Add Array.at(index) to lualib (#1484)

lolleko · web-flow · commit cf79978a00c0 · 2023-09-12T20:58:21.000+02:00
diff --git a/src/LuaLib.ts b/src/LuaLib.ts
@@ -5,6 +5,7 @@ import { LuaTarget } from "./CompilerOptions";
 import { getOrUpdate } from "./utils";
 
 export enum LuaLibFeature {
+    ArrayAt = "ArrayAt",
     ArrayConcat = "ArrayConcat",
     ArrayEntries = "ArrayEntries",
     ArrayEvery = "ArrayEvery",
diff --git a/src/lualib/ArrayAt.ts b/src/lualib/ArrayAt.ts
@@ -0,0 +1,13 @@
+// https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.prototype.at
+// Technically the specs also allow non-numeric types to be passed as index.
+// However, TypeScript types the `Array.at` index param as number so we also expect only numbers
+// This behaviour also matches the implementation of other Array functions in lualib.
+export function __TS__ArrayAt<T>(this: T[], relativeIndex: number): T | undefined {
+    const absoluteIndex = relativeIndex < 0 ? this.length + relativeIndex : relativeIndex;
+
+    if (absoluteIndex >= 0 && absoluteIndex < this.length) {
+        return this[absoluteIndex];
+    }
+
+    return undefined;
+}
diff --git a/src/transformation/builtins/array.ts b/src/transformation/builtins/array.ts
@@ -88,6 +88,8 @@ export function transformArrayPrototypeCall(
 
     const expressionName = calledMethod.name.text;
     switch (expressionName) {
+        case "at":
+            return transformLuaLibFunction(context, LuaLibFeature.ArrayAt, node, caller, ...params);
         case "concat":
             return transformLuaLibFunction(context, LuaLibFeature.ArrayConcat, node, caller, ...params);
         case "entries":
diff --git a/test/unit/builtins/array.spec.ts b/test/unit/builtins/array.spec.ts
@@ -272,6 +272,36 @@ test("tuple.forEach", () => {
     `.expectToMatchJsResult();
 });
 
+describe("at", () => {
+    test("valid index", () => {
+        util.testFunction`
+            const array = [1, 2, 3, 4];
+            return array.at(2);
+        `.expectToMatchJsResult();
+    });
+
+    test("invalid index", () => {
+        util.testFunction`
+            const array = [1, 2, 3, 4];
+            return array.at(10);
+        `.expectToMatchJsResult();
+    });
+
+    test("valid negative index", () => {
+        util.testFunction`
+            const array = [1, 2, 3, 4];
+            return array.at(-2);
+        `.expectToMatchJsResult();
+    });
+
+    test("invalid negative index", () => {
+        util.testFunction`
+            const array = [1, 2, 3, 4];
+            return array.at(-10);
+        `.expectToMatchJsResult();
+    });
+});
+
 test("array.forEach (%p)", () => {
     util.testFunction`
         const array = [0, 1, 2, 3];
