阅读 Promise of Stream in nodejs 的 Props

Question

basically what I want to achieve is check in a middleware whether an uploaded file has the correct image type (png for example). This is what I have come up with till now:

export const fileCheckMiddleware = (req, res, next) => {
  const acceptedImageTypes = ["image/gif", "image/jpeg", "image/png"];

  const oldWrite = res.write;
  const oldEnd = res.end;

  const chunks = [];

  res.write = (...restArgs) => {
  chunks.push(new Buffer(restArgs[0]));
  oldWrite.apply(res, restArgs);
 };

  res.end = async (...restArgs) => {
if (restArgs[0]) {
  chunks.push(new Buffer(restArgs[0]));
}

const body = Buffer.concat(chunks).toString("utf8");

try {
  let parsedBody = {};
  try {
    parsedBody = JSON.parse(body);
  } catch (err) {
    parsedBody = { data: { unparsedBody: body } };
  }

  const { variables } = req.body;

  console.log("\x1b[1m%s\x1b[0m", "LOG variables", variables.file);
  if (variables.file) {
    console.log("\x1b[1m%s\x1b[0m", "LOG type", typeof variables.file);
  }
} catch (err) {}
oldEnd.apply(res, restArgs);
   };
  next();
};

The logged type of variables.file is an object. And the result of the console.log is this:

LOG variables Promise {
 { filename: 'trump.jpeg',
   mimetype: 'image/jpeg',
   encoding: '7bit',
   createReadStream: [Function: createReadStream] } }

So how can I access the mimetype here? I tried to map over the keys, variables.file["Promise"],...

Answer 1

Promise is not a key of variables.file , it's the type of variables.file . That means your code starts executing as soon as the HTTP request starts, and the file is received asynchronously, so you have to do something like:

variables.file.then(file => {
    // Do whatever you want with the file
    next();
});

Or declare the surrounding function as async and do this:

const file = await variables.file;
// Do whatever you want with the file
next();