getHeader.js

function getHeader(url, header) {
	return new Promise(function(resolve, reject) {
		var http = new XMLHttpRequest();
	    	http.open('HEAD', url);
    		http.onreadystatechange = function() {
			if (this.readyState !== this.DONE) return;

			if (this.status != 200) {
				reject(this.status);
			} else {
				var value = header ? this.getResponseHeader(header) : this.getAllResponseHeaders();
				resolve(value);
			}
    		};
		http.send();
	});
}

// usage:
getHeader('http://....', 'Content-Type').then(function(contentType) {
	// HTTP OK (200)
}, function() {
	// rejected because HTTP non-OK
});


getHeader('http://....').then(function(allHeaders) {
	// Response.getAllResponseHeaders()
});