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

Question

I am writing a library which may set headers. I want to give a custom error message if headers have already been sent, instead of just letting it fail with the "Can't set headers after they are sent" message given by Node.js. So how to check if headers have already been sent?

powerboy would you be able to accept the other answer, because the currently accepted answer doesn't work anymore? See @fiatjaf 's comment. — Willem Mulder, Oct 31 '15 at 20:54

Willem Mulder · Answer 1 · 2015-06-29T09:11:45.803

218

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.

edited Jun 29 '15 at 09:11

answered Jun 06 '14 at 08:46

Willem Mulder

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This should be made the accepted answer seeing as the current accepted answer no longer works. – mantagen Jun 27 '17 at 18:07

score 75 · Accepted Answer · edited Apr 16 '18 at 19:54

75

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

edited Apr 16 '18 at 19:54

MalcolmOcean

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answered Aug 19 '12 at 22:20

Niko

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Note that this is not the same as http://nodejs.org/docs/latest/api/http.html#http_response_headerssent In express it seems to return the headers sent.. rather than a bool of if the headers have been sent. – Lee Olayvar Apr 20 '13 at 18:47
Everyday I admire how is nodejs easy concept. – Bruno Nov 29 '13 at 02:44
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@powerboy Node currently supports res.headersSent natively, so it's probably a good idea to to start using that property. – Willem Mulder Jun 06 '14 at 08:47
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As of May 17, 2014, res.headerSent has been deprecated by senchalabs/connect in favour of res.headersSent. – Kunal Kapadia Nov 28 '14 at 12:43
11

it's `headersSent`not `headerSent` – Onur Yıldırım Jun 16 '17 at 21:59
I've edited the answer to reflect the headersSent thing, as it is accepted but is now misleading. – MalcolmOcean Apr 17 '18 at 18:09
@MalcolmOcean setTimeout of how much? – Emilio May 06 '18 at 16:34
Seems it doesn't work to just use zero... I have mine set to 250ms to be safe, but fewer would probably be fine. – MalcolmOcean May 07 '18 at 23:16
i flagged this answer for moderation. `res.headersSent` answer should be the accepted solution as this one no longer works. – Mark Shust at M.academy Jun 12 '19 at 14:06
Where is this documented that headersSent is set asynchronously? – chetan Mar 11 '22 at 20:51

Manohar Reddy Poreddy · Answer 3 · 2019-01-21T16:33:31.133

16

Others answers point to Node.js or Github websites.

Below is from Expressjs website: https://expressjs.com/en/api.html#res.headersSent

app.get('/', function (req, res) {
  console.log(res.headersSent); // false
  res.send('OK');
  console.log(res.headersSent); // true
});

edited Jan 21 '19 at 16:33

answered Aug 13 '18 at 05:24

Manohar Reddy Poreddy

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

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