Question

我是Java的新手，正在尝试了解Promise是如何工作的。我已经编写了以下代码来调用REST API并解决响应。

const buyFlightTicket = () => {
    return new Promise( (resolve, reject) => {
        setTimeout( () => {
            let commentResponse = fetch('https://jsonplaceholder.typicode.com/comments/1'); 
            commentResponse
                .then(response => {
                    if (response.status == 200) {
                        resolve(response.json());
                    } else {
                        reject("Failed to fetch comment: " + response.statusText);
                    }
                })
                .reject("Failed to fetch comment");
        }, 3000);
    })
}

buyFlightTicket()
.then( (responseData) => console.log(responseData))
.catch( (error) => console.log(error));

我能够记录响应数据，但出现错误：即将响应记录到控制台之前的未知错误。

是什么导致此错误？

此外，如何在不使用Promise的情况下使用回调重写此代码？

Answer 1

您应该使用catch，而不是.reject

            commentResponse
                .then(response => {
                    if (response.status == 200) {
                        resolve(response.json());
                    } else {
                        reject("Failed to fetch comment: " + response.statusText);
                    }
                })
                .catch(() => "Failed to fetch comment");

Answer 2

您可以使用访存，它还会返回一个诺言

const buyFlightTicket = () => {
    let commentResponse = fetch('https://jsonplaceholder.typicode.com/comments/1'); 
    return commentResponse
        .then(response => {
            if (response.status == 200) {
                return (response.json());
            } else {
                return new Error("Failed to fetch comment: " + response.statusText);
            }
        })
}

buyFlightTicket()
.then( (responseData) => console.log(responseData))
.catch( (error) => console.log(error));

如果浏览器不支持Promise，则可以添加polyfill或使用xhr

    var buyFlightTicket = function(cb, errorCb) {
        var xhr = new XMLHttpRequest();
        xhr.open('GET', 'https://jsonplaceholder.typicode.com/comments/', true);
        xhr.onreadystatechange = function() {
            if (xhr.readyState === 4){
                if (xhr.status === 200){
                  cb(xhr.responseText)
                } else {
                  errorCb(xhr.statusText);
                }
            }
        };


        xhr.send(null);
        xhr.onerror = function (e) {
            console.error(xhr.statusText);
        };
        console.log('end')
    }
    buyFlightTicket(successCb, errorCb)
    function successCb(res) {
        console.log(res);
    }
    function errorCb(err) { console.log(err) }

使用Javascript时出现未知错误

2 个答案: