What is the default Content-Type in XMLHttpRequest?

Question

I have created a javascript library and I am setting manually the Content-Type right now. I would love to know what is the default Content-Type . The XHR documentation for the send() method is confusing at best:

4. If body is null, go to the next step. Otherwise ...

BodyInit

If body is a string, set encoding to UTF-8 .

Set request body and Content-Type to the result of extracting body.

Where, the result of extracting body for a text is:

• Blob
• BufferSource
• FormData
• URLSearchParams
• USVString

So, which of these types would it be? The body passed to send() is in the default, escaped format of a=b&c=d . Is it a URLSearchParams ? or a USVString ? or a Blob ? There's a description on each one but it's too cryptic for me to understand (I only know that it's probably not a FormData ).

Note: please provide some official documentation if possible

Answer 1

Directly from the horse's mouth , if you are sending a string their last test expects "text/plain;charset=UTF-8" in accordance with the XMLHttpRequest Level 1 specification .

So you will have to refer to the step four of the send method to know exactly what is expected depending on the data.