Get the value of an HTTP response header in JavaScript

response-header.js

/**
 * Read the value of a header in the current document.
 *
 * This uses a [single] XMLHTTPRequest to do a HEAD of the current document 
 * and fetch HTTP response header values. Note that the implementation is 
 * rather stupid in that it spins waiting for the XMLHTTPRequest to complete
 * if it hasn't been called yet.
 *
 * @param name string
 *     The name of the header.
 * @return string
 *     The value (or undefined).
 */
var readHeader = (function() {

  // Hidden cache for the headers...
  var _request = undefined;

  return function(name) {
    //
    // We have a request cached...
    ///
    if (_request) {
      return _request.getResponseHeader(name);
    }

    //
    // We need to get the request...
    //
    else {
      // Do the request and wait for it to complete.
      _request = new XMLHttpRequest();
      _request.open("HEAD", window.location, true);
      _request.send(null)
      while (_request.readyState != 4) {};
      
      return _request.getResponseHeader(name);
    }
  }
})();