ad0eb6cdb1975ba1bc38c6e Mon Sep 17 00:00:00 2001
From: whilenot <git@whilenot.dev>
Date: Wed, 1 Jul 2026 11:37:04 +0200
Subject: [PATCH 1/2] Exhaustiveness checking with runtime error and
 assertNever

---
 .../copy/en/handbook-v2/Narrowing.md           | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/packages/documentation/copy/en/handbook-v2/Narrowing.md b/packages/documentation/copy/en/handbook-v2/Narrowing.md
index c7a552ecf9b9..612944c7b072 100644
--- a/packages/documentation/copy/en/handbook-v2/Narrowing.md
+++ b/packages/documentation/copy/en/handbook-v2/Narrowing.md
@@ -708,9 +708,9 @@ In those cases, TypeScript will use a `never` type to represent a state which sh
 
 # Exhaustiveness checking
 
-The `never` type is assignable to every type; however, no type is assignable to `never` (except `never` itself). This means you can use narrowing and rely on `never` turning up to do exhaustive checking in a `switch` statement.
+The `never` type is assignable to every type; however, no type is assignable to `never` (except `never` itself). It is good practice to create a `assertNever` helper function that would raise a runtime error when called. This means you can use narrowing and rely on `never` turning up to do exhaustive checking in a `switch` statement.
 
-For example, adding a `default` to our `getArea` function which tries to assign the shape to `never` will not raise an error when every possible case has been handled.
+For example, adding a `default` to our `getArea` function which tries to call our `assertNever` function that expects shape to be `never` will not raise an error when every possible case has been handled.
 
 ```ts twoslash
 interface Circle {
@@ -732,10 +732,13 @@ function getArea(shape: Shape) {
     case "square":
       return shape.sideLength ** 2;
     default:
-      const _exhaustiveCheck: never = shape;
-      return _exhaustiveCheck;
+      assertNever(shape);
   }
 }
+
+function assertNever(value: never) {
+  throw new Error(`Missing case for value: ${value}`);
+}
 ```
 
 Adding a new member to the `Shape` union, will cause a TypeScript error:
@@ -766,8 +769,11 @@ function getArea(shape: Shape) {
     case "square":
       return shape.sideLength ** 2;
     default:
-      const _exhaustiveCheck: never = shape;
-      return _exhaustiveCheck;
+      assertNever(shape);
   }
 }
+
+function assertNever(value: never) {
+  throw new Error(`Missing case for value: ${value}`);
+}
 ```

From acb8701c2b1ef3098c06c7e714dcbb6cbb9e7663 Mon Sep 17 00:00:00 2001
From: whilenot <git@whilenot.dev>
Date: Thu, 2 Jul 2026 10:37:21 +0200
Subject: [PATCH 2/2] Never return on never assertion

---
 packages/documentation/copy/en/handbook-v2/Narrowing.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/documentation/copy/en/handbook-v2/Narrowing.md b/packages/documentation/copy/en/handbook-v2/Narrowing.md
index 612944c7b072..5dd774920ba0 100644
--- a/packages/documentation/copy/en/handbook-v2/Narrowing.md
+++ b/packages/documentation/copy/en/handbook-v2/Narrowing.md
@@ -736,7 +736,7 @@ function getArea(shape: Shape) {
   }
 }
 
-function assertNever(value: never) {
+function assertNever(value: never): never {
   throw new Error(`Missing case for value: ${value}`);
 }
 ```
@@ -773,7 +773,7 @@ function getArea(shape: Shape) {
   }
 }
 
-function assertNever(value: never) {
+function assertNever(value: never): never {
   throw new Error(`Missing case for value: ${value}`);
 }
 ```