Seeing how dependency injection is done in angular 4. On entering to see more deeply the documentation of angular.io there is a "useExisting" option instead of the traditional "useClass":
// Forma tradicional
[ NewLogger,
// Not aliased! Creates two instances of 'NewLogger'
{ provide: OldLogger, useClass: NewLogger}]
// Esta es la que no entiendo
[ NewLogger,
// Alias OldLogger w/ reference to NewLogger
{ provide: OldLogger, useExisting: NewLogger}]
Does anyone know the difference between injecting with "useClass" to "useExisting"?