XMLHttpRequest中onreadystatechange和onload的使用区别

两种写法：

onreadystatechange

var xhr = new XMLHttpRequest();
xhr.open('GET', 'https://api.example.com/data.json', true);
xhr.onreadystatechange = function() {
  if (xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {
  //也可以写作 xhr.readyState === 4
    console.log(xhr.responseText);
  }
};
xhr.send();

onload

const xhr = new XMLHttpRequest()
xhr.addEventListener('load', () => {
    if(xhr.status === 200){
        console.log(JSON.parse(xhr.responseText))
    }
})
xhr.open('GET', 'https://api.example.com/data.json')
xhr.send()
}

onload 等价于 readyState === 4