Updated 28

Author: mattpocock

src/06-identity-functions/28-constraints-with-const-annotations.solution.1.ts

@@ -0,0 +1,47 @@
+import { it } from "vitest";
+import { Equal, Expect } from "../helpers/type-utils";
+
+type FruitsConstraint = readonly {
+  name: string;
+  price: number;
+}[];
+
+export const narrowFruits = <const TFruits extends FruitsConstraint>(
+  t: TFruits
+) => t;
+
+const fruits = narrowFruits([
+  {
+    name: "apple",
+    price: 1,
+  },
+  {
+    name: "banana",
+    price: 2,
+  },
+]);
+
+type tests = [
+  Expect<
+    Equal<
+      typeof fruits,
+      readonly [
+        {
+          readonly name: "apple";
+          readonly price: 1;
+        },
+        {
+          readonly name: "banana";
+          readonly price: 2;
+        }
+      ]
+    >
+  >
+];
+
+it("Should ONLY let you pass an array of fruits", () => {
+  const notAllowed = narrowFruits([
+    // @ts-expect-error
+    "not allowed",
+  ]);
+});

src/06-identity-functions/28-constraints-with-const-annotations.solution.ts renamed to src/06-identity-functions/28-constraints-with-const-annotations.solution.2.ts

@@ -1,8 +1,13 @@
 import { it } from "vitest";
 import { Equal, Expect } from "../helpers/type-utils";
 
-export const narrowFruits = <const TFruits extends readonly { name: string; price: number }[]>(
-  t: TFruits,
+export const narrowFruits = <
+  const TFruits extends ReadonlyArray<{
+    name: string;
+    price: number;
+  }>
+>(
+  t: TFruits
 ) => t;
 
 const fruits = narrowFruits([
@@ -28,10 +33,10 @@ type tests = [
         {
           readonly name: "banana";
           readonly price: 2;
-        },
+        }
       ]
     >
-  >,
+  >
 ];
 
 it("Should ONLY let you pass an array of fruits", () => {