以编程方式在JavaScript中为Content-Type设置http请求标头

Question

I'm writing some code to handle sending post data for my application and I wanted to make it so that I can send custom headers from another function if I need them. My question is, can I default something like "Content-Type" the way my code example does below and then overwrite it, or do I need to check the custom headers being sent, and if Content-type is not set, set it to the default. Basically, during the creation of the post request, can you overwrite headers programmatically?

xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.setRequestHeader("Content-length", formData.length);    
// check for custom headers
if ((headers !== null) && (headers !== undefined)) {
    for(var k in headers) {
        if(headers.hasOwnProperty(k) {
            xmlhttp.setRequestHeader(k.toString(), headers[k]);
        }
    }
}

I'm sending a different "Content-Type" like JSON perhaps in the "headers" object. If I do setRequestHeader on Content-Type again does it overwrite or does it send 2 content-type headers in the post request?

edit : I don't know why I asked this on StackOverflow, I just realized I could probably test this by logging my headers with a form handler, which I'm off to do, I'll leave the question up anyway.

Answer 1

According to MDN :

Sets the value of an HTTP request header. You must call setRequestHeader()after open(), but before send(). If this method is called several times with the same header, the values are merged into one single request header.

So calling setRequestHeader() multiple times will yield the following:

Content-Type: application/x-www-form-urlencoded, application/json