test: add promise API test for appendFile()

Apply the first of five test cases in test-fs-append-file to the
promise-based API in addition to the callback-based API.
PR-URL: #20739
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: Trott

test/parallel/test-fs-append-file.js

@@ -27,8 +27,6 @@ const join = require('path').join;
 
 consttmpdir=require('../common/tmpdir');
 
-constfilename=join(tmpdir.path,'append.txt');
-
 constcurrentFileData='ABCD';
 
 constn=220;
@@ -44,18 +42,33 @@ let ncallbacks = 0;
 
 tmpdir.refresh();
 
-// test that empty file will be created and have content added
-fs.appendFile(filename,s,function(e){
-assert.ifError(e);
+constthrowNextTick=(e)=>{process.nextTick(()=>{throwe;});};
 
-ncallbacks++;
+// test that empty file will be created and have content added (callback API)
+{
+constfilename=join(tmpdir.path,'append.txt');
 
-fs.readFile(filename,function(e,buffer){
+fs.appendFile(filename,s,common.mustCall(function(e){
 assert.ifError(e);
-ncallbacks++;
-assert.strictEqual(Buffer.byteLength(s),buffer.length);
-});
-});
+
+fs.readFile(filename,common.mustCall(function(e,buffer){
+assert.ifError(e);
+assert.strictEqual(Buffer.byteLength(s),buffer.length);
+}));
+}));
+}
+
+// test that empty file will be created and have content added (promise API)
+{
+constfilename=join(tmpdir.path,'append-promise.txt');
+
+fs.promises.appendFile(filename,s)
+.then(common.mustCall(()=>fs.promises.readFile(filename)))
+.then((buffer)=>{
+assert.strictEqual(Buffer.byteLength(s),buffer.length);
+})
+.catch(throwNextTick);
+}
 
 // test that appends data to a non empty file
 constfilename2=join(tmpdir.path,'append2.txt');
@@ -151,9 +164,8 @@ assert.throws(
 {code: 'ERR_INVALID_CALLBACK'});
 
 process.on('exit',function(){
-assert.strictEqual(12,ncallbacks);
+assert.strictEqual(10,ncallbacks);
 
-fs.unlinkSync(filename);
 fs.unlinkSync(filename2);
 fs.unlinkSync(filename3);
 fs.unlinkSync(filename4);