improve readFileUtf8 binding

lib/binding.js

@@ -844,28 +844,43 @@ Binding.prototype.readdir = function (
   });
 };
 
-Binding.prototype.readFile = function (filepath, options, callback, ctx) {
-  markSyscall(ctx, 'readFile');
-
-  return maybeCallback(normalizeCallback(callback), ctx, this, function () {
-    filepath = deBuffer(filepath);
-    const item = this._system.getItem(filepath);
-
-    return item.getContent();
-  });
-};
+/**
+ * Read file as utf8 string.
+ * @param {string} name file to write.
+ * @param {number} flags Flags.
+ * @return {string} the file content.
+ */
+Binding.prototype.readFileUtf8 = function (name, flags) {
+  const fd = this.open(name, flags);
+  const descriptor = this.getDescriptorById(fd);
 
-Binding.prototype.readFileSync = function (filepath, options, ctx) {
-  return this.readFile(filepath, options, undefined, ctx);
-};
-Binding.prototype.readFileUtf8 = function (filepath, options, ctx) {
-  return this.readFile(filepath, options, undefined, ctx).toString('utf-8');
+  if (!descriptor.isRead()) {
+    throw new FSError('EBADF');
+  }
+  const file = descriptor.getItem();
+  if (file instanceof Directory) {
+    throw new FSError('EISDIR');
+  }
+  if (!(file instanceof File)) {
+    // deleted or not a regular file
+    throw new FSError('EBADF');
+  }
+  const content = file.getContent();
+  return content.toString('utf8');
 };
 
+/**
+ * Write a utf8 string.
+ * @param {string} filepath file to write.
+ * @param {string} data data to write to filepath.
+ * @param {number} flags Flags.
+ * @param {number} mode Mode.
+ */
 Binding.prototype.writeFileUtf8 = function (filepath, data, flags, mode) {
   const destFd = this.open(filepath, flags, mode);
   this.writeBuffer(destFd, data, 0, data.length);
 };
+
 /**
  * Create a directory.
  * @param {string} pathname Path to new directory.