incorporate review comments

lib/string_decoder.js

@@ -215,9 +215,9 @@ function utf8Text(buf, i) {
 
 // For UTF-8, a replacement character is added when ending on a partial
 // character.
-function utf8End(buf) {
-  const r = (buf && buf.length ? this.write(buf) : '');
-  if (this.lastNeed)
+function utf8End(ctx, buf) {
+  const r = (buf && buf.length ? ctx.write(buf) : '');
+  if (ctx.lastNeed)
     return r + '\ufffd';
   return r;
 }
@@ -249,11 +249,11 @@ function utf16Text(buf, i) {
 
 // For UTF-16LE we do not explicitly append special replacement characters if we
 // end on a partial character, we simply let v8 handle that.
-function utf16End(buf) {
-  const r = (buf && buf.length ? this.write(buf) : '');
-  if (this.lastNeed) {
-    const end = this.lastTotal - this.lastNeed;
-    return r + this.lastChar.toString('utf16le', 0, end);
+function utf16End(ctx, buf) {
+  const r = (buf && buf.length ? ctx.write(buf) : '');
+  if (ctx.lastNeed) {
+    const end = ctx.lastTotal - ctx.lastNeed;
+    return r + ctx.lastChar.toString('utf16le', 0, end);
   }
   return r;
 }
@@ -274,10 +274,10 @@ function base64Text(buf, i) {
 }
 
 
-function base64End(buf) {
-  const r = (buf && buf.length ? this.write(buf) : '');
-  if (this.lastNeed)
-    return r + this.lastChar.toString('base64', 0, 3 - this.lastNeed);
+function base64End(ctx, buf) {
+  const r = (buf && buf.length ? ctx.write(buf) : '');
+  if (ctx.lastNeed)
+    return r + ctx.lastChar.toString('base64', 0, 3 - ctx.lastNeed);
   return r;
 }
 
@@ -286,23 +286,18 @@ function simpleWrite(buf) {
   return buf.toString(this.encoding);
 }
 
-function simpleEnd(buf) {
-  return (buf && buf.length ? this.write(buf) : '');
+function simpleEnd(ctx, buf) {
+  return (buf && buf.length ? ctx.write(buf) : '');
 }
 
-function end(buf) {
-  let result = '';
-
-  if (this.encoding === 'utf16le')
-    result = utf16End.call(this, buf);
-  else if (this.encoding === 'utf8')
-    result = utf8End.call(this, buf);
-  else if (this.encoding === 'base64')
-    result = base64End.call(this, buf);
-  else
-    result = simpleEnd.call(this, buf);
-
-  this.reset();
+const endMappings = new Map([
+  ['utf16le', utf16End],
+  ['utf8', utf8End],
+  ['base64', base64End]
+]);
 
+function end(buf) {
+  const result = (endMappings.get(this.encoding) || simpleEnd)(this, buf);
+  this.reset(this.encoding);
   return result;
 }