如何在纯javascript中应用CORS头

var xhr = new XMLHttpRequest();
xhr.open('GET', 'http://example.com/api/data', true);
xhr.setRequestHeader('Content-Type', 'application/json');
xhr.setRequestHeader('Access-Control-Allow-Origin', '*');
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    var response = JSON.parse(xhr.responseText);
    // 处理响应数据
  }
};
xhr.send();

fetch('http://example.com/api/data', {
  method: 'GET',
  headers: {
    'Content-Type': 'application/json',
    'Access-Control-Allow-Origin': '*'
  }
})
.then(function(response) {
  if (response.ok) {
    return response.json();
  }
  throw new Error('Network response was not ok.');
})
.then(function(data) {
  // 处理响应数据
})
.catch(function(error) {
  console.log('Error:', error.message);
});