stream: fix undefined in Readable object mode

Fixes `this.push(undefined)`.
Fixes: #13753
PR-URL: #13760
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Calvin Metcalf <calvin.metcalf@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: addaleax

lib/_stream_readable.js

@@ -215,8 +215,8 @@ function readableAddChunk(stream, chunk, encoding, addToFront, skipChunkCheck) {
 stream.emit('error',er);
 }elseif(state.objectMode||chunk&&chunk.length>0){
 if(typeofchunk!=='string'&&
-Object.getPrototypeOf(chunk)!==Buffer.prototype&&
-!state.objectMode){
+!state.objectMode&&
+Object.getPrototypeOf(chunk)!==Buffer.prototype){
 chunk=Stream._uint8ArrayToBuffer(chunk);
 }
 

test/parallel/test-stream-objectmode-undefined.js

@@ -0,0 +1,44 @@
+'use strict';
+constcommon=require('../common');
+constassert=require('assert');
+const{ Readable, Writable, Transform }=require('stream');
+
+{
+conststream=newReadable({
+objectMode: true,
+read: common.mustCall(()=>{
+stream.push(undefined);
+stream.push(null);
+})
+});
+
+stream.on('data',common.mustCall((chunk)=>{
+assert.strictEqual(chunk,undefined);
+}));
+}
+
+{
+conststream=newWritable({
+objectMode: true,
+write: common.mustCall((chunk)=>{
+assert.strictEqual(chunk,undefined);
+})
+});
+
+stream.write(undefined);
+}
+
+{
+conststream=newTransform({
+objectMode: true,
+transform: common.mustCall((chunk)=>{
+stream.push(chunk);
+})
+});
+
+stream.on('data',common.mustCall((chunk)=>{
+assert.strictEqual(chunk,undefined);
+}));
+
+stream.write(undefined);
+}