Using Fetch

【Using Fetch】

　　This kind of functionality was previously achieved using XMLHttpRequest. Fetch provides a better alternative that can be easily used.

　　需要注意以下2点：

　　1）The Promise returned from fetch() won’t reject on HTTP error status even if the response is an HTTP 404 or 500. Instead, it will resolve normally (with ok status set to false), and it will only reject on network failure or if anything prevented the request from completing.

　　2）By default, fetch won't send or receive any cookies from the server, resulting in unauthenticated requests if the site relies on maintaining a user session (to send cookies, the credentials init option must be set).

var myImage = document.querySelector('img');

fetch('flowers.jpg').then(function(response) {
  return response.blob();
}).then(function(myBlob) {
  var objectURL = URL.createObjectURL(myBlob);
  myImage.src = objectURL;
});

　　下面的示例，控制了HTTP Header

var myHeaders = new Headers();

var myInit = { method: 'GET',
               headers: myHeaders,
               mode: 'cors',
               cache: 'default' };

fetch('flowers.jpg', myInit).then(function(response) {
  return response.blob();
}).then(function(myBlob) {
  var objectURL = URL.createObjectURL(myBlob);
  myImage.src = objectURL;
});

【credentials】

fetch('https://example.com', {
  credentials: 'include'  
})

// The calling script is on the origin 'https://example.com'

fetch('https://example.com', {
  credentials: 'same-origin'  
})

fetch('https://example.com', {
  credentials: 'omit'  
})

【错误处理】

fetch('flowers.jpg').then(function(response) {
  if(response.ok) {
    return response.blob();
  }
  throw new Error('Network response was not ok.');
}).then(function(myBlob) { 
  var objectURL = URL.createObjectURL(myBlob); 
  myImage.src = objectURL; 
}).catch(function(error) {
  console.log('There has been a problem with your fetch operation: ' + error.message);
});

【Request对象】

　　Request() accepts exactly the same parameters as the fetch() method. You can even pass in an existing request object to create a copy of it:

var myHeaders = new Headers();

var myInit = { method: 'GET',
               headers: myHeaders,
               mode: 'cors',
               cache: 'default' };

var myRequest = new Request('flowers.jpg', myInit);

fetch(myRequest).then(function(response) {
  return response.blob();
}).then(function(myBlob) {
  var objectURL = URL.createObjectURL(myBlob);
  myImage.src = objectURL;
});

【Headers】

　　All of the Headers methods throw a TypeError if a header name is used that is not a valid HTTP Header name. 

　　A good use case for headers is checking whether the content type is correct before you process it further. 

fetch(myRequest).then(function(response) {
    var contentType = response.headers.get("content-type");
    if(contentType && contentType.includes("application/json")) {
      return response.json();
    }
    throw new TypeError("Oops, we haven't got JSON!");
  })
  .then(function(json) { /* process your JSON further */ })
  .catch(function(error) { console.log(error); });

【Response】

• Response.status — An integer (default value 200) containing the response status code.
• Response.statusText — A string (default value "OK"),which corresponds to the HTTP status code message.
• Response.ok — seen in use above, this is a shorthand for checking that status is in the range 200-299 inclusive. This returns a Boolean.

　　

参考：https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch