Javascript:如何将附加变量传递给 Promise .catch 函数

Question

我已经找到了如何使用 Promise.all 将附加变量传递给 Promise .then 函数。但是如何将这些变量传递给 .catch 函数呢？

我的抽象代码:

// global variable url
var url = [url1, url2, url3];

for (i = 0, i < url.length, i++) {
    // run a promise.all to pass some additional variables to the next .then
    Promise.all([xhr(url[i]), i]).then(function([response, i]) {
        // do something with the response and variable i
        alert(url[i] + " responded: " + response);

    }).catch(function(e) {
        // I need variable i also to be available here!
        alert(url[i] + " failed to respond");

    });
}

function xhr(url) {
    return new Promise(function(resolve, reject) {
        // xmlhttprequest with url
        // resolves with the response
        // or rejects with an error message

    });
}

这只是一个非常抽象的代码，用于解释我也需要变量 i 在 .catch 函数中可用。

如何实现这一目标？

编辑:

为了回应答案和我对他们的评论，这里是我在 for 循环中使用 let 的完整代码。我认为现在这是一个非常巧妙的使用 promise 来并行搜索不同的搜索引擎并使用第一个有用的命中。我使用 Promise.any - 一个倒置的 Promise.all - 从一系列 promise 中获取第一个已解决的 promise 。我使用 for 循环来准备 Promise.any 的 promise 。 nzbDonkeySettings 是一个带有我的设置的全局变量:

// modifiy the promise prototype to add function "any"
// from https://stackoverflow.com/questions/39940152/get-first-fulfilled-promise
Promise.prototype.invert = function() {
    return new Promise((res, rej) => this.then(rej, res));
};
Promise.any = function(ps)  {
    return Promise.all(ps.map((p) => p.invert())).invert();
};

// function to search and download the nzb file
function searchNZB(nzbHeader) {

    // prepare the promises for all search engines
    var getNZB = []; // the promise array
    for (let i = 0; i < nzbDonkeySettings.searchengines.length; i++) {
        if (nzbDonkeySettings.searchengines[i].active) { // but only for "active" search engines
            getNZB[i] = new Promise(function(resolve, reject) {

                // first search for the nzb header
                var nzbSearchURL = nzbDonkeySettings.searchengines[i].searchURL.replace(/%s/, encodeURI(nzbHeader));
                var options = {
                    "url": nzbSearchURL,
                    "responseType": "text",
                    "timeout": 20000
                };
                xhr(options).then(function(response) {
                    // if we have a response, check if we have a result
                    var re = new RegExp(nzbDonkeySettings.searchengines[i].searchPattern, "i");
                    if (re.test(response)) {
                        // if we have a result, generate the url for the nzb file
                        var nzbDownloadURL = nzbDonkeySettings.searchengines[i].downloadURL.replace(/%s/, response.match(re)[nzbDonkeySettings.searchengines[i].searchGroup]);
                        // and download the nzb file
                        var options = {
                            "url": nzbDownloadURL,
                            "responseType": "text",
                            "timeout": 120000
                        };
                        xhr(options).then(function(response) {
                            // if we have a response, check if it is a nzb file
                            if (response.match(/<nzb.*>/i)) {
                                // if it is a nzb file, resolve with the nzb file
                                resolve(response);
                            } else {
                                // if it is not a nzb file, reject
                                reject(new Error(nzbDonkeySettings.searchengines[i].name + ": " + "the downloaded file is not a valid nzb file"));    
                            }
                        }).catch(function(e) {
                            // if the download failed, reject
                            reject(new Error(nzbDonkeySettings.searchengines[i].name + ": " + "an error occurred while trying to download the nzb file"));  
                        });
                    } else {
                        // if we have no result, reject
                        reject(new Error(nzbDonkeySettings.searchengines[i].name + ": " + "no nzb file found"));
                    }   
                }).catch(function(e) {
                    // if we have no response, reject
                    reject(new Error(nzbDonkeySettings.searchengines[i].name + ": " + "an error occurred while searching for the nzb file"));   
                });

            });
        }
    }

    // return a promise
    return new Promise(function(resolve, reject) {

        // lets "race" the promises and get the result from the first resolved promise
        Promise.any(getNZB).then(function(nzbFile) {
            // resolve with the nzbFile
            resolve(nzbFile);
        }).catch(function(e) {
            // if we have no results, reject
            reject(new Error("no search engine returned any usable result"));   
        });     

    });

}

Answer 1

像这样使用let:

for (let i = 0, i < url.length, i++) {

let 为 for 循环的每次迭代创建一个单独的变量 i，因此即使 for 循环在您的 xhr() 请求运行时继续运行，循环的每次调用仍然可以访问其自己的 i 副本。

<小时/>

此外，根本没有理由在循环内使用Promise.all()。对于循环的每次迭代，您只有一个 xhr() Promise:

// global variable url
var url = [url1, url2, url3];

for (i = 0, i < url.length, i++) {
    // run a promise.all to pass some additional variables to the next .then
    xhr(url[i]).then(function([response, i]) {
        // do something with the response and variable i
        alert(url[i] + " responded: " + response);

    }).catch(function(e) {
        // I need variable i also to be available here!
        alert(url[i] + " failed to respond");

    });
}

现在，如果您想知道所有 xhr() 调用何时完成，您可以这样做:

// global variable url
var url = [url1, url2, url3];

Promise.all(url.map((u, i) => {
    return xhr(u).catch(err => {
        console.log(u + " failed to respond");
        return null;
    });
})).then(results => {
    console.log(results);
}).catch(err => {
    console.log(err);
});