fs: use fast paths for UTF-8 aliases

benchmark/fs/bench-writeFileSync.js

@@ -7,7 +7,10 @@ tmpdir.refresh();
 
 // Some variants are commented out as they do not show a change and just slow
 const bench = common.createBenchmark(main, {
-  encoding: ['utf8'],
+  // Include valid case variants to measure the fast-path trade-off.
+  encoding: [
+    'utf8', 'utf-8', 'UTF8', 'UTF-8', 'Utf8', 'Utf-8',
+  ],
   useFd: ['true', 'false'],
   length: [1024, 102400, 1024 * 1024],
 

benchmark/fs/readFileSync.js

@@ -4,7 +4,10 @@ const common = require('../common.js');
 const fs = require('fs');
 
 const bench = common.createBenchmark(main, {
-  encoding: ['undefined', 'utf8', 'ascii'],
+  // Include valid case variants to measure the fast-path trade-off.
+  encoding: [
+    'undefined', 'utf8', 'utf-8', 'UTF8', 'UTF-8', 'Utf8', 'Utf-8', 'ascii',
+  ],
   path: ['existing', 'non-existing'],
   hasFileDescriptor: ['true', 'false'],
   n: [1e4],

lib/fs.js

@@ -531,8 +531,7 @@ function readFileSync(path, options) {
   validateReadFileBufferOptions(options);
   const hasUserBuffer = options.buffer !== undefined;
 
-  if ((options.encoding === 'utf8' || options.encoding === 'utf-8') &&
-      !hasUserBuffer) {
+  if (isUtf8Encoding(options.encoding) && !hasUserBuffer) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }
@@ -2906,7 +2905,7 @@ function writeFileSync(path, data, options) {
   const flag = options.flag || 'w';
 
   // C++ fast path for string data and UTF8 encoding
-  if (typeof data === 'string' && (options.encoding === 'utf8' || options.encoding === 'utf-8')) {
+  if (typeof data === 'string' && isUtf8Encoding(options.encoding)) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }
@@ -3196,8 +3195,15 @@ if (isWindows) {
   };
 }
 
+function isUtf8Encoding(encoding) {
+  return encoding === 'utf8' ||
+    encoding === 'utf-8' ||
+    encoding === 'UTF8' ||
+    encoding === 'UTF-8';
+}
+
 function encodeRealpathResult(result, options) {
-  if (!options || !options.encoding || options.encoding === 'utf8')
+  if (!options || !options.encoding || isUtf8Encoding(options.encoding))
     return result;
   const asBuffer = Buffer.from(result);
   if (options.encoding === 'buffer') {

test/parallel/test-fs-readfile-utf8-fast-path.js

@@ -28,6 +28,14 @@ describe('fs.readFileSync utf8 simdutf dispatch', () => {
     assert.strictEqual(fs.readFileSync(p, 'utf8'), '');
   });
 
+  it('UTF-8 encoding aliases', () => {
+    const buf = Buffer.from('hello 中文 — 🚀', 'utf8');
+    const p = writeFile('encoding-aliases.txt', buf);
+    for (const encoding of ['utf8', 'utf-8', 'UTF8', 'UTF-8']) {
+      assert.strictEqual(fs.readFileSync(p, encoding), buf.toString('utf8'));
+    }
+  });
+
   it('ascii small', () => {
     const buf = Buffer.from('hello');
     expectMatches(writeFile('tiny-ascii.txt', buf), buf);

test/parallel/test-fs-realpath-buffer-encoding.js

@@ -8,7 +8,7 @@ const string_dir = fs.realpathSync(fixtures.fixturesDir);
 const buffer_dir = Buffer.from(string_dir);
 
 const encodings = ['ascii', 'utf8', 'utf16le', 'ucs2',
-                   'base64', 'binary', 'hex'];
+                   'base64', 'binary', 'hex', 'UTF8', 'UTF-8'];
 const expected = {};
 for (const encoding of encodings) {
   expected[encoding] = buffer_dir.toString(encoding);

test/parallel/test-fs-write-file-sync.js

@@ -119,6 +119,16 @@ tmpdir.refresh();
   }
 }
 
+// Test writeFileSync with UTF-8 encoding aliases
+{
+  const utf8Data = 'hello world! 中文 — 🚀';
+  for (const encoding of ['utf8', 'utf-8', 'UTF8', 'UTF-8']) {
+    const file = tmpdir.resolve(`testWriteFileSyncEncoding_${encoding}.txt`);
+    fs.writeFileSync(file, utf8Data, { encoding });
+    assert.strictEqual(fs.readFileSync(file, 'utf8'), utf8Data);
+  }
+}
+
 // Test writeFileSync with an invalid input
 {
   const file = tmpdir.resolve('testWriteFileSyncInvalid.txt');