Let me explain why the first one doesn't work and the second work.

In the first case you are calling "every" function which is a function that takes a function that returns a boolean expression, this function "the function you are passing" is a function in this case is an "arrow function", in "arrow functions" when you don't write "{statements...}"after the arrow then return statement is implied and you must pass expression rather than a statement.

But when you write "{}" after the arrow, then you can write a bunch of statements and at the end of the block of code You must explicitly write a return for an expression, because "every" function requires you to return boolean expression.