Rethrowing error in promise catch

I found the following code in a tutorial:

promise.then(function(result){
    //some code
}).catch(function(error) {
    throw(error);
});

I'm a bit confused: does the catch call accomplish anything? It seems to me that it doesn't have any effect, since it simply throws the same error that was caught. I base this on how a regular try/catch works.

There is no point to a naked catch and throw as you show. It does not do anything useful except add code and slow execution. So, if you're going to .catch() and rethrow, there should be something you want to do in the .catch(), otherwise you should just remove the .catch() entirely.

The usual point for that general structure is when you want to execute something in the .catch() such as log the error or clean up some state (like close files), but you want the promise chain to continue as rejected.

promise.then(function(result){
    //some code
}).catch(function(error) {
    // log and rethrow 
    console.log(error);
    throw error;
});

In a tutorial, it may be there just to show people where they can catch errors or to teach the concept of handling the error, then rethrowing it.

---

Some of the useful reasons for catching and rethrowing are as follows:

1. You want to log the error, but keep the promise chain as rejected.
2. You want to turn the error into some other error (often for easier error processing at the end of the chain). In this case, you would rethrow a different error.
3. You want to do a bunch of processing before the promise chain continues (such as close/free resources) but you want the promise chain to stay rejected.
4. You want a spot to place a breakpoint for the debugger at this point in the promise chain if there's a failure.
5. You want to handle a specific error or set of errors, but rethrow others so that they propagate back to the caller.

But, a plain catch and rethrow of the same error with no other code in the catch handler doesn't do anything useful for normal running of the code.

Both .then() and .catch() methods return Promises, and if you throw an Exception in either handler, the returned promise is rejected and the Exception will be caught in the next reject handler.

In the following code, we throw an exception in the first .catch(), which is caught in the second .catch() :

new Promise((resolve, reject) => {
    console.log('Initial');

    resolve();
})
.then(() => {
    throw new Error('Something failed');
        
    console.log('Do this'); // Never reached
})
.catch(() => {
    console.log('Something failed');
    throw new Error('Something failed again');
})
.catch((error) => {
    console.log('Final error : ', error.message);
});

The second .catch() returns a Promised that is fulfilled, the .then() handler can be called :

new Promise((resolve, reject) => {
    console.log('Initial');

    resolve();
})
.then(() => {
    throw new Error('Something failed');
        
    console.log('Do this'); // Never reached
})
.catch(() => {
    console.log('Something failed');
    throw new Error('Something failed again');
})
.catch((error) => {
    console.log('Final error : ', error.message);
})
.then(() => {
    console.log('Show this message whatever happened before');
});

Useful reference : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises#Chaining_after_a_catch

Hope this helps!

There is no important difference if you leave out the catch method call completely.

The only thing it adds is an extra microtask, which in practice means you'll notice the rejection of the promise later than is the case for a promise that fails without the catch clause.

The next snippet demonstrates this:

var p;
// Case 1: with catch
p = Promise.reject('my error 1')
       .catch(function(error) {
          throw(error);
       });

p.catch( error => console.log(error) );
// Case 2: without catch
p = Promise.reject('my error 2');

p.catch( error => console.log(error) );

Note how the second rejection is reported before the first. That is about the only difference.

So it sounds like your question is, "In the promise chain, what does the .catch() method do?"

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/throw

The throw statement "will stop (the statements after throw won't be executed), and control will be passed to the first catch block in the call stack. If no catch block exists among caller functions, the program will terminate."

In the promise chain, the .then() method will return some type of data chunk. This return of the chunk will complete the promise. The successful return of the data completes the promise. You can think of the .catch() method in the same way. .catch() however will handle unsuccessful data retrieves. The throw statement completes the promise. Occasionally, you will see developers use .catch((err) => {console.log(err))} which would also complete the promise chain.

You actually don't need to re throw it, just leave the Promise.catch empty otherwise it will consider as un handle the reject and then wrap the code in a try catch and it will catch the error automatically which is passing down.

try{
  promise.then(function(result){
    //some code
  }).catch(function(error) {
    //no need for re throwing or any coding. but leave this as this otherwise it will consider as un handled
  });
}catch(e){
  console.log(e);
  //error can handle in here
}

In the promise chain, it is better to use .catch

ex in function f2: .then(...).catch(e => reject(e));

• test1 - with try catch
• test2 - without try or .catch
• test3 - with .catch

function f1() {
	return new Promise((resolve, reject) => {
		throw new Error('test');
	});
}

function f2() {
	return new Promise((resolve, reject) => {
		f1().then(value => {
			console.log('f1 ok ???');
		}).catch(e => reject(e));
	});
}

function test1() {
    console.log('test1 - with try catch - look in F12');
    try {
      f2().then(() => { // Uncaught (in promise) Error: test
        console.log('???'); });
    } catch (e) {
      console.log('this error dont catched');
    }
}

function test2() {
    console.log('test2 - without try or .catch - look in F12');
    f2(); // Uncaught (in promise) Error: test
}

function test3() {
  console.log('test3 - with .catch');
  f2().then(value => {
    console.log('??');
  }).catch(e => {
    console.log(' now its ok, error ', e);
  })
}

setTimeout(() => { test1(); 
  setTimeout(() => { test2(); 
    setTimeout(() => { test3(); 
    }, 100);
  }, 100);
}, 100);