Question

预期的行为：element.urls[j] == url，因此触发了fs.readFile。最终，代码到达fs.writeFile并执行resolve(reply)。

当前行为：代码按预期运行，直到fs.readFile。 fs.readFile开始同步运行，这意味着循环继续迭代，直到其中的代码块执行完毕。这导致我的循环过早结束，因此在fs.writeFile完成并尝试执行resolve(reply)时，它无济于事，因为诺言在循环结束时已被reject(reply)拒绝了一旦到达数组末尾。

这是我的代码，请原谅console.log()s。正在使用它们进行调试：

    //Loop through monitor and access the "urls" key of each json
    let i = 0;
    for (let element of monitor) {
        //Checks if the url is in the array
        for (j = 0; j < element.urls.length; j++) {
            if (element.urls[j] == url) {
                console.log("yes the url is in the array");
                //The URL is in the array
                let item = monitor[i].product;
                //Removes the URL from the array
                monitor[i].urls.splice(j, 1);
                //Updates config.json with the new data
                console.log("removed the url from the arrauy");
                //Code splits into two branches here for some reason
                fs.readFile('./config.json', function (err, data) {
                    console.log("file is opened time to update");
                    var json = JSON.parse(data);
                    json.monitor = monitor;
                    fs.writeFile("./config.json", JSON.stringify(json), (err) => {
                        console.log("file updated");
                        let reply;
                        if (err) {
                            //There was an error trying to save the data
                            reply = [
                                {
                                    "name": "An error occured trying to remove the url from array",
                                    "value": `Error: ${err}`
                                }
                            ];
                            //Exit
                            reject(reply);
                            return;
                        }
                        //Success
                        reply = [
                            {
                                "name": "Successfully removed url from array",
                                "value": `I will now stop monitoring the url supplied`
                            },
                            {
                                "name": "Item name",
                                "value": item
                            },
                            {
                                "name": "URL supplied",
                                "value": url
                            }
                        ];
                        //Exit
                        resolve(reply);
                        return;
                    });
                });
            } else {
                console.log("monitor length");
                console.log(monitor.length-1);
                console.log("value of i");
                console.log(i);
                console.log("length of elements urls");
                console.log(element.urls.length-1);
                console.log("value of j");
                console.log(j);
                //Check if this is the final iteration
                if (i == monitor.length-1 && j == element.urls.length-1) {
                    console.log("ugh");
                    //This is the last iteration, the url supplied is not monitored by the bot
                    reply = [
                        {
                            "name": "The bot was never monitoring that URL",
                            "value": `The URL supplied isn't one that the bot currently monitors`
                        }
                    ];
                    //Exit
                    reject(reply);
                    return;
                }
            }
        }
        console.log("incrementing");
        //Increment i
        i++;
    };

以下是控制台输出，以便可以更好地可视化代码路径：

monitor length
1
value of i
0
length of elements urls
1
value of j
0
yes the url is in the array
removed the url from the arrauy
incrementing   <----- Shows that the loop is incrementing already
monitor length   <----- Start of the loop again
1
value of i
1
length of elements urls
0
value of j
0
ugh
?????   <----- Comes from outside the function, shows that the function this loop is in has already ended
file is opened time to update   <----- This comes from after fs.writeFile() is done opening the file
file updated   <----- fs.writeFile() is done saving the file to disk

如您所见，第一个循环中断了，fs.readFile()同步运行。

非常感谢您的投入。现在已经过去了几个小时了。

Answer 1

fs.readFile()是非阻塞的。这意味着您的for循环会继续运行，同时会从您的循环中启动所有fs.readFile()操作，并且它们同时都在运行中。然后，for循环完成，有时稍后您的fs.readFile()操作完成并调用其回调，并且它们可以按任何顺序完成。因此，在完成for循环时，尚未调用任何fs.readFile()完成回调，并且可以按任何顺序调用它们。那应该可以解释您在输出中看到的内容。

要使它们按顺序运行，最简单的方法是切换为使用promise和readFile的promise接口。从结构上看，如下所示：

// function needs to be async
async function someFunction() {
    for (...) {
        try {
            let data = await fs.promises.readFile(...);
            // do something with data
        } catch(e) {
            // handle error
        }
    }
    // all files have been processed here
}

这将按顺序处理文件，暂停for循环，直到完成每个连续的readFile（）并依次运行异步操作。并且，在您的for循环结束时，所有异步操作都将完成。

这使用await在for循环中对异步操作进行排序，因此必须将父函数标记为async，并且它将返回一个承诺，指示何时完成所有操作。

Answer 2

您可以在FileSystem模块中使用Sync方法。

阅读： https://nodejs.org/api/fs.html#fs_fs_readfilesync_path_options-> fs.readFileSync（path [，options]）

写作： https://nodejs.org/api/fs.html#fs_fs_writefilesync_file_data_options-> fs.writeFileSync（file，data [，options]）

for循环在fs.readfile完成之前进行迭代

2 个答案: