XMLHttpRequest()

http://godpage.tistory.com/entry/Ajax%EC%A0%95%EB%A6%AC-2-XMLHttpRequest%EC%9D%98-%EB%A9%94%EC%84%9C%EB%93%9C%EC%99%80-%ED%94%84%EB%A1%9C%ED%8D%BC%ED%8B%B0

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/onreadystatechange