Why does XMLHttpRequest.setRequestHeader() set undefined or null type as string?

I've found XMLHttpRequest.setRequestHeader() sets undefined value as "undefined". For example, XMLHttpRequest.setRequestHeader("x-custom-header", undefined) results passing "undefined" string value of x-custom-header to server while I expect not passing x-custom-header. I guess this will make much chaos for frontend developers, but there's any reason for this. Is there any reason for this?