Setting xmlhttpRequest.setRequestHeader for an ajax call?

When I have an ajax call why should i set xmlhttpRequest.setRequestHeader ? is it not obvious from the ajax object t开发者_如何学编程hat we are sending an ajax request.

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You have to understand first that AJAX request are HTTP request. When you add

xmlhttp.setRequestHeader('Content-Type','XXXXXXXXX');

it adds an header that define what kind of data will be contained in the body of the request. application/x-www-form-urlencoded means that the data will be form data. The data format could be something else. It's just that form data is the most common thing you will send to a server.