Node.js readFile async callback UnhandledPromiseRejectionWarning-CodePudding

I am trying to supply an async function to node.js's fs.readFile callback, but I am not too sure how to handle the errors thrown by the callback. Normally, with non-async callback, you would do:

// error is handled

fs.readFile('/file_does_not_exist.txt', 'utf8', (err, data) => {
  if (err) throw err
  // do things here
})

And if a readFile error occurs, it is handled properly. But as soon as I use an async function as a callback:

// UnhandledPromiseRejectionWarning

fs.readFile('/file_does_not_exist.txt', 'utf8', async (err, data) => {
  if (err) throw err
  // await for things here
})

Then it throws UnhandledPromiseRejectionWarning when a readFile error occurs. How would you try to handle the err in this scenario? I have tried try catch in the body of the callback, but it doesn't work. The only other solution is to wrap the async function inside another function.

CodePudding user response：

fs.readFile('/file_does_not_exist.txt', 'utf8', async (err, data) => {
         //if (err) throw err
        /* replace it to */ if (err){
                //handle error here
         }else{
                // await for things here
        }
})

CodePudding user response：

It's expected that you get an UnhandledPromiseRejectionWarning error, because throwing an error isn't the same as handling it.

If your goal is to exit the process:

if (err) {
  process.exit(1);
}

If you want to continue running the program but somehow handle the error upstream, you'll end up mixing callbacks and promises, which typically leads to convoluted code.

Instead, I would suggest using fs/promises, the Promise-based implementation of fs:

const fs = require('node:fs/promises');
… 
async function readFile() {
  try {
    const data = await fs.readFile('/file_does_not_exist.txt', 'utf8');
    // process file data
  } catch(e) {
    console.error('caught error reading file', e);
    // here you can cleanly exit the process or re-throw `e` to properly handle it upstream
  }
}