To do this, make a HEAD request and then check for a 404 error in your callback function; Listing 11 shows a sample callback.

要实现这种功能，生成一个HEAD请求，然后在回调函数中检查404错误；清单11给出了一个简单的回调函数。

youdao

Still, sometimes it's useful to know exactly what is available; you never know when creativity will strike and you'll need the HEAD request!

不过，有时确切地了解目前什么可用也是非常有用的；您永远不会知道何时创造力就会迸发或者何时需要 HEAD 请求！

youdao

What is the value in making a HEAD request?

生成 HEAD 请求的价值是什么呢？

youdao

According to media reports, the head of General Motors,Rick Wagoner, has agreed to step down at the request of the White House, as part of a restructuring plan.

VOA: standard.2009.03.30

Mr Snow told AMs Mr Colman agreed to a request that the head of human resources was not involved in his departure.

BBC: Public Accounts Committee

The company says Mr. Conner appeared at the Jan. 11 news conference at the FAA's request because as the head of jetliner production he is Boeing's main liaison to the regulator.

WSJ: The Two Men Behind the 787

That request was denied by the head of AEG Live.

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