test: add tests for WriteStream.prototype.flush

Author: himself65

test/parallel/test-fs-write-stream-fsync.js

@@ -0,0 +1,41 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const fs = require('fs');
+
+const tmpdir = require('../common/tmpdir');
+
+tmpdir.refresh();
+
+const file = path.join(tmpdir.path, 'write.txt');
+
+function checkContent(stream, expected) {
+  const actual = fs.readFileSync(file, 'utf-8');
+  assert.strictEqual(actual, expected);
+}
+
+{
+  const stream = fs.createWriteStream(file);
+  stream.write('data', common.mustCall(() => {
+    stream.flush();
+    checkContent(stream, 'data');
+    stream.write('data');
+    stream.flush(common.mustCall(() => {
+      checkContent(stream, 'datadata');
+      stream.destroy();
+    }));
+  }));
+}
+
+{
+  const stream = fs.createWriteStream(file);
+  stream.write('data');
+  checkContent(stream, '');
+  stream.flush();
+  checkContent(stream, 'data');
+  stream.write('data');
+  stream.flush();
+  checkContent(stream, 'datadata');
+  stream.destroy();
+}