unify non-icu TextDecoder wih icu-based a bit

Author: ChALkeR

lib/internal/encoding.js

@@ -492,7 +492,7 @@ function makeTextDecoderJS() {
   const kBOMSeen = Symbol('BOM seen');
 
   function hasConverter(encoding) {
-    return encoding === 'utf-8' || encoding === 'utf-16le' || isSinglebyteEncoding(encoding);
+    return encoding === 'utf-8' || encoding === 'utf-16le';
   }
 
   class TextDecoder {
@@ -501,27 +501,30 @@ function makeTextDecoderJS() {
       validateObject(options, 'options', kValidateObjectAllowObjectsAndNull);
 
       const enc = getEncodingFromLabel(encoding);
-      if (enc === undefined || !hasConverter(enc))
+      if (enc === undefined)
         throw new ERR_ENCODING_NOT_SUPPORTED(encoding);
 
       let flags = 0;
       if (options !== null) {
-        if (options.fatal) {
-          throw new ERR_NO_ICU('"fatal" option');
-        }
+        flags |= options.fatal ? CONVERTER_FLAGS_FATAL : 0;
         flags |= options.ignoreBOM ? CONVERTER_FLAGS_IGNORE_BOM : 0;
       }
 
       this[kDecoder] = true;
-      // StringDecoder will normalize WHATWG encoding to Node.js encoding.
-      this[kHandle] = new (lazyStringDecoder())(enc);
       this[kFlags] = flags;
       this[kEncoding] = enc;
+      this[kIgnoreBOM] = Boolean(options?.ignoreBOM);
+      this[kFatal] = Boolean(options?.fatal);
       this[kBOMSeen] = false;
       this[kMethod] = undefined;
 
-      if (isSinglebyteEncoding(this.encoding)) {
-        this[kMethod] = createSinglebyteDecoder(this.encoding, this[kFatal]);
+      if (isSinglebyteEncoding(enc)) {
+        this[kMethod] = createSinglebyteDecoder(enc, this[kFatal]);
+      } else {
+        if (!hasConverter(enc)) throw new ERR_ENCODING_NOT_SUPPORTED(encoding);
+        if (this[kFatal]) throw new ERR_NO_ICU('"fatal" option');
+        // StringDecoder will normalize WHATWG encoding to Node.js encoding.
+        this[kHandle] = new (lazyStringDecoder())(enc);
       }
     }
 
@@ -546,9 +549,7 @@ function makeTextDecoderJS() {
         this[kHandle].end(input) :
         this[kHandle].write(input);
 
-      if (result.length > 0 &&
-          !this[kBOMSeen] &&
-          !(this[kFlags] & CONVERTER_FLAGS_IGNORE_BOM)) {
+      if (result.length > 0 && !this[kBOMSeen] && !this[kIgnoreBOM]) {
         // If the very first result in the stream is a BOM, and we are not
         // explicitly told to ignore it, then we discard it.
         if (result[0] === '\ufeff') {