[ReadonlyArray] .at() fix undefined issue

src/entrypoints/array-at.d.ts

@@ -1,9 +1,18 @@
 /// <reference path="utils.d.ts" />
 
 interface ReadonlyArray<T> {
-	at<I extends number> (index: I): `${I}` extends `-${infer J extends number}`
-		? `${TSReset.Subtract<this["length"], J>}` extends `-${number}`
-			? undefined
-			: this[TSReset.Subtract<this["length"], J>]
-		: this[I]
-}
+  at<
+    const N extends number,
+    I extends number = `${N}` extends `${infer J extends number}.${number}`
+      ? J
+      : N,
+  >(
+    index: N,
+  ): TSReset.Equal<I, number> extends true
+    ? T | undefined
+    : `${I}` extends `-${infer J extends number}`
+    ? `${TSReset.Subtract<this["length"], J>}` extends `-${number}`
+      ? undefined
+      : this[TSReset.Subtract<this["length"], J>]
+    : this[I];
+}

src/entrypoints/utils.d.ts

@@ -15,15 +15,25 @@ declare namespace TSReset {
     ? symbol
     : T;
 
-  type Length<T extends any[]> = T extends { length: infer L }
-    ? L
-    : never
-
-  type BuildTuple<L extends number, T extends any[] = []> = T extends { length: L }
+  type BuildTuple<L extends number, T extends any[] = []> = T extends {
+    length: L;
+  }
     ? T
-    : BuildTuple<L, [...T, any]>
-
-  type Subtract<A extends number, B extends number> = BuildTuple<A> extends [...(infer U), ...BuildTuple<B>]
-    ? Length<U>
-    : never
+    : BuildTuple<L, [...T, unknown]>;
+
+  // Extra `A extends number` and `B extends number` needed for union types to work Such as Subtract<10 | 20, 1>
+  type Subtract<A extends number, B extends number> = A extends number
+    ? B extends number
+      ? BuildTuple<A> extends [...infer U, ...BuildTuple<B>]
+        ? U["length"]
+        : never
+      : never
+    : never;
+
+  type Equal<X, Y> = (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y
+    ? 1
+    : 2
+    ? true
+    : false;
+  type NotEqual<X, Y> = true extends Equal<X, Y> ? false : true;
 }

src/tests/array-at.ts

@@ -1,42 +1,60 @@
 import { doNotExecute, Equal, Expect } from "./utils";
 
 doNotExecute(async () => {
-  const arr = [false, 1, '2'] as const;
+  const arr = [false, 1, "2"] as const;
 
-  const a = arr.at(0)
-  const b = arr.at(1)
-  const c = arr.at(2)
-  const d = arr.at(3)
+  const a = arr.at(0);
+  const b = arr.at(1);
+  const c = arr.at(2);
+  const d = arr.at(3);
+  const e = arr.at(1.5);
   type tests = [
     Expect<Equal<typeof a, false>>,
     Expect<Equal<typeof b, 1>>,
-    Expect<Equal<typeof c, '2'>>,
+    Expect<Equal<typeof c, "2">>,
     Expect<Equal<typeof d, undefined>>,
-  ]
+    Expect<Equal<typeof e, 1>>,
+  ];
 });
 
 doNotExecute(async () => {
-  const arr = [false, 1, '2'] as const;
+  const arr = [false, 1, "2"] as const;
 
-  const a = arr.at(-1)
-  const b = arr.at(-2)
-  const c = arr.at(-3)
-  const d = arr.at(-4)
+  const a = arr.at(-1);
+  const b = arr.at(-2);
+  const c = arr.at(-3);
+  const d = arr.at(-4);
+  const e = arr.at(-1.5);
   type tests = [
-    Expect<Equal<typeof a, '2'>>,
+    Expect<Equal<typeof a, "2">>,
     Expect<Equal<typeof b, 1>>,
     Expect<Equal<typeof c, false>>,
     Expect<Equal<typeof d, undefined>>,
-  ]
+    Expect<Equal<typeof e, "2">>,
+  ];
 });
 
 doNotExecute(async () => {
-  const arr = [false, 1, '2'] as const;
-  const index: number = 1
-  const a = arr.at(index)
-  // WARN: with `"strictNullChecks": true,` the correct type here should include `undefined`
-  // but the current implementation does not type this as including `undefined`
-  type tests = [
-    Expect<Equal<typeof a, false | 1 | "2">>,
-  ]
-});
+  const arr = [false, 1, "2"] as const;
+
+  const index = 0 as 0 | 1
+
+  const a = arr.at(index);
+  type tests = [Expect<Equal<typeof a, false | 1>>];
+});
+
+doNotExecute(async () => {
+  const arr = [false, true, 1, "2"] as const;
+
+  const index = -1 as -1 | -2
+
+  const a = arr.at(index);
+  type tests = [Expect<Equal<typeof a, "2" | 1>>];
+});
+
+doNotExecute(async () => {
+  const arr = [false, 1, "2"] as const;
+  const index = 1 as number;
+  const a = arr.at(index);
+  type tests = [Expect<Equal<typeof a, false | 1 | "2" | undefined>>];
+});