Because vasya.sell is just the function ( Function Object ), unexecuted, unbound.

If you want to execute that function with this bound to vasya, you have to bind it and then later execute it, or execute the function as vasya.sell($something) in one expression.

There is a difference between .every( vasya.sell ) and .every( personInLine => vasya.sell(personInLine) ). Maybe there need not have been a difference, but there is. There's something to be said for ( and against ) both possibilities.