fix(expect): name matcher in toMatchObject failure message

expect/_build_message.ts

@@ -49,3 +49,38 @@ export function buildNotEqualErrorMessage<T>(
   const msgPrefix = msg ? `${msg}: ` : "";
   return `${msgPrefix}Expected actual: ${actualString} not to be: ${expectedString}.`;
 }
+
+export function buildToMatchObjectErrorMessage<T>(
+  actual: T,
+  expected: T,
+  options: EqualErrorMessageOptions = {},
+): string {
+  const { formatter = format, msg } = options;
+  const msgPrefix = msg ? `${msg}: ` : "";
+  const actualString = formatter(actual);
+  const expectedString = formatter(expected);
+
+  let message = `${msgPrefix}expect(received).toMatchObject(expected)`;
+
+  const stringDiff = isString(actual) && isString(expected);
+  const diffResult = stringDiff
+    ? diffStr(actual, expected)
+    : diff(actualString.split("\n"), expectedString.split("\n"));
+  const diffMsg = buildMessage(diffResult, { stringDiff }).join("\n");
+  message = `${message}\n${diffMsg}`;
+
+  return message;
+}
+
+export function buildNotToMatchObjectErrorMessage<T>(
+  actual: T,
+  expected: T,
+  options: EqualErrorMessageOptions = {},
+): string {
+  const { formatter = format, msg } = options;
+  const actualString = formatter(actual);
+  const expectedString = formatter(expected);
+
+  const msgPrefix = msg ? `${msg}: ` : "";
+  return `${msgPrefix}expect(received).not.toMatchObject(expected)\n\nExpected: not ${expectedString}\nReceived: ${actualString}`;
+}

expect/_matchers.ts

@@ -27,7 +27,8 @@ import {
 } from "./_utils.ts";
 import {
   buildEqualErrorMessage,
-  buildNotEqualErrorMessage,
+  buildNotToMatchObjectErrorMessage,
+  buildToMatchObjectErrorMessage,
 } from "./_build_message.ts";
 import {
   escapeStringForJs,
@@ -595,15 +596,18 @@ export function toMatchObject(
 
   const triggerError = () => {
     if (context.isNot) {
-      const defaultMessage = buildNotEqualErrorMessage(received, expected);
+      const defaultMessage = buildNotToMatchObjectErrorMessage(
+        received,
+        expected,
+      );
       throw new AssertionError(
         context.customMessage
           ? `${context.customMessage}: ${defaultMessage}`
           : defaultMessage,
       );
     } else {
       const subset = getObjectSubset(received, expected, context.customTesters);
-      const defaultMessage = buildEqualErrorMessage(subset, expected);
+      const defaultMessage = buildToMatchObjectErrorMessage(subset, expected);
       throw new AssertionError(
         context.customMessage
           ? `${context.customMessage}: ${defaultMessage}`

expect/_to_match_object_test.ts

@@ -121,14 +121,24 @@ Deno.test("expect().toMatchObject() throws the correct error messages", () => {
       () => expect({ a: 1 }).toMatchObject({ a: 2 }),
       AssertionError,
     );
+    // The matcher name must appear in the message, not a generic equality
+    // phrase such as "Values are not equal." which misleads users into
+    // thinking strict equality was checked.
+    assertMatch(e.message, /expect\(received\)\.toMatchObject\(expected\)/);
+    assertNotMatch(e.message, /Values are not equal/);
     assertNotMatch(e.message, /not to be/);
   }
   {
     const e = assertThrows(
       () => expect({ a: 1 }).not.toMatchObject({ a: 1 }),
       AssertionError,
     );
-    assertMatch(e.message, /not to be/);
+    assertMatch(
+      e.message,
+      /expect\(received\)\.not\.toMatchObject\(expected\)/,
+    );
+    assertNotMatch(e.message, /Values are equal/);
+    assertNotMatch(e.message, /Expected actual: .* not to be:/);
   }
 });