{"json_modified": "2016-09-09T11:14:24.618576", "uuid": "ae01a53a-e831-4246-ba5b-3f423734e8ab", "title": "If-None-Match", "url": "/en-US/docs/Web/HTTP/Headers/If-None-Match", "tags": ["Reference", "Conditional Requests", "HTTP Header", "HTTP", "Request header"], "translations": [], "modified": "2016-09-09T11:14:23", "label": "If-None-Match", "localization_tags": [], "locale": "en-US", "id": 192713, "last_edit": "2016-09-09T11:14:17", "summary": "The <strong><code>If-None-Match</code></strong> HTTP request header makes the request conditional. For <a href=\"/en-US/docs/Web/HTTP/Methods/GET\" title=\"The GET method requests a representation of the specified resource. Requests using GET should only retrieve data.\"><code>GET</code></a> and <a href=\"/en-US/docs/Web/HTTP/Methods/HEAD\" title=\"The HTTP HEAD method requests the headers that are returned if the specified resource would be requested with an HTTP GET method. Such a request can be done before deciding to download a large resource to save bandwidth, for example.\"><code>HEAD</code></a> methods, the server will send back the requested resource, with a <a href=\"/en-US/docs/Web/HTTP/Status/200\" title=\"The HTTP 200 OK success status response code indicates that the request has succeeded. A 200 response is cacheable by default.\"><code>200</code></a> status, only if doesn't have a <a href=\"/en-US/docs/Web/HTTP/Headers/ETag\" title=\"The ETag HTTP response header is an identifier for a specific version of a resource. It allows caches to be more efficient, and saves bandwidth, as a web server does not need to send a full response if the content has not changed. On the other side, if the content has changed, etags are useful to help prevent simultaneous updates of a resource from overwriting each other (&quot;mid-air collisions&quot;).\"><code>ETag</code></a> matching the given one. For other methods, the request will be processed only if the eventually existing resource's <a href=\"/en-US/docs/Web/HTTP/Headers/ETag\" title=\"The ETag HTTP response header is an identifier for a specific version of a resource. It allows caches to be more efficient, and saves bandwidth, as a web server does not need to send a full response if the content has not changed. On the other side, if the content has changed, etags are useful to help prevent simultaneous updates of a resource from overwriting each other (&quot;mid-air collisions&quot;).\"><code>ETag</code></a> doesn't match any of the values listed.", "sections": [{"id": "Quick_Links", "title": null}, {"id": "Syntax", "title": "Syntax"}, {"id": "Directives", "title": "Directives"}, {"id": "Examples", "title": "Examples"}, {"id": "Specifications", "title": "Specifications"}, {"id": "Browser_compatibility", "title": "Browser compatibility"}, {"id": "See_also", "title": "See also"}], "slug": "Web/HTTP/Headers/If-None-Match", "review_tags": []}