Express.js - How to check if headers have already been sent?

EDIT: as of express 4.x, you need to use res.headersSent. Note also that you may want to use setTimeout before checking, as it isn't set to true immediately following a call to res.send(). Source

Simple: Connect's Response class provides a public property "headerSent".

res.headerSent is a boolean value that indicates whether the headers have already been sent to the client.

From the source code:

/**
   * Provide a public "header sent" flag
   * until node does.
   *
   * @return {Boolean}
   * @api public
   */

  res.__defineGetter__('headerSent', function(){
    return this._header;
  });

https://github.com/senchalabs/connect/blob/master/lib/patch.js#L22

Node supports the res.headersSent these days, so you could/should use that. It is a read-only boolean indicating whether the headers have already been sent.

if(res.headersSent) { ... }

See http://nodejs.org/api/http.html#http_response_headerssent

Note: this is the preferred way of doing it, compared to the older Connect 'headerSent' property that Niko mentions.