Allow customMessage to be a function that returns message

README.md

@@ -39,7 +39,7 @@ Expect takes at most one argument.
 
 ## Solution
 
-`jest-expect-message` allows you to call `expect` with a second argument of a `String` message.
+`jest-expect-message` allows you to call `expect` with a second argument of a `String` message or `function`.
 
 For example the same test as above:
 
@@ -49,6 +49,15 @@ test('returns 2 when adding 1 and 1', () => {
 });
 ```
 
+Or you can pass a function that generates and returns the extra message:
+
+```js
+test('returns 2 when adding 1 and 1', () => {
+  const context = 2;
+  expect(1 + 1, () => (`Woah this should be ${context}!`)).toBe(3);
+});
+```
+
 With `jest-expect-message` this will fail with your custom error message:
 
 ```sh

src/__snapshots__/withMessage.test.js.snap

@@ -1,5 +1,12 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`withMessage() calls custom message function when matcher fails 1`] = `
+"Custom message:
+  custom message from function
+
+expected ACTUAL to be 1"
+`;
+
 exports[`withMessage() throws error with custom message when matcher fails 1`] = `
 "Custom message:
   should fail

src/withMessage.js

@@ -9,7 +9,7 @@ class JestAssertionError extends Error {
   }
 }
 
-const wrapMatcher = (matcher, customMessage, config) => {
+const wrapMatcher = (matcher, customMessageOrFn, config) => {
   const newMatcher = (...args) => {
     try {
       const result = matcher(...args);
@@ -30,7 +30,8 @@ const wrapMatcher = (matcher, customMessage, config) => {
         throw error;
       }
       const { matcherResult } = error;
-
+
+      const customMessage = typeof customMessageOrFn === 'function' ? customMessageOrFn() : customMessageOrFn;
       if (typeof customMessage !== 'string' || customMessage.length < 1) {
         throw new JestAssertionError(matcherResult, newMatcher);
       }

src/withMessage.test.js

@@ -98,6 +98,37 @@ describe('withMessage()', () => {
     }
   });
 
+  test('calls custom message function when matcher fails', () => {
+    expect.assertions(5);
+    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 }));
+
+    const customMessageFn = jest.fn(() => 'custom message from function');
+    try {
+      withMessage(expectMock)(ACTUAL, customMessageFn).toBe(1);
+    } catch (e) {
+      expect(e.matcherResult).toMatchObject({
+        actual: ACTUAL,
+        expected: 1,
+        pass: false
+      });
+      expect(e.message).toMatchSnapshot();
+      expect(customMessageFn).toHaveBeenCalled();
+      expect(expectMock).toHaveBeenCalledWith(ACTUAL);
+      expect(toBeMock).toHaveBeenCalledWith(1);
+    }
+  });
+
   test('throws error with custom message when not matcher fails', () => {
     expect.assertions(4);
     const originalError = new Error('Boo');

types/index.d.ts

@@ -2,7 +2,7 @@ declare namespace jest {
   interface Expect {
     <T = any>(
       actual: T,
-      message?: string,
+      message?: string | (() => string),
       options?: { showMatcherMessage?: boolean; showPrefix?: boolean; showStack?: boolean }
     ): JestMatchers<T>;
   }