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>

Author: hiroppy

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

@@ -8,15 +8,11 @@ var assert = require('assert');
 varSD=require('string_decoder').StringDecoder;
 varencodings=['base64','hex','utf8','utf16le','ucs2'];
 
-varbufs=['☃💩','asdf'].map(function(b){
-returnBuffer.from(b);
-});
+constbufs=['☃💩','asdf'].map((b)=>Buffer.from(b));
 
 // also test just arbitrary bytes from 0-15.
-for(vari=1;i<=16;i++){
-varbytes=newArray(i).join('.').split('.').map(function(_,j){
-returnj+0x78;
-});
+for(leti=1;i<=16;i++){
+constbytes=newArray(i).join('.').split('.').map((_,j)=>j+0x78);
 bufs.push(Buffer.from(bytes));
 }
 
@@ -25,7 +21,7 @@ encodings.forEach(testEncoding);
 console.log('ok');
 
 functiontestEncoding(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(()=>{
+newStringDecoder(1);
+},/^Error:Unknownencoding:1$/);
+
+assert.throws(()=>{
+newStringDecoder('test');
+},/^Error:Unknownencoding:test$/);
+
 // 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){
-vardecoder=newStringDecoder(encoding);
-varoutput='';
-sequence.forEach(function(write){
+sequences.forEach((sequence)=>{
+constdecoder=newStringDecoder(encoding);
+letoutput='';
+sequence.forEach((write)=>{
 output+=decoder.write(input.slice(write[0],write[1]));
 });
 output+=decoder.end();