One fundamental difficulty is how do you know what last-modified date or ETag to send? The browser has cache information that it uses for sending requests, but it won't share that information with JavaScript. Fortunately, jQuery keeps track of the Last-Modified and ETag headers from Ajax responses, so you can use ifModified:true to have jQuery set those header values the next time it sends a request for that resource.

有两点需要注意:

• 304 响应不携带数据.这是设计使然.假设是，如果您选择使用缓存，您应该已经在缓存中拥有数据的副本！如果没有从服务器获取数据是一个问题(即，因为您还没有该数据)为什么要使用缓存?当您手头有旧数据并且只需要新数据时，应该使用缓存；因此，使用 304 取回任何数据应该不是问题.

• 304 responses do not carry data. This is by design. The assumption is that if you have elected to use caching, you should have a copy of the data already in your cache! If getting no data from the sever is a problem (i.e., because you don't already have that data) why are you using caching? Caching should be used when you have the old data on hand and only want new data; thus, getting back no data with a 304 should not be a problem.

jQuery 必须存储上一个请求的最后修改日期或 ETag(与 If-None-Match 一起使用).流程是这样的:

jQuery must have a last-modified date or an ETag (to use with If-None-Match) stored from a previous request. The process goes like this:

• 第一次获取:jQuery 没有缓存信息，所以它不会发送 If-Modified-Since 或 If-None-Match.当响应返回时，服务器可能会宣布最后修改的数据或 ETag，jQuery 将其存储以供将来使用.

• First fetch: jQuery has no cache information, so it doesn't send If-Modified-Since or If-None-Match. When the response comes back, the server may announce a last-modified data or an ETag, which jQuery stores for future use.

后续提取:jQuery 拥有上次提取的缓存信息并将该数据转发到服务器.如果资源没有改变，Ajax 请求会得到 304 响应.如果资源已更改，Ajax 请求会收到 200 响应，以及新的缓存信息供 jQuery 用于下一次获取.

Subsequent fetches: jQuery has cache information from the last fetch and forwards that data to the server. If the resource has not changed, the Ajax request gets a 304 response. If the resource has changed, the Ajax request gets a 200 response, along with new cache information for jQuery to use for its next fetch.

jQuery 不在页面重新加载之间保留缓存信息(例如，在 cookie 中).因此，页面重新加载后第一次获取资源永远不会是 304，因为 jQuery 没有要发送的缓存信息(即，我们重置回第一次获取"情况).jQuery 没有理由不能持久化缓存信息，但目前它没有.

jQuery does not persist cache information (e.g., in cookies) between page reloads, however. Therefore, the first fetch of a resource after a page reload will never be a 304, because jQuery has no cache information to send (i.e., we reset back to the "first fetch" case). There is no reason why jQuery couldn't persist cache information, but at present it doesn't.

这里的底线是您可以使用缓存标头来获取 JavaScript 304 响应，但您不能访问 浏览器自己的 ETag 或特定资源的上次修改日期.因此，浏览器本身可能知道有关资源的缓存信息，但您的 JavaScript 代码不知道.在这种情况下，浏览器将使用其缓存标头来获得真正的 304 响应，但会将 200 响应转发到您的 JavaScript 代码，因为 JavaScript 没有发送任何缓存信息.

The bottom line here is that you can use cache headers to get a JavaScript 304 response, but you can't access the browser's own ETag or last-modified date for a particular resource. So, the browser itself might know caching information about a resource, but your JavaScript code does not. In that case, the browser will use its cache headers to potentially get a real 304 response, but forward a 200 response to your JavaScript code, because JavaScript didn't send any cache information.

不可能使 JavaScript 304 请求与实际的网络 304 响应完全一致，因为您的浏览器知道的缓存信息和您的 JavaScript 代码知道的缓存信息可能以不可预知的方式不同.但是，大多数情况下正确获取 304 请求足以满足大多数实际开发需求.

It is not possible to make JavaScript 304 requests align perfectly with actual network 304 responses, because the cache information known by your browser and the cache information known by your JavaScript code may differ in unpredictable ways. However, getting 304 requests correctly most of the time is good enough for most practical development needs.

这是一个用 Node.js 编写的简短服务器示例(但它应该足够简单，可以移植到其他语言):

Here's a brief server example written in Node.js (but it should be simple enough to port to other langauges):

require("http").createServer(function (req, res) {
  console.log(req.headers["if-modified-since"]);

  // always send Last-Modifed header
  var lastModDate = "Fri, 13 Feb 2013 13:43:19 GMT";
  res.setHeader("Last-Modified", lastModDate);

  // if the request has a If-Modified-Since header,
  //   and it's newer than the last modification,
  //   then send a 304 response; otherwise send 200
  if(req.headers["if-modified-since"] &&
     Date.parse(lastModDate) <= Date.parse(req.headers["if-modified-since"])) {
    console.log("304 -- browser has it cached");
    res.writeHead(304, {'Content-Type': 'text/plain'});
    res.end();
  } else {
    console.log("200 -- browser needs it fresh");
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('some content');
  }

}).listen(8080);

运行此服务器时，您可以在浏览器中加载页面并在浏览器控制台中执行两种不同的测试:

When running this server, you can load the page in your browser and perform two different tests in your browser console:

var xhr = new XMLHttpRequest();
xhr.open("GET", "/");
xhr.send();
xhr.onload = function() { console.log(xhr.status); }

此脚本将始终看到 200 响应，即使浏览器提供 If-Modified-Since 请求标头并获得 304(这将发生在第一个请求之后，在浏览器看到服务器的 Last-Modifed 响应头之后).

This script will always see a 200 response, even if the browser supplies a If-Modified-Since request header and gets a 304 (which will happen all requests after the first, after the browser sees the server's Last-Modifed response header).

相比之下，此脚本将总是看到 304 响应:

By contrast, this script will always see 304 response:

var xhr = new XMLHttpRequest();
xhr.open("GET", "/");
xhr.setRequestHeader("If-Modified-Since", "Fri, 15 Feb 2013 13:43:19 GMT");
xhr.send();
xhr.onload = function() { console.log(xhr.status); }

脚本提供自己的 If-Modified-Since 请求标头(服务器最后修改日期后两天)；它不依赖于浏览器为 If-Modified-Since 提供的任何内容，因此允许(根据 XHR 规范)查看 304 响应.

The script supplies its own If-Modified-Since request header (two days after the server's last-modified date); it does not rely on whatever the browser supplies for If-Modified-Since, and therefore is allowed (per the XHR spec) to see 304 responses.

最后，这个脚本总是会看到一个200:

Finally, this script will always see a 200:

var xhr = new XMLHttpRequest();
xhr.open("GET", "/");
xhr.setRequestHeader("If-Modified-Since", "Fri, 12 Feb 2013 13:43:19 GMT");
xhr.send();
xhr.onload = function() { console.log(xhr.status); }

这是因为脚本使用了早于服务器最后修改日期的 If-Modified-Since，所以服务器总是发送 200.服务器不会发送 304 因为它假定客户端没有最新版本的缓存副本(即，客户端宣布自 2 月 12 日以来它看到了更改，但是有一个2 月 13 日的更改，客户显然没有看到).

This is because the script uses a If-Modified-Since that is prior to the server's last-modified date, so the server always sends a 200. The server won't send a 304 because it assumes the client doesn't have a cached copy of the most recent version (i.e., the client announces that it's seen changes since Feb 12, but there was a change on Feb 13 that the client apparently hasn't seen).

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