Recognize typical assertion errors and use their formatting

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()));
 			});