fix(at): throw error when using at to access undefined property

src/askvm/resources/core/__tests__/at.test.ts

@@ -0,0 +1,64 @@
+import { runUntyped } from '../../..';
+import { parse } from '../../../../askcode';
+import * as core from '..';
+import { at } from '../at';
+
+const values = {};
+
+function ask(code: string) {
+  return runUntyped(
+    {
+      resources: { ...core, at },
+      values,
+    },
+    parse(code)
+  );
+}
+
+describe(`at`, function () {
+  it(`Should retrieve a defined property from an object.`, async function () {
+    await expect(
+      ask(
+        `ask(const('cocoObject',object('title','Coco')),call(get('at'),get('cocoObject'),'title'))`
+      )
+    ).resolves.toStrictEqual(`Coco`);
+  });
+
+  it(`Should throw when trying to access an undefined property of an object.`, async function () {
+    await expect(
+      ask(
+        `ask(const('myobject',object('title','Coco')),call(get('at'),get('myobject'),'description'))`
+      )
+    ).rejects.toThrow(
+      `Requested property key was not defined so it is not possible to get its value.`
+    );
+  });
+
+  it(`Should retrieve an element from a list.`, async function () {
+    await expect(
+      ask(
+        `ask(const('yummyList',list('Coco','Papaya','Mango')),call(get('at'),get('yummyList'),0))`
+      )
+    ).resolves.toStrictEqual(`Coco`);
+  });
+
+  it(`Should throw when accessing a nonnegative index which is out of lists bounds.`, async function () {
+    await expect(
+      ask(
+        `ask(const('yummyList',list('coco','papaya','mango')),call(get('at'),get('yummyList'),3))`
+      )
+    ).rejects.toThrow(
+      `Sorry, but index 3 is out of bounds for a list with 3 elements.`
+    );
+  });
+
+  it(`Should throw when accessing a nonnegative index which is out of lists bounds.`, async function () {
+    await expect(
+      ask(
+        `ask(const('yummyList',list('coco','papaya','mango')),call(get('at'),get('yummyList'),-1))`
+      )
+    ).rejects.toThrow(
+      `Sorry, but index -1 is out of bounds for a list with 3 elements.`
+    );
+  });
+});

src/askvm/resources/core/at.ts

@@ -3,6 +3,23 @@ import { any, resource, typed } from '../../lib';
 export const at = resource({
   type: any,
   async resolver(listOrObject: any, key: any): Promise<any> {
+    if (
+      Array.isArray(listOrObject) &&
+      Number.isInteger(key) &&
+      (listOrObject.length <= key || key < 0)
+    )
+      throw Error(
+        `Sorry, but index ${key} is out of bounds for a list with ${listOrObject.length} elements.`
+      );
+
+    if (
+      listOrObject !== null &&
+      typeof listOrObject === 'object' &&
+      !(key in listOrObject)
+    )
+      throw Error(
+        'Requested property key was not defined so it is not possible to get its value.'
+      );
     return typed(listOrObject[key]);
   },
 });