Back

Build a Javascript async function error handler

Fri, Sep 13 2019 • 2 min read
Nathaniel

Background

If you are a Js developer, you probably have used async and await, and you love them.
No more callback hell or .then chains. Just like writing a synchronous program.
Util you run into UnhandledPromiseRejectionWarning or Uncaught (in promise) Error
You begin to wrap every piece of code using try.. catch.., but that seems a lot of work.
But I've got good news, you can write a custom async function error handler:

Solution

const asyncHandler = fn => async (...args) => {
  try {
    await fn(...args);
  } catch (err) {
    console.log(err);
  }
};
It takes all types of functions with any number of arguments.
use it like this:
const asyncHandler = fn => async (...args) => {
  try {
    await fn(...args);
  } catch (err) {
    console.log(err);
  }
};


function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}


const yourFunction = async () => {
  await sleep(2000);
  throw 'something wrong';
  return 'success';
}


(async ()=> {
  await yourFunction(); // will cause Uncaught error
  await asyncHandler(yourFunction)(); // will handle the error properly
})();

Return values

Of course when you need the result of the async function, you can:
const asyncHandler = fn => async (...args) => {
  try {
    return await fn(...args);
  } catch (err) {
    console.log(err);
  }
};


function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}


const yourFunction = async () => {
  await sleep(2000);
  // throw 'something wrong';
  return 'success';
}


(async ()=> {
  const result = await asyncHandler(yourFunction)(); //will handle the error properly
  console.log(result); // => 'success'
})();

Custom error handling

if you need some custom error handling, for example, to use it in express.js
you can do simply by adding some of your custom logic
const asyncHandler = fn => async (...args) => {
  try {
    await fn(...args);
  } catch (err) {
    logger.error(err);
    const resFn = args.find(arg => arg.name === 'res');
    if (resFn) {
      let errors = {
        message: 'Internal Sever Error',
        error: err
      };


      if (err instanceof mongoose.Error.ValidationError) {
        errors = {
          message: 'Mongoose Model Validation Error',
          error: err
        };
      }
      if (err instanceof mongoose.mongo.MongoError) {
        errors = {
          message: 'MongDB Error',
          error: err
        };
      }


      resFn.status(500).json(errors);
    }
  }
};
later you use it in express routers like this:
router.get('/item',
  asyncHandler(async (req, res) => {
    // router logic here
  })
)

Comments(0)

Continue with
to comment