filename-case: Allow acronyms in pascal case (#3125)

sindresorhus · web-flow · commit b70cb644ea16 · 2026-06-12T01:07:08.000+02:00
diff --git a/docs/rules/filename-case.md b/docs/rules/filename-case.md
@@ -44,6 +44,7 @@ Set the `checkDirectories` option to `false` to only check filenames.
 ### `pascalCase`
 
 - `FooBar.js`
+- `FAQPage.js`
 - `FooBar.Test.js`
 - `FooBar.TestUtils.js`
 
diff --git a/rules/filename-case.js b/rules/filename-case.js
@@ -22,6 +22,21 @@ const isIgnoredChar = char => !/^[\w-]$/.test(char);
 const ignoredByDefault = new Set(['index.js', 'index.mjs', 'index.cjs', 'index.ts', 'index.tsx', 'index.vue']);
 const isLowerCase = string => string === string.toLowerCase();
 const disjunctionListFormat = new Intl.ListFormat('en-US', {type: 'disjunction'});
+const alphanumericRegex = /^[\da-z]+$/i;
+const leadingAcronymRegex = /^[A-Z]{3,}(?=\d*[A-Z](?:[a-z]|\d+[a-z]))/;
+
+function pascalCaseWithLeadingAcronym(string) {
+	if (alphanumericRegex.test(string)) {
+		const leadingAcronym = leadingAcronymRegex.exec(string)?.[0];
+		const suffix = leadingAcronym && string.slice(leadingAcronym.length);
+
+		if (suffix && pascalCase(suffix) === suffix) {
+			return string;
+		}
+	}
+
+	return pascalCase(string);
+}
 
 const cases = {
 	camelCase: {
@@ -37,7 +52,7 @@ const cases = {
 		name: 'snake case',
 	},
 	pascalCase: {
-		fn: pascalCase,
+		fn: pascalCaseWithLeadingAcronym,
 		name: 'pascal case',
 	},
 };
diff --git a/test/filename-case.js b/test/filename-case.js
@@ -140,6 +140,14 @@ ruleTest({
 		testCase('src/foo/.test_utils.js', 'kebabCase'),
 		testCase('Src/Foo/Foo.js', 'pascalCase'),
 		testCase('Src/Foo/FooBar.js', 'pascalCase'),
+		testCase('Src/Foo/FAQPage.js', 'pascalCase'),
+		testCase('Src/Foo/DIYWidget.js', 'pascalCase'),
+		testCase('Src/Foo/URL2Path.js', 'pascalCase'),
+		testCase('Src/Foo/FAQI18n.js', 'pascalCase'),
+		testCase('Src/Foo/URL2I18n.js', 'pascalCase'),
+		testCase('Src/FAQPage/Foo.js', 'pascalCase'),
+		testCase('Src/URL2Path/Foo.js', 'pascalCase'),
+		testCase('Src/URL2I18n/Foo.js', 'pascalCase'),
 		testCase('Src/Foo/Foo.test.js', 'pascalCase'),
 		testCase('Src/Foo/FooBar.test.js', 'pascalCase'),
 		testCase('Src/Foo/FooBar.test-utils.js', 'pascalCase'),
@@ -464,6 +472,61 @@ ruleTest({
 			'pascalCase',
 			'Filename is not in pascal case. Rename it to `FooBar.test-utils.js`.',
 		),
+		testCase(
+			'Src/Foo/PageFAQ.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `PageFaq.js`.',
+		),
+		testCase(
+			'Src/Foo/FAQ-Page.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `FaqPage.js`.',
+		),
+		testCase(
+			'Src/Foo/FAQpage.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `FaQpage.js`.',
+		),
+		testCase(
+			'Src/Foo/FAQPageFOO.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `FaqPageFoo.js`.',
+		),
+		testCase(
+			'Src/FAQpage/Foo.js',
+			'pascalCase',
+			'Directory name `FAQpage` is not in pascal case. Rename it to `FaQpage`.',
+		),
+		testCase(
+			'Src/Foo/URL2path.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `Url2path.js`.',
+		),
+		testCase(
+			'Src/Foo/UIPath.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `UiPath.js`.',
+		),
+		testCase(
+			'Src/Foo/UI2Path.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `Ui2Path.js`.',
+		),
+		testCase(
+			'Src/Foo/FOO2.js',
+			'pascalCase',
+			'Filename is not in pascal case. Rename it to `Foo2.js`.',
+		),
+		testCase(
+			'src/foo/FAQPage.js',
+			'camelCase',
+			'Filename is not in camel case. Rename it to `faqPage.js`.',
+		),
+		testCase(
+			'src/foo/FAQPage.js',
+			undefined,
+			'Filename is not in kebab case. Rename it to `faq-page.js`.',
+		),
 		testCase(
 			'src/foo/_FOO-BAR.js',
 			'camelCase',
