我试图从回调中返回值，并将结果分配给函数内的局部变量并返回该变量，但这些方法都没有实际返回响应 - 它们都返回 undefined 或其他变量 result 的初始值为。
接受回调的异步函数示例（使用 jQuery 的 ajax 函数）：
function foo() {
var result;
$.ajax({
url: '...',
success: function(response) {
result = response;
// return response; // <- I tried that one as well
}
});
return result; // It always returns `undefined`
}
使用 Node.js 的示例：
function foo() {
var result;
fs.readFile("path/to/file", function(err, data) {
result = data;
// return data; // <- I tried that one as well
});
return result; // It always returns `undefined`
}
使用 then 承诺块的示例：
function foo() {
var result;
fetch(url).then(function(response) {
result = response;
// return response; // <- I tried that one as well
});
return result; // It always returns `undefined`
}
function foo() {
var httpRequest = new XMLHttpRequest();
httpRequest.open('GET', "/echo/json");
httpRequest.send();
return httpRequest.responseText;
}
var result = foo(); // Always ends up being 'undefined'
菲利克斯·克林做得很好工作为使用 jQuery for AJAX 的人编写答案，但我决定为不使用 jQuery 的人提供替代方案。
由于 a=5 部分尚未执行，因此返回的 a 值为 undefined。 AJAX 的行为是这样的，您在服务器有机会告诉您的浏览器该值是什么之前就返回了该值。
此问题的一个可能的解决方案是重新主动编写代码，告诉您的程序在计算完成后要做什么。
function onComplete(a){ // When the code completes, do this
alert(a);
}
function getFive(whenDone){
var a;
setTimeout(function(){
a=5;
whenDone(a);
},10);
}
var request = new XMLHttpRequest();
request.open('GET', 'yourURL', false); // `false` makes the request synchronous
request.send(null);
if (request.status === 200) {// That's HTTP for 'ok'
console.log(request.responseText);
}
var result = foo();
// Code that depends on `result` goes here
变成：
foo(function(result) {
// Code that depends on `result`
});
这里我们传递了一个匿名函数，但我们也可以轻松传递对现有函数的引用，使其看起来像：
function myHandler(result) {
// Code that depends on `result`
}
foo(myHandler);
有关如何完成此类回调设计的更多详细信息，请查看 Felix 的回答。
现在，让我们定义 foo 本身以进行相应的操作
function foo(callback) {
var httpRequest = new XMLHttpRequest();
httpRequest.onload = function(){ // When the request is loaded
callback(httpRequest.responseText);// We're calling our method
};
httpRequest.open('GET', "/echo/json");
httpRequest.send();
}
// Using 'superagent' which will return a promise.
var superagent = require('superagent')
// This is isn't declared as `async` because it already returns a promise
function delay() {
// `delay` returns a promise
return new Promise(function(resolve, reject) {
// Only `delay` is able to resolve or reject the promise
setTimeout(function() {
resolve(42); // After 3 seconds, resolve the promise with value 42
}, 3000);
});
}
async function getAllBooks() {
try {
// GET a list of book IDs of the current user
var bookIDs = await superagent.get('/user/books');
// wait for 3 seconds (just for the sake of this example)
await delay();
// GET information about each book
return superagent.get('/books/ids='+JSON.stringify(bookIDs));
} catch(error) {
// If any of the awaited promises was rejected, this catch block
// would catch the rejection reason
return null;
}
}
// Start an IIFE to use `await` at the top level
(async function(){
let books = await getAllBooks();
console.log(books);
})();
function delay() {
// `delay` returns a promise
return new Promise(function(resolve, reject) {
// Only `delay` is able to resolve or reject the promise
setTimeout(function() {
resolve(42); // After 3 seconds, resolve the promise with value 42
}, 3000);
});
}
delay()
.then(function(v) { // `delay` returns a promise
console.log(v); // Log the value once it is resolved
})
.catch(function(v) {
// Or do something else if it is rejected
// (it would not happen in this example, since `reject` is not called).
});
