javascript - How to get the content of ajax request header using js?

Question

1. How to get ajax request header information in js

2. When I use XMLHttpRequest.header in the complete method of ajax, the value obtained is undefined. Is it undefined without this parameter?

3. If my above idea is wrong, please give me some advice

Answer 1

Maybe this:
var xhr = new XMLHttpRequest();
xhr.open('get','',false);
var headers = xhr.getAllResponseHeaders();
xfr.send();

Answer 2

var request = new XMLHttpRequest();
request.open("GET", "foo.txt", true);
request.send();
request.onreadystatechange = function() {
if(this.readyState == this.HEADERS_RECEIVED) {

console.log(request.getAllResponseHeaders());

}
}