fix: encoding utf8 strings in node 20

lib/binding.js

@@ -722,7 +722,7 @@ Binding.prototype.writeString = function (
       callback(err, written, returned && string);
     };
   }
-  return this.writeBuffer(fd, buffer, 0, string.length, position, wrapper, ctx);
+  return this.writeBuffer(fd, buffer, 0, buffer.length, position, wrapper, ctx);
 };
 
 /**
@@ -878,7 +878,7 @@ Binding.prototype.readFileUtf8 = function (name, flags) {
  */
 Binding.prototype.writeFileUtf8 = function (filepath, data, flags, mode) {
   const destFd = this.open(filepath, flags, mode);
-  this.writeBuffer(destFd, data, 0, data.length);
+  this.writeString(destFd, data, null, 'utf8');
 };
 
 /**

test/lib/encoding.spec.js

@@ -0,0 +1,39 @@
+const helper = require('../helper.js');
+const assert = helper.assert;
+const fs = require('fs');
+const mock = require('../../lib/index.js');
+
+const CHARS = [
+  // // 1 utf-16, 1 utf-8 byte
+  'A',
+
+  // 1 utf-16 code unit, 3 utf-8 bytes
+  '’',
+
+  // // 2 utf-16 code units, 4 utf-8 bytes
+  '😄',
+];
+
+const ENCODINGS = ['utf8', 'utf16le', 'latin1'];
+
+for (const encoding of ENCODINGS) {
+  for (const char of CHARS) {
+    describe(`Encoding (${encoding} ${char})`, () => {
+      const buffer = Buffer.from(char, encoding);
+
+      beforeEach(() => mock());
+      afterEach(() => mock.restore());
+
+      beforeEach(() => fs.writeFileSync('file', char, {encoding}));
+
+      it(`writes ${buffer.length} bytes`, () => {
+        assert.strictEqual(fs.statSync('file').size, buffer.length);
+      });
+
+      it('reads the written value (buffer)', () => {
+        const out = fs.readFileSync('file');
+        assert.sameOrderedMembers(Array.from(out), Array.from(buffer));
+      });
+    });
+  }
+}