Express.js - How to check if headers have already been sent?
Javascriptnode.jsExpressResponse HeadersJavascript Problem Overview
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?
Javascript Solutions
Solution 1 - Javascript
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.
Solution 2 - Javascript
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][1]
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
[1]: https://github.com/expressjs/express/issues/2109 "Documentation refers to quot;res.headerSentquot;"
Solution 3 - Javascript
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
});