From 44d3fb008d30b638402819fa73d9bc9efb5070c9 Mon Sep 17 00:00:00 2001
From: JanMalch <25508038+JanMalch@users.noreply.github.com>
Date: Fri, 30 Oct 2020 12:48:13 +0100
Subject: [PATCH] feat: add optional message to unreachable

---
 README.md     | 6 +++++-
 index.test.ts | 8 +++++++-
 index.ts      | 8 ++++++--
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 70256bc..f132f5b 100644
--- a/README.md
+++ b/README.md
@@ -209,10 +209,14 @@ Asserts that a code branch is unreachable. If it is, the compiler will throw a t
 If this function is reached at runtime, an error will be thrown.
 
 ```ts
-function unreachable(value: never): never;
+function unreachable(
+  value: never,
+  message = 'Reached an unreachable case'
+): never;
 ```
 
 - `value` - a value
+- `message` - an optional message for the error
 
 **Example:**
 
diff --git a/index.test.ts b/index.test.ts
index 75bc233..5dbf0b1 100644
--- a/index.test.ts
+++ b/index.test.ts
@@ -122,7 +122,13 @@ describe('unreachable', () => {
       'Reached an unreachable case'
     );
   });
-
+  it('should always throw an error at runtime with the given message', () => {
+    expectError(
+      () => unreachable(true as never, 'Test'),
+      AssertionError,
+      'Test'
+    );
+  });
   it('should not throw an error when the switch is exhaustive', () => {
     enum MyEnum {
       A,
diff --git a/index.ts b/index.ts
index e237155..2509830 100644
--- a/index.ts
+++ b/index.ts
@@ -256,6 +256,7 @@ export function error(
  * Asserts that a code branch is unreachable. If it is, the compiler will throw a type error.
  * If this function is reached at runtime, an error will be thrown.
  * @param value a value
+ * @param message an optional message for the error
  * @throws AssertionError in any case
  * @example
  * function myFun(foo: MyEnum): string {
@@ -267,6 +268,9 @@ export function error(
  *   }
  * }
  */
-export function unreachable(value: never): never {
-  throw new AssertionError('Reached an unreachable case');
+export function unreachable(
+  value: never,
+  message = 'Reached an unreachable case'
+): never {
+  throw new AssertionError(message);
 }