Angular’s $q.reject() vs deferred.reject()

1) $q.reject() is a shortcut to create a deferred and then reject it immediately; I often use this in an errorFn if I can’t handle the error.

2) Nothing, promise.catch(errorFn) is just syntactic sugar for promise.then(null, errorFn), just like the success and error methods of the $http service, so you can write code like the following:

promise.
    then(function(result){
        // handle success
        return result;
    }, function errorHandler1(error){
        // handle error, exactly as if this was a separate catch in the chain.

    }).catch(function errorHandler2(error){
        // handle errors from errorHandler1
    });

3) This is exactly where $q.reject can come in handy:

promise.
    catch(function(error){
       //Decide you can't handle the error
       return $q.reject(error); //This forwards the error to the next error handler;
    }).catch(function(error){
       // Here you may handle the error or reject it again.
       return 'An error occurred'; 
       //Now other errorFn in the promise chain won't be called, 
       // but the successFn calls will.
    }).catch(function(error){
       // This will never be called because the previous catch handles all errors.
    }).then(function(result){
       //This will always be called with either the result of promise if it was successful, or 
       //'An error occured' if it wasn't
    });