test_runner: support function mocking

This commit allows tests in the test runner to mock functions
and methods.

PR-URL: #45326
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: cjihrig

doc/api/test.md

@@ -352,6 +352,89 @@ Otherwise, the test is considered to be a failure. Test files must be
 executable by Node.js, but are not required to use the `node:test` module
 internally.
 
+## Mocking
+
+The `node:test` module supports mocking during testing via a top-level `mock`
+object. The following example creates a spy on a function that adds two numbers
+together. The spy is then used to assert that the function was called as
+expected.
+
+```mjs
+import assert from 'node:assert';
+import { mock, test } from 'node:test';
+
+test('spies on a function', () => {
+  const sum = mock.fn((a, b) => {
+    return a + b;
+  });
+
+  assert.strictEqual(sum.mock.calls.length, 0);
+  assert.strictEqual(sum(3, 4), 7);
+  assert.strictEqual(sum.mock.calls.length, 1);
+
+  const call = sum.mock.calls[0];
+  assert.deepStrictEqual(call.arguments, [3, 4]);
+  assert.strictEqual(call.result, 7);
+  assert.strictEqual(call.error, undefined);
+
+  // Reset the globally tracked mocks.
+  mock.reset();
+});
+```
+
+```cjs
+'use strict';
+const assert = require('node:assert');
+const { mock, test } = require('node:test');
+
+test('spies on a function', () => {
+  const sum = mock.fn((a, b) => {
+    return a + b;
+  });
+
+  assert.strictEqual(sum.mock.calls.length, 0);
+  assert.strictEqual(sum(3, 4), 7);
+  assert.strictEqual(sum.mock.calls.length, 1);
+
+  const call = sum.mock.calls[0];
+  assert.deepStrictEqual(call.arguments, [3, 4]);
+  assert.strictEqual(call.result, 7);
+  assert.strictEqual(call.error, undefined);
+
+  // Reset the globally tracked mocks.
+  mock.reset();
+});
+```
+
+The same mocking functionality is also exposed on the [`TestContext`][] object
+of each test. The following example creates a spy on an object method using the
+API exposed on the `TestContext`. The benefit of mocking via the test context is
+that the test runner will automatically restore all mocked functionality once
+the test finishes.
+
+```js
+test('spies on an object method', (t) => {
+  const number = {
+    value: 5,
+    add(a) {
+      return this.value + a;
+    },
+  };
+
+  t.mock.method(number, 'add');
+  assert.strictEqual(number.add.mock.calls.length, 0);
+  assert.strictEqual(number.add(3), 8);
+  assert.strictEqual(number.add.mock.calls.length, 1);
+
+  const call = number.add.mock.calls[0];
+
+  assert.deepStrictEqual(call.arguments, [3]);
+  assert.strictEqual(call.result, 8);
+  assert.strictEqual(call.target, undefined);
+  assert.strictEqual(call.this, number);
+});
+```
+
 ## `run([options])`
 
 <!-- YAML
@@ -644,6 +727,281 @@ describe('tests', async () => {
 });
 ```
 
+## Class: `MockFunctionContext`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+The `MockFunctionContext` class is used to inspect or manipulate the behavior of
+mocks created via the [`MockTracker`][] APIs.
+
+### `ctx.calls`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* {Array}
+
+A getter that returns a copy of the internal array used to track calls to the
+mock. Each entry in the array is an object with the following properties.
+
+* `arguments` {Array} An array of the arguments passed to the mock function.
+* `error` {any} If the mocked function threw then this property contains the
+  thrown value. **Default:** `undefined`.
+* `result` {any} The value returned by the mocked function.
+* `stack` {Error} An `Error` object whose stack can be used to determine the
+  callsite of the mocked function invocation.
+* `target` {Function|undefined} If the mocked function is a constructor, this
+  field contains the class being constructed. Otherwise this will be
+  `undefined`.
+* `this` {any} The mocked function's `this` value.
+
+### `ctx.callCount()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Returns: {integer} The number of times that this mock has been invoked.
+
+This function returns the number of times that this mock has been invoked. This
+function is more efficient than checking `ctx.calls.length` because `ctx.calls`
+is a getter that creates a copy of the internal call tracking array.
+
+### `ctx.mockImplementation(implementation)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `implementation` {Function|AsyncFunction} The function to be used as the
+  mock's new implementation.
+
+This function is used to change the behavior of an existing mock.
+
+The following example creates a mock function using `t.mock.fn()`, calls the
+mock function, and then changes the mock implementation to a different function.
+
+```js
+test('changes a mock behavior', (t) => {
+  let cnt = 0;
+
+  function addOne() {
+    cnt++;
+    return cnt;
+  }
+
+  function addTwo() {
+    cnt += 2;
+    return cnt;
+  }
+
+  const fn = t.mock.fn(addOne);
+
+  assert.strictEqual(fn(), 1);
+  fn.mock.mockImplementation(addTwo);
+  assert.strictEqual(fn(), 3);
+  assert.strictEqual(fn(), 5);
+});
+```
+
+### `ctx.mockImplementationOnce(implementation[, onCall])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `implementation` {Function|AsyncFunction} The function to be used as the
+  mock's implementation for the invocation number specified by `onCall`.
+* `onCall` {integer} The invocation number that will use `implementation`. If
+  the specified invocation has already occurred then an exception is thrown.
+  **Default:** The number of the next invocation.
+
+This function is used to change the behavior of an existing mock for a single
+invocation. Once invocation `onCall` has occurred, the mock will revert to
+whatever behavior it would have used had `mockImplementationOnce()` not been
+called.
+
+The following example creates a mock function using `t.mock.fn()`, calls the
+mock function, changes the mock implementation to a different function for the
+next invocation, and then resumes its previous behavior.
+
+```js
+test('changes a mock behavior once', (t) => {
+  let cnt = 0;
+
+  function addOne() {
+    cnt++;
+    return cnt;
+  }
+
+  function addTwo() {
+    cnt += 2;
+    return cnt;
+  }
+
+  const fn = t.mock.fn(addOne);
+
+  assert.strictEqual(fn(), 1);
+  fn.mock.mockImplementationOnce(addTwo);
+  assert.strictEqual(fn(), 3);
+  assert.strictEqual(fn(), 4);
+});
+```
+
+### `ctx.restore()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+Resets the implementation of the mock function to its original behavior. The
+mock can still be used after calling this function.
+
+## Class: `MockTracker`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+The `MockTracker` class is used to manage mocking functionality. The test runner
+module provides a top level `mock` export which is a `MockTracker` instance.
+Each test also provides its own `MockTracker` instance via the test context's
+`mock` property.
+
+### `mock.fn([original[, implementation]][, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `original` {Function|AsyncFunction} An optional function to create a mock on.
+  **Default:** A no-op function.
+* `implementation` {Function|AsyncFunction} An optional function used as the
+  mock implementation for `original`. This is useful for creating mocks that
+  exhibit one behavior for a specified number of calls and then restore the
+  behavior of `original`. **Default:** The function specified by `original`.
+* `options` {Object} Optional configuration options for the mock function. The
+  following properties are supported:
+  * `times` {integer} The number of times that the mock will use the behavior of
+    `implementation`. Once the mock function has been called `times` times, it
+    will automatically restore the behavior of `original`. This value must be an
+    integer greater than zero. **Default:** `Infinity`.
+* Returns: {Proxy} The mocked function. The mocked function contains a special
+  `mock` property, which is an instance of [`MockFunctionContext`][], and can
+  be used for inspecting and changing the behavior of the mocked function.
+
+This function is used to create a mock function.
+
+The following example creates a mock function that increments a counter by one
+on each invocation. The `times` option is used to modify the mock behavior such
+that the first two invocations add two to the counter instead of one.
+
+```js
+test('mocks a counting function', (t) => {
+  let cnt = 0;
+
+  function addOne() {
+    cnt++;
+    return cnt;
+  }
+
+  function addTwo() {
+    cnt += 2;
+    return cnt;
+  }
+
+  const fn = t.mock.fn(addOne, addTwo, { times: 2 });
+
+  assert.strictEqual(fn(), 2);
+  assert.strictEqual(fn(), 4);
+  assert.strictEqual(fn(), 5);
+  assert.strictEqual(fn(), 6);
+});
+```
+
+### `mock.method(object, methodName[, implementation][, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `object` {Object} The object whose method is being mocked.
+* `methodName` {string|symbol} The identifier of the method on `object` to mock.
+  If `object[methodName]` is not a function, an error is thrown.
+* `implementation` {Function|AsyncFunction} An optional function used as the
+  mock implementation for `object[methodName]`. **Default:** The original method
+  specified by `object[methodName]`.
+* `options` {Object} Optional configuration options for the mock method. The
+  following properties are supported:
+  * `getter` {boolean} If `true`, `object[methodName]` is treated as a getter.
+    This option cannot be used with the `setter` option. **Default:** false.
+  * `setter` {boolean} If `true`, `object[methodName]` is treated as a setter.
+    This option cannot be used with the `getter` option. **Default:** false.
+  * `times` {integer} The number of times that the mock will use the behavior of
+    `implementation`. Once the mocked method has been called `times` times, it
+    will automatically restore the original behavior. This value must be an
+    integer greater than zero. **Default:** `Infinity`.
+* Returns: {Proxy} The mocked method. The mocked method contains a special
+  `mock` property, which is an instance of [`MockFunctionContext`][], and can
+  be used for inspecting and changing the behavior of the mocked method.
+
+This function is used to create a mock on an existing object method. The
+following example demonstrates how a mock is created on an existing object
+method.
+
+```js
+test('spies on an object method', (t) => {
+  const number = {
+    value: 5,
+    subtract(a) {
+      return this.value - a;
+    },
+  };
+
+  t.mock.method(number, 'subtract');
+  assert.strictEqual(number.subtract.mock.calls.length, 0);
+  assert.strictEqual(number.subtract(3), 2);
+  assert.strictEqual(number.subtract.mock.calls.length, 1);
+
+  const call = number.subtract.mock.calls[0];
+
+  assert.deepStrictEqual(call.arguments, [3]);
+  assert.strictEqual(call.result, 2);
+  assert.strictEqual(call.error, undefined);
+  assert.strictEqual(call.target, undefined);
+  assert.strictEqual(call.this, number);
+});
+```
+
+### `mock.reset()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+This function restores the default behavior of all mocks that were previously
+created by this `MockTracker` and disassociates the mocks from the
+`MockTracker` instance. Once disassociated, the mocks can still be used, but the
+`MockTracker` instance can no longer be used to reset their behavior or
+otherwise interact with them.
+
+After each test completes, this function is called on the test context's
+`MockTracker`. If the global `MockTracker` is used extensively, calling this
+function manually is recommended.
+
+### `mock.restoreAll()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+This function restores the default behavior of all mocks that were previously
+created by this `MockTracker`. Unlike `mock.reset()`, `mock.restoreAll()` does
+not disassociate the mocks from the `MockTracker` instance.
+
 ## Class: `TapStream`
 
 <!-- YAML
@@ -979,6 +1337,8 @@ added:
 [`--test-name-pattern`]: cli.md#--test-name-pattern
 [`--test-only`]: cli.md#--test-only
 [`--test`]: cli.md#--test
+[`MockFunctionContext`]: #class-mockfunctioncontext
+[`MockTracker`]: #class-mocktracker
 [`SuiteContext`]: #class-suitecontext
 [`TestContext`]: #class-testcontext
 [`context.diagnostic`]: #contextdiagnosticmessage

lib/internal/test_runner/mock.js

@@ -0,0 +1,295 @@
+'use strict';
+const {
+  ArrayPrototypePush,
+  ArrayPrototypeSlice,
+  Error,
+  FunctionPrototypeCall,
+  ObjectDefineProperty,
+  ObjectGetOwnPropertyDescriptor,
+  Proxy,
+  ReflectApply,
+  ReflectConstruct,
+  ReflectGet,
+  SafeMap,
+} = primordials;
+const {
+  codes: {
+    ERR_INVALID_ARG_TYPE,
+    ERR_INVALID_ARG_VALUE,
+  },
+} = require('internal/errors');
+const { kEmptyObject } = require('internal/util');
+const {
+  validateBoolean,
+  validateFunction,
+  validateInteger,
+  validateObject,
+} = require('internal/validators');
+
+function kDefaultFunction() {}
+
+class MockFunctionContext {
+  #calls;
+  #mocks;
+  #implementation;
+  #restore;
+  #times;
+
+  constructor(implementation, restore, times) {
+    this.#calls = [];
+    this.#mocks = new SafeMap();
+    this.#implementation = implementation;
+    this.#restore = restore;
+    this.#times = times;
+  }
+
+  get calls() {
+    return ArrayPrototypeSlice(this.#calls, 0);
+  }
+
+  callCount() {
+    return this.#calls.length;
+  }
+
+  mockImplementation(implementation) {
+    validateFunction(implementation, 'implementation');
+    this.#implementation = implementation;
+  }
+
+  mockImplementationOnce(implementation, onCall) {
+    validateFunction(implementation, 'implementation');
+    const nextCall = this.#calls.length;
+    const call = onCall ?? nextCall;
+    validateInteger(call, 'onCall', nextCall);
+    this.#mocks.set(call, implementation);
+  }
+
+  restore() {
+    const { descriptor, object, original, methodName } = this.#restore;
+
+    if (typeof methodName === 'string') {
+      // This is an object method spy.
+      ObjectDefineProperty(object, methodName, descriptor);
+    } else {
+      // This is a bare function spy. There isn't much to do here but make
+      // the mock call the original function.
+      this.#implementation = original;
+    }
+  }
+
+  trackCall(call) {
+    ArrayPrototypePush(this.#calls, call);
+  }
+
+  nextImpl() {
+    const nextCall = this.#calls.length;
+    const mock = this.#mocks.get(nextCall);
+    const impl = mock ?? this.#implementation;
+
+    if (nextCall + 1 === this.#times) {
+      this.restore();
+    }
+
+    this.#mocks.delete(nextCall);
+    return impl;
+  }
+}
+
+const { nextImpl, restore, trackCall } = MockFunctionContext.prototype;
+delete MockFunctionContext.prototype.trackCall;
+delete MockFunctionContext.prototype.nextImpl;
+
+class MockTracker {
+  #mocks = [];
+
+  fn(
+    original = function() {},
+    implementation = original,
+    options = kEmptyObject,
+  ) {
+    if (original !== null && typeof original === 'object') {
+      options = original;
+      original = function() {};
+      implementation = original;
+    } else if (implementation !== null && typeof implementation === 'object') {
+      options = implementation;
+      implementation = original;
+    }
+
+    validateFunction(original, 'original');
+    validateFunction(implementation, 'implementation');
+    validateObject(options, 'options');
+    const { times = Infinity } = options;
+    validateTimes(times, 'options.times');
+    const ctx = new MockFunctionContext(implementation, { original }, times);
+    return this.#setupMock(ctx, original);
+  }
+
+  method(
+    object,
+    methodName,
+    implementation = kDefaultFunction,
+    options = kEmptyObject,
+  ) {
+    validateObject(object, 'object');
+    validateStringOrSymbol(methodName, 'methodName');
+
+    if (implementation !== null && typeof implementation === 'object') {
+      options = implementation;
+      implementation = kDefaultFunction;
+    }
+
+    validateFunction(implementation, 'implementation');
+    validateObject(options, 'options');
+
+    const {
+      getter = false,
+      setter = false,
+      times = Infinity,
+    } = options;
+
+    validateBoolean(getter, 'options.getter');
+    validateBoolean(setter, 'options.setter');
+    validateTimes(times, 'options.times');
+
+    if (setter && getter) {
+      throw new ERR_INVALID_ARG_VALUE(
+        'options.setter', setter, "cannot be used with 'options.getter'"
+      );
+    }
+
+    const descriptor = ObjectGetOwnPropertyDescriptor(object, methodName);
+    let original;
+
+    if (getter) {
+      original = descriptor?.get;
+    } else if (setter) {
+      original = descriptor?.set;
+    } else {
+      original = descriptor?.value;
+    }
+
+    if (typeof original !== 'function') {
+      throw new ERR_INVALID_ARG_VALUE(
+        'methodName', original, 'must be a method'
+      );
+    }
+
+    const restore = { descriptor, object, methodName };
+    const impl = implementation === kDefaultFunction ?
+      original : implementation;
+    const ctx = new MockFunctionContext(impl, restore, times);
+    const mock = this.#setupMock(ctx, original);
+    const mockDescriptor = {
+      __proto__: null,
+      configurable: descriptor.configurable,
+      enumerable: descriptor.enumerable,
+    };
+
+    if (getter) {
+      mockDescriptor.get = mock;
+      mockDescriptor.set = descriptor.set;
+    } else if (setter) {
+      mockDescriptor.get = descriptor.get;
+      mockDescriptor.set = mock;
+    } else {
+      mockDescriptor.writable = descriptor.writable;
+      mockDescriptor.value = mock;
+    }
+
+    ObjectDefineProperty(object, methodName, mockDescriptor);
+
+    return mock;
+  }
+
+  reset() {
+    this.restoreAll();
+    this.#mocks = [];
+  }
+
+  restoreAll() {
+    for (let i = 0; i < this.#mocks.length; i++) {
+      FunctionPrototypeCall(restore, this.#mocks[i]);
+    }
+  }
+
+  #setupMock(ctx, fnToMatch) {
+    const mock = new Proxy(fnToMatch, {
+      __proto__: null,
+      apply(_fn, thisArg, argList) {
+        const fn = FunctionPrototypeCall(nextImpl, ctx);
+        let result;
+        let error;
+
+        try {
+          result = ReflectApply(fn, thisArg, argList);
+        } catch (err) {
+          error = err;
+          throw err;
+        } finally {
+          FunctionPrototypeCall(trackCall, ctx, {
+            arguments: argList,
+            error,
+            result,
+            // eslint-disable-next-line no-restricted-syntax
+            stack: new Error(),
+            target: undefined,
+            this: thisArg,
+          });
+        }
+
+        return result;
+      },
+      construct(target, argList, newTarget) {
+        const realTarget = FunctionPrototypeCall(nextImpl, ctx);
+        let result;
+        let error;
+
+        try {
+          result = ReflectConstruct(realTarget, argList, newTarget);
+        } catch (err) {
+          error = err;
+          throw err;
+        } finally {
+          FunctionPrototypeCall(trackCall, ctx, {
+            arguments: argList,
+            error,
+            result,
+            // eslint-disable-next-line no-restricted-syntax
+            stack: new Error(),
+            target,
+            this: result,
+          });
+        }
+
+        return result;
+      },
+      get(target, property, receiver) {
+        if (property === 'mock') {
+          return ctx;
+        }
+
+        return ReflectGet(target, property, receiver);
+      },
+    });
+
+    ArrayPrototypePush(this.#mocks, ctx);
+    return mock;
+  }
+}
+
+function validateStringOrSymbol(value, name) {
+  if (typeof value !== 'string' && typeof value !== 'symbol') {
+    throw new ERR_INVALID_ARG_TYPE(name, ['string', 'symbol'], value);
+  }
+}
+
+function validateTimes(value, name) {
+  if (value === Infinity) {
+    return;
+  }
+
+  validateInteger(value, name, 1);
+}
+
+module.exports = { MockTracker };

lib/internal/test_runner/test.js

@@ -32,6 +32,7 @@ const {
   AbortError,
 } = require('internal/errors');
 const { getOptionValue } = require('internal/options');
+const { MockTracker } = require('internal/test_runner/mock');
 const { TapStream } = require('internal/test_runner/tap_stream');
 const {
   convertStringToRegExp,
@@ -112,6 +113,11 @@ class TestContext {
     this.#test.diagnostic(message);
   }
 
+  get mock() {
+    this.#test.mock ??= new MockTracker();
+    return this.#test.mock;
+  }
+
   runOnly(value) {
     this.#test.runOnlySubtests = !!value;
   }
@@ -239,6 +245,7 @@ class Test extends AsyncResource {
     this.#outerSignal?.addEventListener('abort', this.#abortHandler);
 
     this.fn = fn;
+    this.mock = null;
     this.name = name;
     this.parent = parent;
     this.cancelled = false;
@@ -593,6 +600,7 @@ class Test extends AsyncResource {
     }
 
     this.#outerSignal?.removeEventListener('abort', this.#abortHandler);
+    this.mock?.reset();
 
     if (this.parent !== null) {
       this.parent.activeSubtests--;

lib/test.js

@@ -1,5 +1,5 @@
 'use strict';
-const { ObjectAssign } = primordials;
+const { ObjectAssign, ObjectDefineProperty } = primordials;
 const { test, describe, it, before, after, beforeEach, afterEach } = require('internal/test_runner/harness');
 const { run } = require('internal/test_runner/runner');
 
@@ -14,3 +14,20 @@ ObjectAssign(module.exports, {
   run,
   test,
 });
+
+let lazyMock;
+
+ObjectDefineProperty(module.exports, 'mock', {
+  __proto__: null,
+  configurable: true,
+  enumerable: true,
+  get() {
+    if (lazyMock === undefined) {
+      const { MockTracker } = require('internal/test_runner/mock');
+
+      lazyMock = new MockTracker();
+    }
+
+    return lazyMock;
+  },
+});

test/parallel/test-runner-mocking.js

@@ -14,7 +14,7 @@ const jsPrimitives = {
 const jsGlobalObjectsUrl = `${jsDocPrefix}Reference/Global_Objects/`;
 const jsGlobalTypes = [
   'AggregateError', 'Array', 'ArrayBuffer', 'DataView', 'Date', 'Error',
-  'EvalError', 'Function', 'Map', 'Object', 'Promise', 'RangeError',
+  'EvalError', 'Function', 'Map', 'Object', 'Promise', 'Proxy', 'RangeError',
   'ReferenceError', 'RegExp', 'Set', 'SharedArrayBuffer', 'SyntaxError',
   'TypeError', 'TypedArray', 'URIError', 'Uint8Array',
 ];