Rethrowing error in promise catch

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.

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!