denoland/deno · error · NodeError

ERR_METHOD_NOT_IMPLEMENTED

ERR_METHOD_NOT_IMPLEMENTED

Error message

The _write() method is not implemented

What it means

streams.Writable is abstract: the base prototype's _write() throws ERR_METHOD_NOT_IMPLEMENTED unless an _writev exists. The error surfaces on the first chunk the stream tries to flush (delivered to the write callback / 'error' event, destroying the stream), not at construction time.

Source

Thrown at ext/node/polyfills/internal/streams/writable.js:912

    } while (i < buffered.length && (state[kState] & kWriting) === 0);

    if (i === buffered.length) {
      resetBuffer(state);
    } else if (i > 256) {
      buffered.splice(0, i);
      state.bufferedIndex = 0;
    } else {
      state.bufferedIndex = i;
    }
  }
  state[kState] &= ~kBufferProcessing;
}

Writable.prototype._write = function (chunk, encoding, cb) {
  if (this._writev) {
    this._writev([{ chunk, encoding }], cb);
  } else {
    throw new ERR_METHOD_NOT_IMPLEMENTED("_write()");
  }
};

Writable.prototype._writev = null;

Writable.prototype.end = function (chunk, encoding, cb) {
  const state = this._writableState;

  if (typeof chunk === "function") {
    cb = chunk;
    chunk = null;
    encoding = null;
  } else if (typeof encoding === "function") {
    cb = encoding;
    encoding = null;
  }

  let err;

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Pass the implementation in options: new Writable({ write(chunk, enc, cb) { /* store */ cb(); } })
2. Implement _write(chunk, enc, cb) (or _writev) on the subclass prototype
3. Use a concrete stream (fs.createWriteStream, net.Socket) when you only need output

Example fix

// before
class Sink extends Writable {}
new Sink().end('x'); // ERR_METHOD_NOT_IMPLEMENTED on flush

// after
class Sink extends Writable {
  _write(chunk, _enc, cb) { process.stdout.write(chunk, cb); }
}
new Sink().end('x'); // ok

function makeWritable(writeImpl) {
  if (typeof writeImpl !== 'function') {
    throw new Error('Writable requires a write(chunk, enc, cb) implementation');
  }
  return new Writable({ write: writeImpl });
}

const hasWriteImpl = (s) =>
  typeof s._write === 'function' &&
  (s._write !== Writable.prototype._write || typeof s._writev === 'function');

ws.on('error', (err) => {
  if (err.code === 'ERR_METHOD_NOT_IMPLEMENTED') {
    console.error('Implement _write(chunk, enc, cb) on this Writable subclass');
  }
});

When it happens

Trigger: new Writable({highWaterMark: 16}) with no write option, followed by .write()/end(); class MyOut extends Writable {} that adds a constructor but never defines _write/_writev; a method misnamed write() or _write2 instead of _write.

Common situations: Porting stream samples; refactors that rename or drop _write; test fakes/mocks of Writable without a write implementation.

Related errors

• ERR_STREAM_NULL_VALUES
• ERR_INVALID_ARG_TYPE
• ERR_METHOD_NOT_IMPLEMENTED
• ERR_INVALID_ARG_TYPE
• ERR_CONSOLE_WRITABLE_STREAM

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/ccf8379753e13701. Report an issue: GitHub.