• Version: 12.16.1
• Platform: Windows
• Subsystem: http

response.writeHead(statusCode[, statusMessage][, headers])

works as expected if statusMessage is omitted, supplying a default value.
However, if undefined is passed as a placeholder then it unexpectedly ignores the headers argument.

Why would I want this? Imagine a function like:

function send({ statusCode, statusMessage, headers, body } = { statusCode: 404 }, res) {
  return new Promise((resolve, reject) => {
    res.writeHead(statusCode, statusMessage, headers)
      .end(body)
      .on('finish', resolve)
      .on('error', reject)
  })
}

You can see I depend on a single signature to work whether statusMessage is supplied or not. Otherwise, I have to:

  if (statusMessage)
    res.writeHead(statusCode, statusMessage, headers)
  else
    res.writeHead(statusCode, headers)

Not the end of the world, but it lacks elegance.

Maybe this is not a bug, but wrong expectations?
Perhaps. But this expectation aligns with how javascript default parameters behave.