理解 js 中的 fetch

问题描述

我查看了一些资源，但并没有真正了解 fetch 方法.

I checked out a few resources but don't really get the fetch method.

有 2 个 then-s 有什么意义?第一个和第二个 then 有什么用?为什么 return 在第一个?

What is the point in having 2 then-s? What is the first and the second then good for? Why is a return in the first one?

fetch('http://some-site.com/cors-enabled/some.json')
  .then(function(response) {
    return response.text();
  })
  .then(function(text) {
    console.log('Request successful', text);
  })
  .catch(function(error) {
    log('Request failed', error)
  });

推荐答案

如需更详细的答案，请点击文档链接fetch 和 promises

For a much detailed answer please follow the documentation links fetch and promises

'fetch' 函数，执行异步操作，并始终返回一个 Promise 对象.

The 'fetch' function, perform an asynchronous operation, and always return a Promise object.

我在这里的目的只是为了澄清一些你似乎没有抓住的东西，并帮助你更好地理解文档.

My purpose, here, is just to clarify something you seems to had not catched and help you to understand better the documentation.

'.then' 是 Promise 的对象方法，在异步操作完成时调用一些代码.

The '.then' are Promise's object method, that call some code when the asynchronous operation is done.

'.then' 如果 Promise 被认为是'resolved'，则按顺序调用，否则调用'.catch' 来处理错误.

'.then' are call sequentially if the Promise is considered 'resolved', otherwise the '.catch' is call to handle the error.

传递给'.then'的函数中的return语句是第二个排队的'.then'的输入.

The return statement in the function passed to '.then' is the imput of the second queued '.then'.

在您发布的示例中，有 2 个排队的 '.then' 只是为了表明可以执行此操作以及如何执行.

In the example you post, there are 2 queued '.then' just to show it is possible to do it and how to do.

不需要使用多个 '.then'，它很有用，因为您可以将在检索数据中执行的操作拆分为不同的步骤，以便在遇到错误时中止承诺.

There is no need to use more than one '.then', it is just useful as you could split up the action performed in retrieving data in different steps so you could abort the promise, if you got an error.

这有助于编写干净的代码和调试，您可以概括一些操作并重用，并且 alzò 有更多的 défaillance 错误.

This helps to write clean code and debugging, ad you could generalize some actions and reuse, and alzò have more défaillance errors.

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