How to set multiple headers data with XMLHttpRequest in async mode?

setRequestHeader sets one header and takes two arguments (the name and the value).

If you want to set multiple headers, then call setRequestHeader multiple times. Don't add extra arguments to the first call.

In case you don't want to set multiple headers explicitly you can use

function setHeaders(headers){
  for(let key in headers){
    xhr.setRequestHeader(key, headers[key]) 
  }
}
setHeaders({"Host":"api.domain.com","X-Requested-With":"XMLHttpRequest","contentType":"application/json"})