This is a meta-issue to keep to track of all usages of _writableState and _readableState outside of streams source. These properties are considered private and should not be used unless absolutely necessary. Usage of them can indicate a few things:

• the code can be rewritten using existing documented API to achieve the same result;
• streams lack some consumer functionality and new public API should be introduced;
• streams lack some implementor functionality and new protected API should be introduced;
• documentation needs to be added for some parts of private state for implementors;
• it is an optimization that is and always be possible only in core.

The list of all _writableState and _readableState usages:

src/node.js

• L564 stdin._readableState.reading = false. Added in commit: bb56dcc by @isaacs; (src: nix stdin _readableState.reading manipulation #454)
• L573 stdin._readableState.reading = false. Added in commit: bb56dcc by @isaacs; (src: nix stdin _readableState.reading manipulation #454)

lib/_debug_agent.js

• L87 this._readableState.objectMode = true (_debug_agent: use readableObjectMode option for client stream #270);

lib/_http_server.js

• L348 socket._readableState.flowing = null (TODO(isaacs): Need a way to reset a stream to fresh state IE, not flowing, and not explicitly paused.). Added in 967b5db by @isaacs;
• L408 var needPause = socket._writableState.needDrain. Added in 085dd30 by @isaacs ;
• L445 req._readableState.resumeScheduled. Not sure where this originated, but in 2efe4ab @indutny added oldMode check here;

lib/_tls_legacy.js

• L421 this._writableState.finished;
• L508 self._readableState.length > 0;

lib/_tls_wrap.js

• L311 self._writableState.errorEmitted (Cleanup stream state in net #465);
• L313 self._writableState.errorEmitted = true (Cleanup stream state in net #465);
• L350 socket._readableState.length;

lib/child_process.js

• L1061 stream._readableState.flowing (child_process: remove redundant condition #511);

lib/crypto.js

• whole LazyTransform thing. Is it really necessary? Maybe it should go to stream? Maybe it should be public? Maybe transforms should be lazy by default?;
• L56 this._writableState.decodeStrings = false;
• L57 this._writableState.defaultEncoding = 'binary';
• L90 var encoding = this._readableState.encoding || 'buffer' (crypto: remove use of this._readableState #610);

lib/fs.js

• L1624 allocNewPool(this._readableState.highWaterMark);

lib/net.js

• L162 this._writableState.decodeStrings = false (Cleanup stream state in net #465);
• L174 this._readableState.flowing = false (Cleanup stream state in net #465);
• L196 this._readableState.ended (Cleanup stream state in net #465);
• L226 self._readableState.ended;
• L242 this._readableState.ended = true (comment: ended should already be true, since this is called after the EOF errno and onread has eof'ed) (Cleanup stream state in net #465);
• L243 this._readableState.endEmitted;
• L362 this._writableState.length;
• L392 this._readableState.endEmitted;
• L405 socket._writableState.length;
• L415 if (this._writableState.finished);
• L429 self._writableState.errorEmitted (Cleanup stream state in net #465);
• L433 self._writableState.errorEmitted = true (Cleanup stream state in net #465);
• L535 self._readableState.length === 0;
• L715 state.getBuffer();
• L842 this._readableState.reading = false;
• L843 this._readableState.ended = false;
• L844 this._readableState.endEmitted = false;
• L845 this._writableState.ended = false;
• L846 this._writableState.ending = false;
• L847 this._writableState.finished = false;
• L848 this._writableState.errorEmitted = false (Cleanup stream state in net #465);

lib/zlib.js

• L423 ws.ended;
• L426 ws.ending;
• L429 ws.needDrain;
• L460 ws.ending || ws.ended;
• L461 ws.length;
• L479 ws.length.

List of used properties:

_readableState

• reading;
• objectMode;
• flowing (boolean?) is used to determine which mode readable stream is in; can be true, false or null; `null is the initial state which means that is implicitly paused;
• resumeScheduled;
• length;
• encoding;
• highWaterMark;
• ended;
• endEmitted;

_writableState

• needDrain;
• ended;
• ending;
• finished;
• errorEmitted;
• decodeStrings;
• defaultEncoding;
• length;
• getBuffer();

/cc @chrisdickinson