javascript - NodeJS 函数返回未定义。

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)

已关闭 5 年前。

我试图构建一个将附加到文件的通用函数。根据结果​​，该函数将返回状态 200 或 500。

初始设置应调用该函数并返回任一状态。调用该方法后，我收到一个空对象。

该函数是类的一部分，并且已添加所有必需的导入。我确实尝试返回 fs.appendFile 但我设法收到相同的结果。

/**
 * A functio which writes a data into a file
 * @param  {[string]} fileName A file location with file name string
 * @param  {[string]} msg      A data to be saved
 * @return {[object]}          A response object with response code and msg.
 */
static toFile(fileName, msg) {
    fs.appendFile(fileName, msg, function(err) {
        let responseObject = {};
        if (err) {
            responseObject.status = 500;
            responseObject.msg = 'Error occured please view ' + fileName;
            return responseObject;
        }
        responseObject.status = 200;
        responseObject.msg = 'Success, file has been created ' + fileName;

        return responseObject;
    });
}

Answer 1

这里的问题是 fs.appendFile 的方式功能有效。该函数是异步的......所以基本上你可以看到你的函数如下:

static toFile(fileName, msg) {
    fs.appendFile(fileName, msg, function(err) {
        let responseObject = {};
        if (err) {
            responseObject.status = 500;
            responseObject.msg = 'Error occured please view ' + fileName;
            return responseObject;
        }
        responseObject.status = 200;
        responseObject.msg = 'Success, file has been created ' + fileName;

        return responseObject; //this wont be assigned to anything.
    });
    return undefined; //this is why you get undefined 
}

您可以使用两种方法:

Callback approach:当您收到响应 obj 时，这将触发您的 cb 函数。

static toFile(fileName, msg, cb) { //cb is a function
    fs.appendFile(fileName, msg, function(err) {
        let responseObject = {};
        if (err) {
            responseObject.status = 500;
            responseObject.msg = 'Error occured please view ' + fileName;
            return responseObject;
        }
        responseObject.status = 200;
        responseObject.msg = 'Success, file has been created ' + fileName;

        cb(responseObject); //instead of returning, call your cb function and use it as you please.
    });
}

Promise approach

这将在解析时返回响应对象，当然你必须处理 toFile 之外的 promise 功能。

   static toFile(fileName, msg) { //cb is a function
    return new Promise((resolve, reject) => {
            fs.appendFile(fileName, msg, function(err) {
                let responseObject = {};
                if (err) {
                    responseObject.status = 500;
                    responseObject.msg = 'Error occured please view ' + fileName;
                    reject(responseObject)
                }
                responseObject.status = 200;
                responseObject.msg = 'Success, file has been created ' + fileName;

                resolve(responseObject)
            });
        }
    }