Duplex stream is not returning whether it is in object mode correctly

[alesmenzel]

• Version: v12.16.0
• Platform: MacOS
• Subsystem: Darwin ... 19.4.0 Darwin Kernel Version 19.4.0: Wed Mar 4 22:28:40 PST 2020; root:xnu-6153.101.6~15/RELEASE_X86_64 x86_64

What steps will reproduce the bug?

// open the node repl
x = new stream.Duplex({writableObjectMode: true})
x.writableObjectMode // undefined - should be true

but the mode is set correctly:

...
_writableState: WritableState {
 objectMode: true,
 ...

it does work on a Writable stream:

x = new stream.Writable({objectMode: true})
x.writableObjectMode // true

How often does it reproduce? Is there a required condition?

100%

What is the expected behavior?

Return the actual value of whether the stream is in object mode.

[ArfatSalman]

Version: v14.2.0

x = new stream.Duplex({ writableObjectMode: true })
// console.log(x.writable);

const prop = 'writableObjectMode';
const hasProp = Object.prototype.hasOwnProperty;

function listProtoChain(x) {
 while (x !== null) {
 console.log(hasProp.call(x, prop));
 x = Object.getPrototypeOf(x);
 }
}

listProtoChain(x);

// Output
// false
// false
// false
// false
// false
// false

I was investigating this and for some reason this code was giving false 6 times. Which leads me to conclude that property writableObjectMode was not found any where in the chain. Which was surprising since this property is set on Writable.prototype in the _stream_writable.js file. I don't know whether this is correct or not.

If I can get some help in this, and if this is somewhat an easy issue, I would like to make my first contribution. 😊

[jasnell]

Yep, definitely a bug. If you look here you'll find where writableObjectMode is defined on the stream.Writable. Then here where the various property definitions are copied into stream.Duplex. All that would be needed to fix this I think is adding the following into _stream_duplex:

 writableObjectMode:
 ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writableObjectMode'),

Then add a regression test in test/parallel that tests for the presence of the property on stream.Duplex

[ronag]

@jasnell: Ah, sorry, guess this would have been a good first issue and I was a bit too fast :/.

[ArfatSalman]

@jasnell Thanks a lot for the guidelines. I see @ronag has already fixed it! 😊