Implement jest.fn(), expect().toHaveBeenCalled() and expect().toHaveB…

…eenCalledTimes() (facebook#47699)

Summary:

Changelog: [internal]

Implements a `jest.fn()` and a subset of methods in `expect` using them (`.toHaveBeenCalled()` and `.toHaveBeenCalledTimes()`).

Differential Revision: D66118002

jest/integration/runtime/setup.js

@@ -114,6 +114,64 @@ global.it.skip =
       globalModifiers.pop();
     };
 
+global.jest = {
+  fn: createMockFunction,
+};
+
+const MOCK_FN_TAG = Symbol('mock function');
+
+function createMockFunction<TArgs: $ReadOnlyArray<mixed>, TReturn>(
+  initialImplementation?: (...TArgs) => TReturn,
+): JestMockFn<TArgs, TReturn> {
+  let implementation: ?(...TArgs) => TReturn = initialImplementation;
+
+  const mock: JestMockFn<TArgs, TReturn>['mock'] = {
+    calls: [],
+    // $FlowExpectedError[incompatible-type]
+    lastCall: undefined,
+    instances: [],
+    contexts: [],
+    results: [],
+  };
+
+  const mockFunction = function (this: mixed, ...args: TArgs): TReturn {
+    let result: JestMockFn<TArgs, TReturn>['mock']['results'][number] = {
+      isThrow: false,
+      // $FlowExpectedError[incompatible-type]
+      value: undefined,
+    };
+
+    if (implementation != null) {
+      try {
+        result.value = implementation.apply(this, args);
+      } catch (error) {
+        result.isThrow = true;
+        result.value = error;
+      }
+    }
+
+    mock.calls.push(args);
+    mock.lastCall = args;
+    // $FlowExpectedError[incompatible-call]
+    mock.instances.push(new.target ? this : undefined);
+    mock.contexts.push(this);
+    mock.results.push(result);
+
+    if (result.isThrow) {
+      throw result.value;
+    }
+
+    return result.value;
+  };
+
+  mockFunction.mock = mock;
+  // $FlowExpectedError[invalid-computed-prop]
+  mockFunction[MOCK_FN_TAG] = true;
+
+  // $FlowExpectedError[prop-missing]
+  return mockFunction;
+}
+
 // flowlint unsafe-getters-setters:off
 
 class Expect {
@@ -152,7 +210,7 @@ class Expect {
       Math.abs(expected - Number(this.#received)) < Math.pow(10, -precision);
     if (!this.#isExpectedResult(pass)) {
       throw new Error(
-        `expected ${String(this.#received)}${this.#maybeNotLabel()} to be close to ${expected}`,
+        `Expected ${String(this.#received)}${this.#maybeNotLabel()} to be close to ${expected}`,
       );
     }
   }
@@ -171,7 +229,27 @@ class Expect {
     }
     if (!this.#isExpectedResult(pass)) {
       throw new Error(
-        `expected ${String(this.#received)}${this.#maybeNotLabel()} to throw`,
+        `Expected ${String(this.#received)}${this.#maybeNotLabel()} to throw`,
+      );
+    }
+  }
+
+  toHaveBeenCalled(): void {
+    const mock = this.#requireMock();
+    const pass = mock.calls.length > 0;
+    if (!this.#isExpectedResult(pass)) {
+      throw new Error(
+        `Expected ${String(this.#received)}${this.#maybeNotLabel()} to have been called, but it was${this.#isNot ? '' : "n't"}`,
+      );
+    }
+  }
+
+  toHaveBeenCalledTimes(times: number): void {
+    const mock = this.#requireMock();
+    const pass = mock.calls.length === times;
+    if (!this.#isExpectedResult(pass)) {
+      throw new Error(
+        `Expected ${String(this.#received)}${this.#maybeNotLabel()} to have been called ${times} times, but it was called ${mock.calls.length} times`,
       );
     }
   }
@@ -183,6 +261,18 @@ class Expect {
   #maybeNotLabel(): string {
     return this.#isNot ? ' not' : '';
   }
+
+  #requireMock(): JestMockFn<$ReadOnlyArray<mixed>, mixed>['mock'] {
+    // $FlowExpectedError[incompatible-use]
+    if (!this.#received?.[MOCK_FN_TAG]) {
+      throw new Error(
+        `Expected ${String(this.#received)} to be a mock function, but it wasn't`,
+      );
+    }
+
+    // $FlowExpectedError[incompatible-use]
+    return this.#received.mock;
+  }
 }
 
 global.expect = (received: mixed) => new Expect(received);