From 1d6f74eab148440829792c69c93aa817a8ef59d0 Mon Sep 17 00:00:00 2001
From: Devin Riegle <devin.riegle@joinroot.com>
Date: Tue, 24 Nov 2020 11:04:16 -0500
Subject: [PATCH] Add mockFn.mock.lastCall to easily retrieve the last call
 arguments

---
 docs/MockFunctionAPI.md                       | 13 +++++++++
 docs/MockFunctions.md                         |  3 +++
 .../jest-mock/src/__tests__/index.test.ts     | 27 +++++++++++++++++++
 packages/jest-mock/src/index.ts               |  8 ++++++
 4 files changed, 51 insertions(+)

diff --git a/docs/MockFunctionAPI.md b/docs/MockFunctionAPI.md
index 5bb9002d4069..3b6454e75e84 100644
--- a/docs/MockFunctionAPI.md
+++ b/docs/MockFunctionAPI.md
@@ -77,6 +77,19 @@ mockFn.mock.instances[0] === a; // true
 mockFn.mock.instances[1] === b; // true
 ```
 
+### `mockFn.mock.lastCall`
+
+An array containing the call arguments of the last call that was made to this mock function. If the function was not called, it will return `undefined`.
+
+For example: A mock function `f` that has been called twice, with the arguments `f('arg1', 'arg2')`, and then with the arguments `f('arg3', 'arg4')`, would have a `mock.lastCall` array that looks like this:
+
+```js
+[
+  'arg3',
+  'arg4',
+];
+```
+
 ### `mockFn.mockClear()`
 
 Resets all information stored in the [`mockFn.mock.calls`](#mockfnmockcalls) and [`mockFn.mock.instances`](#mockfnmockinstances) arrays.
diff --git a/docs/MockFunctions.md b/docs/MockFunctions.md
index e28ba0960c37..3217a733a7e3 100644
--- a/docs/MockFunctions.md
+++ b/docs/MockFunctions.md
@@ -75,6 +75,9 @@ expect(someMockFunction.mock.instances.length).toBe(2);
 // The object returned by the first instantiation of this function
 // had a `name` property whose value was set to 'test'
 expect(someMockFunction.mock.instances[0].name).toEqual('test');
+
+// The first argument of the last call to the function was 'test'
+expect(someMockFunction.mock.lastCall[0]).toBe('test');
 ```
 
 ## Mock Return Values
diff --git a/packages/jest-mock/src/__tests__/index.test.ts b/packages/jest-mock/src/__tests__/index.test.ts
index d68857294cb4..0be108b2610b 100644
--- a/packages/jest-mock/src/__tests__/index.test.ts
+++ b/packages/jest-mock/src/__tests__/index.test.ts
@@ -1064,6 +1064,33 @@ describe('moduleMocker', () => {
     expect(fn.getMockName()).toBe('myMockFn');
   });
 
+
+  test('jest.fn should provide the correct lastCall', () => {
+    const mock = jest.fn();
+
+    expect(mock.mock.lastCall).toBeUndefined();
+
+    mock('first');
+    mock('second');
+    mock('last', 'call');
+
+    expect(mock).toHaveBeenLastCalledWith('last', 'call');
+    expect(mock.mock.lastCall).toEqual(['last', 'call']);
+  });
+
+  test('lastCall gets reset by mockReset', () => {
+    const mock = jest.fn();
+
+    mock('first');
+    mock('last', 'call');
+
+    expect(mock.mock.lastCall).toEqual(['last', 'call']);
+
+    mock.mockReset();
+
+    expect(mock.mock.lastCall).toBeUndefined();
+  });
+
   test('mockName gets reset by mockReset', () => {
     const fn = jest.fn();
     expect(fn.getMockName()).toBe('jest.fn()');
diff --git a/packages/jest-mock/src/index.ts b/packages/jest-mock/src/index.ts
index 3d3dcd50e199..aeab76303b6a 100644
--- a/packages/jest-mock/src/index.ts
+++ b/packages/jest-mock/src/index.ts
@@ -101,6 +101,10 @@ type MockFunctionState<T, Y extends Array<unknown>> = {
   calls: Array<Y>;
   instances: Array<T>;
   invocationCallOrder: Array<number>;
+  /**
+   * Getter for retrieving the last call arguments
+   */
+  lastCall?: Y;
   /**
    * List of results of calls to the mock function.
    */
@@ -456,6 +460,9 @@ class ModuleMockerClass {
       state = this._defaultMockState();
       this._mockState.set(f, state);
     }
+
+    state.lastCall = state.calls[state.calls.length - 1];
+
     return state;
   }
 
@@ -473,6 +480,7 @@ class ModuleMockerClass {
     Y
   > {
     return {
+      lastCall: undefined,
       calls: [],
       instances: [],
       invocationCallOrder: [],