js async await function 执行顺序

Question

I have node.js server. And I should store files on specific api. My function for it:

exports.bootcampPhotoUpload = asyncHandler(async (req, res, next) => {
  ...
  const fileNames = [];
  files.forEach((item) => {
    item.name = `photo_${bootcamp._id}${path.parse(item.name).ext}`;
    item.mv(`${process.env.FILE_UPLOAD_PATH}/${item.name}`, async err => {
      console.log(item.name); // logs on console second and shows fileName
      fileNames.push(item.name);
    });
  });

  console.log(fileNames); // logs on console firstly and shows []
  const updated = await Bootcamp.findByIdAndUpdate(req.params.id, { photo: fileNames });
  console.log(fileNames); // logs on console third and shows [filenames]
  res.status(200).json({
    success: true,
    data: fileNames
  });
});

And on my model saved:

{
  photo: []
}

item object:

{ name: 'photo_2020-04-18 14.08.56.jpeg',
  data:
   <Buffer ff d8 ff e0 00 10 4a 46 49 46 00 01 01 01 00 48 00 48 00 00 ff db 00 43 00 04 03 03 04 03 03 04 04 03 04 05 04 04 05 06 0a 07 06 06 06 06 0d 09 0a 08 ... >,
  size: 208920,
  encoding: '7bit',
  tempFilePath: '',
  truncated: false,
  mimetype: 'image/jpeg',
  md5: 'f8714b4e2135c3873c939a9464ea380a',
  mv: [Function: mv]
}

For file upload I'm using express-fileupload . There is problem with async/await executing. How can I save it correctly?

Answer 1

Two problems here.

1. Your mv function is async and callback-based, so you cannot use await directly.
2. Even if we turn it to promise-based, await won't work with forEach as we expect.

For the first one, we can use the normal for loop. And for the second one, we can use util.promisify to convert your mv function to promise based. Check here for doc on util.promisify .

Try the below code,

const util = require('util');

const fileNames = [];

for (let i = 0; i < files.length; i++) { // 1. for loop here to support await in code block
    const item = files[i];
    item.name = `photo_${bootcamp._id}${path.parse(item.name).ext}`;
    try {
        await util.promisify(item.mv)(`${process.env.FILE_UPLOAD_PATH}/${item.name}`)   // 2. Promisify here so we can await
        console.log(item.name);
        fileNames.push(item.name);
    } catch (err) {
        // handle err
    }
}

console.log(fileNames);

Note: util.promisify could be slower, so it will be better if you find an alternate method/version of mv which already is promise-based.