javascript - promise 返回未定义

Question

这个问题已经有答案了:

How do I return the response from an asynchronous call? (42 个回答)

Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference (7 个回答)

已关闭 5 年前。

export function getHotOffers() {
    let offers;
    getRequest('/hot-offers').then(x => offers = x);
    alert(offers);
    return JSON.parse(offers);
}

这是我导出的函数。它异步向服务器发出请求并获取 JSON 字符串。当我调试项目时，一切正常，警报返回字符串，但是当我执行代码时，警报返回未定义。我该如何解决这个问题？

Answer 1

这不是 Promise 的工作原理。 getRequest() 之后的代码不会等待 Promise 完成 - 它会立即运行。

export function getHotOffers() {
    let offers;
    getRequest('/hot-offers').then( x =>
       //this only happens after the request is done
       offers = x 
    );
    //this code runs right away, before the request completes
    alert(offers);
    //so does this, so "offers" will be undefined at this point
    return JSON.parse(offers);
}

请求返回后运行的所有代码都应该在回调中，如下所示:

export function getHotOffers() {
    return getRequest('/hot-offers');
}

//somewhere else
getHotOffers().then( offers => {
   useTheOffers( offers );
});