javascript - 在 Promise.prototype.then() 外部声明的对象在其内部使用时返回 undefined

Question

在下面的函数中，我遇到了一个问题，即 options 在 .then() 中被认为是未定义的，即使 console.log(options.destination) 在 storageObj 函数调用链正确打印之前。

/**
 * Returns a promise that resolves an object including the
 * file destination if the requested file is downloaded successfully
 * and rejects if there is an error whle downloading the file.
 *
 * @param {Object} storageObj GCS Storage object instance.
 * @param {String} bucketName Name of the bucket to access in the GCS Storage object.
 * @param {String} filePath Path of the file to access in the given bucketName.
 * @param {String} destinationDir Name of the direcory to download the file to.
 * @returns Promise
 */
function downloadGCSFileToDir(storageObj, bucketName, filePath, destinationDir) {
    return new Promise((resolve, reject) => {
        console.log("filePath is: " + filePath);
        const filePathParts = filePath.split("/");
        const destination = `${destinationDir}/${filePathParts[filePathParts.length - 1]}`;
        const options = { "destination": destination };
        console.log(options.destination);
        storageObj
            .bucket(bucketName)
            .file(filePath)
            .download(options)
            .then(() => {
                console.log(`gs://${bucketName}/${filePath} downloaded to ${options.destination}`);
                resolve({ "destination": options.destination });
            })
            .catch(err => reject(new Error(`downloadGCSFileToDir failed: ${err}`)));
    });
}

但是如果我在 .then() 中使用 destination 这样:

function downloadGCSFileToDir(storageObj, bucketName, filePath, destinationDir) {
    return new Promise((resolve, reject) => {
        console.log("filePath is: " + filePath);
        const filePathParts = filePath.split("/");
        const destination = `${destinationDir}/${filePathParts[filePathParts.length - 1]}`;
        const options = { "destination": destination };
        console.log(options.destination);
        storageObj
            .bucket(bucketName)
            .file(filePath)
            .download(options)
            .then(() => {
                console.log(`gs://${bucketName}/${filePath} downloaded to ${destination}`);
                resolve({ "destination": destination });
            })
            .catch(err => reject(new Error(`downloadGCSFileToDir failed: ${err}`)));
    });
}

它按预期打印。这是为什么？

Answer 1

options 在您的 then 中不是未定义的； options.destination 是。原因是 download 方法 deletes the destination property来自 options 对象。

File.prototype.download = function(options, callback) {
  if (is.fn(options)) {
    callback = options;
    options = {};
  }

  callback = once(callback);

  var destination = options.destination;
  delete options.destination;  // <--- here

  // ...

至于它为什么这样做，你的猜测和我的一样好。