Handling Js promise rejection

Question

How do you handle an error (eg. "new error" below) that is outside of the promise?

function testError() {
    throw new Error("new error") // how to handle this?
    var p123 = new Promise(function(resolve, reject) {
         resolve(123)
    });
    return p123
};

testError().catch(err => {
        return err;  // code doesn't come here
    })
    .then(ok => {
        console.log(ok)
    });

Answer 1

If you're not sure whether a function will throw (or return a value) synchronously, you can call it using .then(). This will wrap it so that the result will be handled consistently no matter how it is produced:

function testError() {
  throw new Error("new error") // how to handle this?
  var p123 = new Promise(function(resolve, reject) {
    resolve(123)
  });
  return p123
};

Promise.resolve()
  .then(testError)
  .catch(err => {
    console.error(err);
    return err; 
  })
  .then(ok => {
    console.log(ok.message)
  });

Answer 2

Since the error doesn't involve the async code, a regular try-catch should do fine here:

try {
  testError().catch(err => {
    return err;  // code doesn't come here
  })
  .then(ok => {
     console.log(ok)
  });
}
catch(e) {
   // 
}

Note that when the async-await pattern finally becomes the native way of resolving promises, the try-catch will also become the native way of handling errors:

try {
    var ok = await testError();
    console.log(ok)
}
catch(e) {
    console.log('e:' +e);
}

As one can easily verify, this one correctly handles both the sync and the async error and is much cleaner than then-catch.

Answer 3

If you can, rewrite your testError function like so

function testError () {
  return new Promise(function (resolve, reject) {
     throw new Error('new error')
     resolve(123)
  })
}

testError().then(ok => console.log(ok),
                 err => console.error(err.message))

1. Run it once to see it throw the error in console.error
2. Comment out the throw line to see the promise resolve successfully