为什么会收到“ UnhandledPromiseRejectionWarning”，这是什么意思？

Question

I am trying to read and write to a CSV file, and I am quite new to javascript. My code seems to make sense to me but I am not sure why it isn't running.

app.post('/name=:name/available=:available/type=:type/subtype=:subtype/ip=:ip', (req, res) => {
  csvtojson().fromFile(csvFilePath).then((objects) => {
    objectsArray = objects;
    const newItem = {
      id: objectsArray.length + 1, 
      name: req.params.name,
      available: req.params.available
    };

    csvWriter.writeRecords(objectsArray).then( () => {
      console.log("Csv File Created!");
      console.log(objectsArray);
      res.json(objectsArray);
    });
   });
});

From what I can see, I should first read the CSV file, then save the contents in an array, then create an object, then write the object into the CSV file. But I get the errors: "UnhandledPromiseRejectionWarning: Error: EBUSY: resource busy or locked", and "UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)"

Answer 1

The UnhandledPromiseRejectionWarning comes becomes one of your two asynchronous operations is encountering an EBUSY error and you have no error handler for that error. The node.js runtime sees that a promise was rejected, but there was no error handler for that and identifies that as a likely inappropriate coding situation (a bug) so it emits the warning.

In Javascript, EVERY promise operation that can ever reject must have an error handler. An error handler for promise rejections can be done with a .catch() , a try/catch if the promise is await ed or with the 2nd argument to .then() .

You can add proper error handlers to your code and that will tell you a bit more about where the error is occurring and will get rid of the UnhandledPromiseRejectionWarning .

app.post('/name=:name/available=:available/type=:type/subtype=:subtype/ip=:ip', (req, res) => {
  csvtojson().fromFile(csvFilePath).then((objects) => {
    const newItem = {
      id: objects.length + 1, 
      name: req.params.name,
      available: req.params.available
    };

    return csvWriter.writeRecords(objects).then( () => {
      console.log("Csv File Created!");
      console.log(objects);
      res.json(objects);
    });
   }).catch(err => {
      console.log(err);
      res.sendStatus(500);
   });
});

If, for debugging purposes, you wanted to log the 2nd promise rejection separately, you could do this:

app.post('/name=:name/available=:available/type=:type/subtype=:subtype/ip=:ip', (req, res) => {
  csvtojson().fromFile(csvFilePath).then((objects) => {
    const newItem = {
      id: objects.length + 1, 
      name: req.params.name,
      available: req.params.available
    };

    return csvWriter.writeRecords(objects).then( () => {
      console.log("Csv File Created!");
      console.log(objects);
      res.json(objects);
    }).catch(err => {
      console.log("csvWriter error", err);
      throw err;
    });
   }).catch(err => {
      console.log(err);
      res.sendStatus(500);
   });
});

In this case, I've returned the 2nd promise which will chain it to the 1st promise so we can then add a single .catch() handler to the 1st promise and it will catch errors from either promise. Then, the error handler sends a 500 status to complete the request handler.

I would guess that the EBUSY error has something to do with either reading from the file represented at csvFilePath or with the file that csvWriter is trying to write to. It sounds like it could be a file exclusivity issue where it's opened for exclusive access while doesn't allow other readers/writers to have access to it.

We can't really help with that specific error without seeing more of the context of your code and understanding while files might have multiple readers/writers that creates some sort of contention.

---

FYI, it also looks like you should get rid of the objectsArray variable and just use objects instead. objectsArray is not declared locally within this function. If it is not declared at all, then it's an implicit global (which is bad). If it is declared at a higher scope, then it's a variable that is potentially shared between users each making the same request (very bad bug).