Expires
Conditional GET requests and 304 responses help pages load faster,
but they still require making a roundtrip between the client and server
to perform the validity check. The Expires
header eliminates the need to check
with the server by making it clear whether the browser can use its
cached copy of a component.
HTTP/1.1 200 OK Content-Type: application/x-javascript Last-Modified: Wed, 22 Feb 2006 04:15:54 GMT Expires: Wed, 05 Oct 2016 19:16:20 GMT
When the browser sees an Expires
header in the response, it saves the
expiration date with the component in its cache. As long as the
component hasn't expired, the browser uses the cached version and avoids
making any HTTP requests. Chapter 5 talks about the Expires
and Cache-Control
headers in more detail.
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