Accessing the web page's HTTP Headers in JavaScript

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How do I access a page's HTTP response headers via JavaScript?

Related to this question, which was modified to ask about accessing two specific HTTP headers.

Related: How do I access the HTTP request header fields via JavaScript?

It's not possible to read the current headers. You could make another request to the same URL and read its headers, but there is no guarantee that the headers are exactly equal to the current.


Use the following JavaScript code to get all the HTTP headers by performing a get request:

var req = new XMLHttpRequest();
req.open('GET', document.location, false);
req.send(null);
var headers = req.getAllResponseHeaders().toLowerCase();
alert(headers);

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Using XmlHttpRequest you can pull up the current page and then examine the http headers of the response.

Best case is to just do a HEAD request and then examine the headers.

For some examples of doing this have a look at http://www.jibbering.com/2002/4/httprequest.html

Just my 2 cents.

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For those looking for a way to parse all HTTP headers into an object that can be accessed as a dictionary headers["content-type"], I've created a function parseHttpHeaders:

function parseHttpHeaders(httpHeaders) {
    return httpHeaders.split("\n")
     .map(x=>x.split(/: */,2))
     .filter(x=>x[0])
     .reduce((ac, x)=>{ac[x[0]] = x[1];return ac;}, {});
}

var req = new XMLHttpRequest();
req.open('GET', document.location, false);
req.send(null);
var headers = parseHttpHeaders(req.getAllResponseHeaders());
// Now we can do:  headers["content-type"]

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Comments
  • @user2284570 — It isn't. this question is about response headers, not request headers.
  • Saeed, maybe not best for the question author.. I guess it's because it does not access the headers of loaded resource, but makes a new request.. obviously he knows the best, what the best answer is, and made it himself
  • Depending on what header you are after you may want to use the 'HEAD' verb.
  • Making a new request will only work if the response values you need are guaranteed to be identical from one request to the next. It will depend on your application, so your mileage with an approach like this will vary.
  • This hack might work in some scenarios, but it won't work at all if the page containing the script was generated in response to a POST request, and it doesn't help if you're trying to determine whether the server encountered an error (HTTP 5XX) while processing the original request.
  • This answer is horribly wrong. The correct answer is "it's not possible". Or to fit this answer "It's not possible, but here's a hack to attempt to simulate it which may or may not work at all for you".
  • How Google detect it as like I explained here: stackoverflow.com/questions/7191242/…
  • RE update: ajax requests were a standard part of web development way back in 2008 as well -_-
  • BOM stands for "Browser Object Model", for those wondering. See stackoverflow.com/questions/2213594/… for some background.
  • 3) you could stash them in the http cookie header, too. Then you wouldn't need to change the document markup.
  • There is a simple way to access the response header elements such as the link element: use document example here: gist.github.com/FunThomas424242/…
  • This approach still requires that you control the server for your JS. No matter how you communicate that info, your code has suddenly been made uncacheable. Why not just make an API for that specific request to avoid corrupting the request for the original asset?
  • you will not be able to modify request header in mozilla for security reasons. mxr.mozilla.org/mozilla1.8.0/source/extensions/xmlextras/base/…
  • You must call open() before using the setRequestHeader() method. developer.mozilla.org/en/…