Recognize typical assertion errors and use their formatting

Co-authored-by: Mark Wubben <mark@novemberborn.net>

Author: Irvenae

lib/test.js

@@ -7,6 +7,22 @@ import concordanceOptions from './concordance-options.js';
 import nowAndTimers from './now-and-timers.cjs';
 import parseTestArgs from './parse-test-args.js';
 
+const hasOwnProperty = (object, prop) => Object.prototype.hasOwnProperty.call(object, prop);
+
+function isExternalAssertError(error) {
+if (typeof error !== 'object' || error === null) {
+return false;
+}
+
+// Match errors thrown by <https://www.npmjs.com/package/expect>.
+if (hasOwnProperty(error, 'matcherResult')) {
+return true;
+}
+
+// Match errors thrown by <https://www.npmjs.com/package/chai> and <https://nodejs.org/api/assert.html>.
+return hasOwnProperty(error, 'actual') && hasOwnProperty(error, 'expected');
+}
+
 function formatErrorValue(label, error) {
 const formatted = concordance.format(error, concordanceOptions);
 return {label, formatted};
@@ -519,11 +535,19 @@ export default class Test {
 
 const result = this.callFn();
 if (!result.ok) {
-this.saveFirstError(new AssertionError({
-message: 'Error thrown in test',
-savedError: result.error instanceof Error && result.error,
-values: [formatErrorValue('Error thrown in test:', result.error)],
-}));
+if (isExternalAssertError(result.error)) {
+this.saveFirstError(new AssertionError({
+message: 'Assertion failed',
+savedError: result.error instanceof Error && result.error,
+values: [{label: 'Assertion failed: ', formatted: result.error.message}],
+}));
+} else {
+this.saveFirstError(new AssertionError({
+message: 'Error thrown in test',
+savedError: result.error instanceof Error && result.error,
+values: [formatErrorValue('Error thrown in test:', result.error)],
+}));
+}
 
 return this.finish();
 }
@@ -564,11 +588,19 @@ export default class Test {
 
 promise
 .catch(error => {
-this.saveFirstError(new AssertionError({
-message: 'Rejected promise returned by test',
-savedError: error instanceof Error && error,
-values: [formatErrorValue('Rejected promise returned by test. Reason:', error)],
-}));
+if (isExternalAssertError(error)) {
+this.saveFirstError(new AssertionError({
+message: 'Assertion failed',
+savedError: error instanceof Error && error,
+values: [{label: 'Assertion failed: ', formatted: error.message}],
+}));
+} else {
+this.saveFirstError(new AssertionError({
+message: 'Rejected promise returned by test',
+savedError: error instanceof Error && error,
+values: [formatErrorValue('Rejected promise returned by test. Reason:', error)],
+}));
+}
 })
 .then(() => resolve(this.finish()));
 });