diff --git a/workspaces/adventure-pack/goodies/typescript/Object.groupBy/index.test.ts b/workspaces/adventure-pack/goodies/typescript/Object.groupBy/index.test.ts
new file mode 100644
index 00000000..0816ade1
--- /dev/null
+++ b/workspaces/adventure-pack/goodies/typescript/Object.groupBy/index.test.ts
@@ -0,0 +1,151 @@
+import { describe, expect, it } from "@jest/globals";
+
+delete (Object as unknown as Record<string, unknown>).groupBy;
+// eslint-disable-next-line import-x/first -- This has to happen after we delete the built-in implementation.
+import "./index.ts";
+
+describe("Object.groupBy", () => {
+  it("groups elements by the result of the callback function", () => {
+    const words = ["zero", "one", "two", "three", "four", "five", "six"];
+    const result = Object.groupBy(words, (word) => String(word.length));
+
+    expect(result).toStrictEqual({
+      "4": ["zero", "four", "five"],
+      "3": ["one", "two", "six"],
+      "5": ["three"],
+    });
+  });
+
+  it("allows duplicate elements", () => {
+    const numbers = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9];
+    const result = Object.groupBy(numbers, (num) =>
+      num % 2 === 0 ? "even" : "odd",
+    );
+
+    expect(result).toStrictEqual({
+      odd: [3, 1, 1, 5, 9, 5, 3, 5, 9, 7, 9],
+      even: [4, 2, 6, 8],
+    });
+  });
+
+  it("respects key order", () => {
+    const words = [
+      "zero",
+      "one",
+      "two",
+      "three",
+      "four",
+      "five",
+      "six",
+      "seven",
+      "eight",
+      "nine",
+      "ten",
+    ];
+    const result = Object.groupBy(
+      words,
+      (word) => word.replace(/[aeiou]/gi, "").length + "!",
+    );
+
+    expect(Object.keys(result)).toStrictEqual(["2!", "1!", "3!"]);
+  });
+
+  it.each([
+    [],
+    [].values(),
+    new Set().values(),
+    new Map().entries(),
+    ""[Symbol.iterator](),
+    (function* () {})(),
+  ])("handles empty iterables", (iterable) => {
+    const result = Object.groupBy(iterable, String);
+
+    expect(result).toStrictEqual({});
+  });
+
+  it("can use the index in the callback function", () => {
+    const letters = ["a", "b", "c", "d"];
+    const result = Object.groupBy(letters, (_, index) =>
+      index % 2 === 0 ? "even" : "odd",
+    );
+
+    expect(result).toStrictEqual({
+      even: ["a", "c"],
+      odd: ["b", "d"],
+    });
+  });
+
+  it("works with a Set", () => {
+    const set = new Set(["apple", "banana", "cherry", "apple"]);
+    const result = Object.groupBy(set, (fruit) => fruit[1]);
+
+    expect(result).toStrictEqual({
+      p: ["apple"],
+      a: ["banana"],
+      h: ["cherry"],
+    });
+  });
+
+  it("works with a Map", () => {
+    const map = new Map([
+      ["a", 1],
+      ["b", 2],
+      ["c", 3],
+    ]);
+    const result = Object.groupBy(map, ([, value]) =>
+      value % 2 === 0 ? "even" : "odd",
+    );
+
+    expect(result).toStrictEqual({
+      even: [["b", 2]],
+      odd: [
+        ["a", 1],
+        ["c", 3],
+      ],
+    });
+  });
+
+  it("works with a generator", () => {
+    const generator = (function* () {
+      yield 3;
+      yield 1;
+      yield 4;
+    })();
+    const result = Object.groupBy(generator, (num) =>
+      num % 2 === 0 ? "even" : "odd",
+    );
+
+    expect(result).toStrictEqual({
+      odd: [3, 1],
+      even: [4],
+    });
+  });
+
+  it("works with a custom iterable", () => {
+    const iterable = {
+      [Symbol.iterator]() {
+        let count = 0;
+        return {
+          next: () =>
+            count < 3 ? { value: ++count } : { done: true, value: undefined },
+        };
+      },
+    } as Iterable<number>;
+
+    const result = Object.groupBy(iterable, (num) =>
+      num % 2 === 0 ? "even" : "odd",
+    );
+
+    expect(result).toStrictEqual({
+      odd: [1, 3],
+      even: [2],
+    });
+  });
+
+  it("throws for non-iterable arguments", () => {
+    expect(() => {
+      // @ts-expect-error Invalid argument.
+      Object.groupBy(123, String);
+    }).toThrow(TypeError);
+  });
+});
diff --git a/workspaces/adventure-pack/goodies/typescript/Object.groupBy/index.ts b/workspaces/adventure-pack/goodies/typescript/Object.groupBy/index.ts
new file mode 100644
index 00000000..8c4369e3
--- /dev/null
+++ b/workspaces/adventure-pack/goodies/typescript/Object.groupBy/index.ts
@@ -0,0 +1,27 @@
+declare global {
+  interface ObjectConstructor {
+    groupBy<V>(
+      iterable: Iterable<V>,
+      callbackFn: (value: V, index: number) => string,
+    ): Partial<Record<string, V[]>>;
+  }
+}
+
+Object.groupBy ??= function <V>(
+  iterable: Iterable<V>,
+  callbackFn: (value: V, index: number) => string,
+): Partial<Record<string, V[]>> {
+  const groups: Partial<Record<string, V[]>> = {};
+
+  let index = 0;
+  for (const value of iterable) {
+    const key = callbackFn(value, index++);
+    (groups[key] ??= []).push(value);
+  }
+
+  return groups;
+};
+
+// Needed to fix the error "Augmentations for the global scope can only be directly nested in external modules or ambient module declarations. ts(2669)"
+// See: https://stackoverflow.com/questions/57132428/augmentations-for-the-global-scope-can-only-be-directly-nested-in-external-modul
+export {};
diff --git a/workspaces/adventure-pack/src/app/__tests__/__snapshots__/equip-test.ts.snap b/workspaces/adventure-pack/src/app/__tests__/__snapshots__/equip-test.ts.snap
index f7a3641e..974d53d3 100644
--- a/workspaces/adventure-pack/src/app/__tests__/__snapshots__/equip-test.ts.snap
+++ b/workspaces/adventure-pack/src/app/__tests__/__snapshots__/equip-test.ts.snap
@@ -2289,6 +2289,26 @@ Object.getUnsafe = function (obj, properties) {
 /////////////////////////// END ADVENTURE PACK CODE ////////////////////////////"
 `;
 
+exports[`App can equip single goody: JavaScript Object.groupBy 1`] = `
+"////////////////////////// BEGIN ADVENTURE PACK CODE ///////////////////////////
+// Adventure Pack commit fake-commit-hash
+// Running at: https://example.com/
+
+Object.groupBy ??= function (iterable, callbackFn) {
+  const groups = {};
+
+  let index = 0;
+  for (const value of iterable) {
+    const key = callbackFn(value, index++);
+    (groups[key] ??= []).push(value);
+  }
+
+  return groups;
+};
+
+/////////////////////////// END ADVENTURE PACK CODE ////////////////////////////"
+`;
+
 exports[`App can equip single goody: JavaScript Object.prototype.entries 1`] = `
 "////////////////////////// BEGIN ADVENTURE PACK CODE ///////////////////////////
 // Adventure Pack commit fake-commit-hash
@@ -4898,6 +4918,38 @@ Object.getUnsafe = function (
 /////////////////////////// END ADVENTURE PACK CODE ////////////////////////////"
 `;
 
+exports[`App can equip single goody: TypeScript Object.groupBy 1`] = `
+"////////////////////////// BEGIN ADVENTURE PACK CODE ///////////////////////////
+// Adventure Pack commit fake-commit-hash
+// Running at: https://example.com/
+
+declare global {
+  interface ObjectConstructor {
+    groupBy<V>(
+      iterable: Iterable<V>,
+      callbackFn: (value: V, index: number) => string,
+    ): Partial<Record<string, V[]>>;
+  }
+}
+
+Object.groupBy ??= function <V>(
+  iterable: Iterable<V>,
+  callbackFn: (value: V, index: number) => string,
+): Partial<Record<string, V[]>> {
+  const groups: Partial<Record<string, V[]>> = {};
+
+  let index = 0;
+  for (const value of iterable) {
+    const key = callbackFn(value, index++);
+    (groups[key] ??= []).push(value);
+  }
+
+  return groups;
+};
+
+/////////////////////////// END ADVENTURE PACK CODE ////////////////////////////"
+`;
+
 exports[`App can equip single goody: TypeScript Object.prototype.entries 1`] = `
 "////////////////////////// BEGIN ADVENTURE PACK CODE ///////////////////////////
 // Adventure Pack commit fake-commit-hash
diff --git a/workspaces/adventure-pack/src/app/__tests__/__snapshots__/render-test.ts.snap b/workspaces/adventure-pack/src/app/__tests__/__snapshots__/render-test.ts.snap
index f0a6e8f0..563003a2 100644
--- a/workspaces/adventure-pack/src/app/__tests__/__snapshots__/render-test.ts.snap
+++ b/workspaces/adventure-pack/src/app/__tests__/__snapshots__/render-test.ts.snap
@@ -1263,6 +1263,20 @@ exports[`App can render goody: JavaScript Object.getUnsafe 1`] = `
 };"
 `;
 
+exports[`App can render goody: JavaScript Object.groupBy 1`] = `
+"Object.groupBy ??= function (iterable, callbackFn) {
+  const groups = {};
+
+  let index = 0;
+  for (const value of iterable) {
+    const key = callbackFn(value, index++);
+    (groups[key] ??= []).push(value);
+  }
+
+  return groups;
+};"
+`;
+
 exports[`App can render goody: JavaScript Object.prototype.entries 1`] = `
 "import "Iterator.prototype.map";
 import "Object.prototype.keys";
@@ -2811,6 +2825,32 @@ Object.getUnsafe = function (
 };"
 `;
 
+exports[`App can render goody: TypeScript Object.groupBy 1`] = `
+"declare global {
+  interface ObjectConstructor {
+    groupBy<V>(
+      iterable: Iterable<V>,
+      callbackFn: (value: V, index: number) => string,
+    ): Partial<Record<string, V[]>>;
+  }
+}
+
+Object.groupBy ??= function <V>(
+  iterable: Iterable<V>,
+  callbackFn: (value: V, index: number) => string,
+): Partial<Record<string, V[]>> {
+  const groups: Partial<Record<string, V[]>> = {};
+
+  let index = 0;
+  for (const value of iterable) {
+    const key = callbackFn(value, index++);
+    (groups[key] ??= []).push(value);
+  }
+
+  return groups;
+};"
+`;
+
 exports[`App can render goody: TypeScript Object.prototype.entries 1`] = `
 "import "Iterator.prototype.map";
 import "Object.prototype.keys";