Add expect with custom message logic

package.json

@@ -64,6 +64,7 @@
     ]
   },
   "jest": {
+    "testEnvironment": "node",
     "setupTestFrameworkScriptFile": "jest-extended"
   }
 }

src/__snapshots__/index.test.js.snap

@@ -0,0 +1,16 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`jest-expect-message should fail with custom message 1`] = `
+"Custom message:
+  Woah this should be false!
+
+expect(received).toBeTruthy()
+
+Received: false"
+`;
+
+exports[`jest-expect-message should fail without custom message 1`] = `
+"expect(received).toBeTruthy()
+
+Received: false"
+`;

src/__snapshots__/withMessage.test.js.snap

@@ -0,0 +1,8 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`withMessage() throws error with custom message when matcher fails 1`] = `
+"Custom message:
+  should fail
+
+expected ACTUAL to be 1"
+`;

src/index.js

@@ -0,0 +1,3 @@
+import withMessage from './withMessage';
+
+global.expect = withMessage(global.expect);

src/index.test.js

@@ -0,0 +1,11 @@
+import './';
+
+describe('jest-expect-message', () => {
+  test('should fail with custom message', () => {
+    expect(() => expect(false, 'Woah this should be false!').toBeTruthy()).toThrowErrorMatchingSnapshot();
+  });
+
+  test('should fail without custom message', () => {
+    expect(() => expect(false).toBeTruthy()).toThrowErrorMatchingSnapshot();
+  });
+});

src/withMessage.js

@@ -0,0 +1,33 @@
+class JestAssertionError extends Error {
+  constructor(result, callsite) {
+    super(result.message());
+    this.matcherResult = result;
+
+    if (Error.captureStackTrace) {
+      Error.captureStackTrace(this, callsite);
+    }
+  }
+}
+
+export default expect => (actual, customMessage) => {
+  const matchers = expect(actual);
+
+  return Object.keys(matchers).reduce((acc, key) => {
+    const matcher = matchers[key];
+    const newMatcher = (...args) => {
+      try {
+        matcher(...args);
+      } catch (error) {
+        if (typeof customMessage !== 'string' || customMessage.length < 1 || !error.matcherResult) {
+          throw error;
+        }
+
+        const { matcherResult } = error;
+        const message = () => 'Custom message:\n  ' + customMessage + '\n\n' + matcherResult.message();
+
+        throw new JestAssertionError(Object.assign({}, matcherResult, { message }), newMatcher);
+      }
+    };
+    return Object.assign({}, acc, { [key]: newMatcher });
+  }, {});
+};

src/withMessage.test.js

@@ -0,0 +1,77 @@
+import withMessage from './withMessage';
+
+describe('withMessage()', () => {
+  const ACTUAL = 'ACTUAL';
+
+  test('does not throw when matcher passes', () => {
+    expect.assertions(1);
+    const toBeMock = jest.fn();
+    const expectMock = jest.fn(() => ({ toBe: toBeMock }));
+
+    withMessage(expectMock)(ACTUAL, 'should fail').toBe(1);
+    expect(1).toBe(1);
+  });
+
+  test.each([undefined, ''])('throws original error when given message: %s', message => {
+    expect.assertions(3);
+    const originalError = new Error('Boo');
+    const toBeMock = jest.fn(() => {
+      throw originalError;
+    });
+    const expectMock = jest.fn(() => ({ toBe: toBeMock }));
+
+    try {
+      withMessage(expectMock)(ACTUAL, message).toBe(1);
+    } catch (e) {
+      expect(e).toBe(originalError);
+      expect(expectMock).toHaveBeenCalledWith(ACTUAL);
+      expect(toBeMock).toHaveBeenCalledWith(1);
+    }
+  });
+
+  test('throws original error when matcher error does not contain matcherResult', () => {
+    expect.assertions(3);
+    const originalError = new Error('Boo');
+    const toBeMock = jest.fn(() => {
+      throw originalError;
+    });
+    const expectMock = jest.fn(() => ({ toBe: toBeMock }));
+
+    try {
+      withMessage(expectMock)(ACTUAL, 'should fail').toBe(1);
+    } catch (e) {
+      expect(e).toBe(originalError);
+      expect(expectMock).toHaveBeenCalledWith(ACTUAL);
+      expect(toBeMock).toHaveBeenCalledWith(1);
+    }
+  });
+
+  test('throws error with custom message when matcher fails', () => {
+    expect.assertions(4);
+    const originalError = new Error('Boo');
+    originalError.matcherResult = {
+      actual: ACTUAL,
+      expected: 1,
+      message: () => 'expected ACTUAL to be 1',
+      pass: false
+    };
+
+    const toBeMock = jest.fn(() => {
+      throw originalError;
+    });
+    const expectMock = jest.fn(() => ({ toBe: toBeMock }));
+
+    try {
+      withMessage(expectMock)(ACTUAL, 'should fail').toBe(1);
+    } catch (e) {
+      expect(e.matcherResult).toMatchObject({
+        actual: ACTUAL,
+        expected: 1,
+        pass: false
+      });
+      expect(e.message).toMatchSnapshot();
+      expect(expectMock).toHaveBeenCalledWith(ACTUAL);
+      expect(toBeMock).toHaveBeenCalledWith(1);
+    }
+  });
+});