Nintendo Switch Manual page 25

The Joy-Con controllers will remain paired when
you detach them from the console, and you'll be
able to use them wirelessly. If they run out of
battery, attach them to the console when it's
charging to recharge them.
Can I detach the Joy-Con controllers and use
them with another console?
Each Joy-Con can only be paired to one console
at a time.
To use a paired Joy-Con with a different console,
you must attach the Joy-Con to that console to
pair it.
You will need to do this again each time you'd
like to change what console you are using the
Joy-Con with.
Playing with the Joy-Con controllers detached from
the console
You can use Joy-Con strap accessories when you
are playing with the Joy-Con controllers detached
from the console.