test: increase coverage of string-decoder

Make use of Arrow Function.
Add normalizeencoding's test.
normalizeEncoding:
https://github.com/nodejs/node/blob/master/lib/string_decoder.js#L9

PR-URL: #10863
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-string-decoder-end.js

@@ -8,15 +8,11 @@ var assert = require('assert');
 var SD = require('string_decoder').StringDecoder;
 var encodings = ['base64', 'hex', 'utf8', 'utf16le', 'ucs2'];
 
-var bufs = [ '☃💩', 'asdf' ].map(function(b) {
-  return Buffer.from(b);
-});
+const bufs = [ '☃💩', 'asdf' ].map((b) => Buffer.from(b));
 
 // also test just arbitrary bytes from 0-15.
-for (var i = 1; i <= 16; i++) {
-  var bytes = new Array(i).join('.').split('.').map(function(_, j) {
-    return j + 0x78;
-  });
+for (let i = 1; i <= 16; i++) {
+  const bytes = new Array(i).join('.').split('.').map((_, j) => j + 0x78);
   bufs.push(Buffer.from(bytes));
 }
 
@@ -25,7 +21,7 @@ encodings.forEach(testEncoding);
 console.log('ok');
 
 function testEncoding(encoding) {
-  bufs.forEach(function(buf) {
+  bufs.forEach((buf) => {
     testBuf(encoding, buf);
   });
 }

test/parallel/test-string-decoder.js

@@ -107,6 +107,14 @@ assert.strictEqual(decoder.write(Buffer.from('3DD8', 'hex')), '');
 assert.strictEqual(decoder.write(Buffer.from('4D', 'hex')), '');
 assert.strictEqual(decoder.end(), '\ud83d');
 
+assert.throws(() => {
+  new StringDecoder(1);
+}, /^Error: Unknown encoding: 1$/);
+
+assert.throws(() => {
+  new StringDecoder('test');
+}, /^Error: Unknown encoding: test$/);
+
 // test verifies that StringDecoder will correctly decode the given input
 // buffer with the given encoding to the expected output. It will attempt all
 // possible ways to write() the input buffer, see writeSequences(). The
@@ -119,10 +127,10 @@ function test(encoding, input, expected, singleSequence) {
   } else {
     sequences = [singleSequence];
   }
-  sequences.forEach(function(sequence) {
-    var decoder = new StringDecoder(encoding);
-    var output = '';
-    sequence.forEach(function(write) {
+  sequences.forEach((sequence) => {
+    const decoder = new StringDecoder(encoding);
+    let output = '';
+    sequence.forEach((write) => {
       output += decoder.write(input.slice(write[0], write[1]));
     });
     output += decoder.end();