How do I catch thrown errors with async / await?

Question

Here's some code:

  import 'babel-polyfill'

  async function helloWorld () {
    throw new Error ('hi')
  }

  helloWorld()

I also went deep and tried this as well:

  import 'babel-polyfill'

  async function helloWorld () {
    throw new Error ('hi')
  }

  async function main () {
    try {
      await helloWorld()
    } catch (e) {
      throw e
    }
  }

  main()

and:

import 'babel-polyfill'

 async function helloWorld () {
   throw new Error ('hi')
 }

try {
 helloWorld()
} catch (e) {
 throw e
}

This works:

import 'babel-polyfill'

async function helloWorld () {
  throw new Error('xxx')
}

helloWorld()
.catch(console.log.bind(console))

Answer 1

So it's kind of tricky, but the reason you're not catching the error is because, at the top level, the entire script can be thought of as a synchronous function. Anything you want to catch asynchronously needs to be wrapped in an async function or using Promises.

So for instance, this will swallow errors:

async function doIt() {
  throw new Error('fail');
}

doIt();

Because it's the same as this:

function doIt() {
  return Promise.resolve().then(function () {
    throw new Error('fail');
  });
}

doIt();

At the top level, you should always add a normal Promise-style catch() to make sure that your errors get handled:

async function doIt() {
  throw new Error('fail');
}

doIt().catch(console.error.bind(console));

In Node, there is also the global unhandledRejection event on process that you can use to catch all Promise errors.

Answer 2

async is meant to be used with Promises . If you reject the promise, then you can catch the error, if you resolve the promise, that becomes the return value of the function.

async function helloWorld () {
  return new Promise(function(resolve, reject){
    reject('error')
  });
}


try {
    await helloWorld();
} catch (e) {
    console.log('Error occurred', e);
}

Answer 3

To catch an error from an async function, you can await the error:

async function helloWorld () {
  //THROW AN ERROR FROM AN ASYNC FUNCTION
  throw new Error('hi')
}

async function main() {
  try {
    await helloWorld()
  } catch(e) {
    //AWAIT THE ERROR WITHIN AN ASYNC FUNCTION
    const error = await e
    console.log(error)
  }
}

main()

Alternatively, you can just await the error message:

async function main() {
  try {
    await helloWorld()
  } catch(e) {
    //AWAIT JUST THE ERROR MESSAGE
    const message = await e.message
    console.log(message)
  }
}