javascript - 掌握 JS Promise

Question

我正在尝试理解 promise ，在本例中是在一个循环中。

我的场景基于将文件上传到 Google 云端硬盘。我的理解是，每个文件都应该上传，然后一旦 promise 得到解决，就上传下一个文件，依此类推。

目前我有一个函数可以上传文件并在完成后返回一个 promise :

# upload.js
const google = require('googleapis');
const drive = google.drive('v3');

function uploadFile(jwtClient, fileMetadata, media) {
  return new Promise((resolve, reject) => {
    drive.files.create({
      auth: jwtClient,
      resource: fileMetadata,
      media,
      fields: 'id, modifiedTime, originalFilename'
    }, (err, uploadedFile) => {
      if (err) reject(err);
        // Promise is resolved with the result of create call
        console.log("File Uploaded: " + uploadedFile.data.originalFilename);
        resolve(uploadedFile)
    });
  });
}

module.exports = uploadFile;

然后我想在循环中使用这个函数，我的想法是，在从 uploadFile 函数返回 Promise 之前，不应发生循环的下一次迭代

const google = require('googleapis');
const uploadFile = require('./components/upload');
const config = require('./gamechanger-creds.json');
const drive = google.drive('v3');
const targetFolderId = "1234"
var excel_files_array = [array if file names];


const jwtClient = new google.auth.JWT(
  config.client_email,
  null,
  config.private_key,
  ['https://www.googleapis.com/auth/drive'],
  null
);

jwtClient.authorize((authErr) => {
 if (authErr) {
  console.log(authErr);
  return;
}

for(var i = 0; i < excel_files_array.length; i++) {
  console.log("File Name is: " + excel_files_array[i]);

  const fileMetadata = {
    name: excel_files_array[i],
    parents: [targetFolderId]
  };

  const media = {
    mimeType: 'application/vnd.ms-excel',
    body: fs.createReadStream('path/to/folder' + excel_files_array[i] )
  };

  uploadFile(jwtClient, fileMetadata, media);

 }
});

运行时我的输出如下

File Name is: arsenal_away.xlsx
File Name is: bournemouth_away.xlsx
File Name is: brighton_away.xlsx
File Name is: burnley_away.xlsx
File Name is: chelsea_away.xlsx
File Name is: crystal_palace_away.xlsx
File Name is: everton_away.xlsx

File Uploaded: undefined
(node:83552) UnhandledPromiseRejectionWarning: Unhandled promise rejection 
(rejection id: 7): Error: Invalid multipart request with 0 mime parts.
(node:83552) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
File Uploaded: undefined
(node:83552) UnhandledPromiseRejectionWarning: Unhandled promise rejection 
(rejection id: 9): Error: Invalid multipart request with 0 mime parts.
File Uploaded: undefined

File Uploaded: bournemouth_away.xlsx
File Uploaded: everton_away.xlsx
File Uploaded: burnley_away.xlsx
File Uploaded: arsenal_away.xlsx
File Uploaded: brighton_away.xlsx
File Uploaded: chelsea_away.xlsx
File Uploaded: crystal_palace_away.xlsx

所以文件上传没有按顺序发生(不确定它们是否应该如此？猜不是因为它都是异步发生的)。

我希望了解如何确保这些文件按顺序上传(如果这确实是最好的方法)，并确保文件上传在移至下一个文件之前解决 promise 。

我还希望能够将脚本的身份验证部分包装成一个 promise ，但到目前为止尚未成功。

Answer 1

只需将async/await放在它们所属的位置即可

async function uploadFile(jwtClient, fileMetadata, media) ...

async function uploadManyFiles(...)
    for(....)
       await uploadFile(...)

这可确保上传按顺序执行。如果您希望它们并行发生，请将 promise 分组到 .all 中:

  await Promise.all(files.map(uploadFile))

使用身份验证与上传完全相同:

async function auth(...)
    return new Promise((resolve, reject) => {
       jwtClient = ...
       jwtClient.authorize((authErr) => {
         if (authErr) {
             reject(authErr);
         else
             resolve(whatever)