Always print actual or expected assertion value

This is especially useful for `t.notThrows()`, which includes the
thrown exception as the `actual` value for the assertion error.

Fixes #1150.

Author: novemberborn

lib/format-assert-error.js

@@ -34,47 +34,55 @@ module.exports = error => {
 			.join('\n\n') + '\n';
 	}
 
-	if (error.actual && error.expected) {
-		if (error.actual.type === error.expected.type) {
-			const type = error.actual.type;
-			if (type === 'array' || type === 'object') {
-				const patch = diff.createPatch('string', error.actual.formatted, error.expected.formatted);
-				const msg = patch
-					.split('\n')
-					.slice(4)
-					.map(cleanUp)
-					.filter(Boolean)
-					.join('\n')
-					.trimRight();
+	if (error.actual && error.expected && error.actual.type === error.expected.type) {
+		const type = error.actual.type;
+		if (type === 'array' || type === 'object') {
+			const patch = diff.createPatch('string', error.actual.formatted, error.expected.formatted);
+			const msg = patch
+				.split('\n')
+				.slice(4)
+				.map(cleanUp)
+				.filter(Boolean)
+				.join('\n')
+				.trimRight();
 
-				return `Difference:\n\n${msg}\n`;
-			}
+			return `Difference:\n\n${msg}\n`;
+		}
 
-			if (type === 'string') {
-				const diffMatchPatch = new DiffMatchPatch();
-				const patch = diffMatchPatch.diff_main(stripAnsi(error.actual.formatted), stripAnsi(error.expected.formatted));
-				const msg = patch
-					.map(part => {
-						if (part[0] === 1) {
-							return chalk.bgGreen.black(part[1]);
-						}
+		if (type === 'string') {
+			const diffMatchPatch = new DiffMatchPatch();
+			const patch = diffMatchPatch.diff_main(stripAnsi(error.actual.formatted), stripAnsi(error.expected.formatted));
+			const msg = patch
+				.map(part => {
+					if (part[0] === 1) {
+						return chalk.bgGreen.black(part[1]);
+					}
 
-						if (part[0] === -1) {
-							return chalk.bgRed.black(part[1]);
-						}
+					if (part[0] === -1) {
+						return chalk.bgRed.black(part[1]);
+					}
 
-						return chalk.red(part[1]);
-					})
-					.join('')
-					.trimRight();
+					return chalk.red(part[1]);
+				})
+				.join('')
+				.trimRight();
 
-				return `Difference:\n\n${msg}\n`;
-			}
+			return `Difference:\n\n${msg}\n`;
 		}
+	}
 
-		return `Actual:\n\n${indentString(error.actual.formatted, 2).trimRight()}\n\n` +
-			`Expected:\n\n${indentString(error.expected.formatted, 2).trimRight()}\n`;
+	let retval = null;
+	if (error.actual) {
+		retval = `Actual:\n\n${indentString(error.actual.formatted, 2).trimRight()}\n`;
+	}
+	if (error.expected) {
+		if (retval) {
+			retval += '\n';
+		} else {
+			retval = '';
+		}
+		retval += `Expected:\n\n${indentString(error.expected.formatted, 2).trimRight()}\n`;
 	}
 
-	return null;
+	return retval;
 };

test/format-assert-error.js

@@ -89,7 +89,7 @@ test('diff strings', t => {
 	t.end();
 });
 
-test('diff different types', t => {
+test('print different types', t => {
 	const err = {
 		actual: {
 			type: 'array',
@@ -109,3 +109,33 @@ test('diff different types', t => {
 	].join('\n'));
 	t.end();
 });
+
+test('print actual even if no expected', t => {
+	const err = {
+		actual: {
+			type: 'array',
+			formatted: prettyFormat([1, 2, 3])
+		}
+	};
+
+	t.is(format(err), [
+		'Actual:\n',
+		`${indentString(err.actual.formatted, 2)}\n`
+	].join('\n'));
+	t.end();
+});
+
+test('print expected even if no actual', t => {
+	const err = {
+		expected: {
+			type: 'array',
+			formatted: prettyFormat([1, 2, 3])
+		}
+	};
+
+	t.is(format(err), [
+		'Expected:\n',
+		`${indentString(err.expected.formatted, 2)}\n`
+	].join('\n'));
+	t.end();
+});