如何从跨域 Ajax 请求访问 Content-Length 标头?
我的 JavaScript 应用程序需要在使用 Ajax 下载资源之前确定资源的长度.通常这不是问题，您只需发出 HEAD 请求并提取 Content-Length.
My JavaScript application needs to determine the length of a resource before downloading it with Ajax. Ordinarily this is not a problem, you just make a HEAD request and extract the Content-Length.
var xhr = $.ajax({type:"HEAD", url: "http://own-domain/file.html"})
xhr.getResponseHeader("Content-Length")
// "2195"
但是，资源存储在与客户端不同的服务器上.(我控制的服务器).所以我使用 CORS 来发出跨域 ajax 请求，并设置服务器来响应 HEAD 请求的预检请求和带有自定义标头的 GET/POST 请求.
However, the resources are stored on a different server to the client. (A server I control). So I'm using CORS to make cross domain ajax requests, and have set up the server to respond to preflighting requests for HEAD requests and GET/POST requests with custom headers.
这在主要方面工作得很好，但在使用 CORS 时，我似乎找不到从 HEAD 响应中提取 Content-Length 的方法:
That is working great in the main, but I can't seem to find a way extract the Content-Length from the HEAD response when working with CORS:
var xhr = $.ajax({type:"HEAD", url: "http://other-domain/file.html"})
xhr.getResponseHeader("Content-Length")
// ERROR: Refused to get unsafe header "Content-Length"
我已经尝试在预检或响应中设置各种标头，例如
I have experimented with setting various headers in the preflighting or in the response, such as
Access-Control-Expose-Headers: Content-Length
规范似乎建议应该使其可用.但无论我做什么，我似乎都无法使 Content-Length 标头对客户端可用.有什么建议吗?
which the specification seems to suggest should make it available. But no matter what I do, I can't seem to make the Content-Length header available to the client. Any suggestions?
(铬 8)
推荐答案
我发现所有浏览器对 CORS 响应标头的支持都存在问题.在 Chrome/Safari 中，我只在 getAllResponseHeaders() 的结果中看到简单的响应标头 (http://www.w3.org/TR/cors/#terminology)，即使是Access-Control-Expose-Headers"标头在响应中设置.在 Firefox 3.6.13 中， getAllResponseHeaders() 不返回任何内容(甚至不返回简单的响应标头).作为一种解决方法，我想您可以重载一个简单的响应标头以包含内容长度，但这可能会导致其他问题，并且仍然无法修复 Firefox.
I've found CORS response header support in all browsers to be buggy. In Chrome/Safari, I only see simple response headers (http://www.w3.org/TR/cors/#terminology) in the result of getAllResponseHeaders(), even when the "Access-Control-Expose-Headers" header is set in the response. And in Firefox 3.6.13, getAllResponseHeaders() doesn't return anything (not even simple response headers). As a workaround, I suppose you could overload one of the simple response headers to include the content-length, but that may cause other issues, and still wouldn't fix Firefox.
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