# Ajax 비동기 통신 설정

## 사용하기

동기 통신과 세팅에서 다른 점은 `.open()`에 `true` 값을 전달 한다. 

```javascript
var xhr = new XMLHttpRequest;
xhr.open('GET', 'ajax/data.txt', true);
xhr.send();

// 서버에서 응답이 올 경우, 이벤트를 감지하여 처리하는 이벤트 핸들링  
xhr.onreadystatechang = function() {
  if((xhr.status === 200 || xhr.status === 304) && xhr.readyState === 4) {
    console.log(xhr, xhr.responseText);
  } else {
    console.warn('통신 실패')
  }
}

```

* `.onreadystatechang` : 서버에서 응답이 올 경우 실행한다. 

## 이벤트 & 통신상태 확인 

![](/files/-MRAvbp5E5CUaNtFHziY)

* 0 : Uninitialized (초기화 되지 않음)
* 1 : Loading
* 2 : Loaded
* 3 : Interactive 
* 4 : Complete (완료)


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