- Since data to display are loaded dynamically, they are not part of the page, and the keywords inside are not used by search engines. - The asynchronous mode may change the page with delays (when the processing on the server take some times), this may be disturbing. - The back button may be deactivated (for ie 6.0 and earlier)
getResponseHeader(headerName) : Returns the value of the specified HTTP header
open( method, URL, async ) :
“ method” parameter can have a value of "GET", "POST", or "HEAD". Other HTTP methods, such as "PUT" and "DELETE" (primarily used in REST applications), may be possible.
“ URL” parameter may be either a relative or complete URL. "async" parameter specifies whether the request should be handled asynchronously or not.
send( content ) : Sends the request.
setRequestHeader( label, value ) : Adds a label/value pair to the HTTP header to be sent.