function factorial2f( n ) {
	return addon( n );
}
Evaluates the double factorial of `n` as a single-precision floating-point number.

@private
@param {number} n - input value
@returns {(NonNegativeInteger|number)} double factorial

@example
var v = factorial2f( 3 );
// returns 3

@example
var v = factorial2f( 4 );
// returns 8

@example
var v = factorial2f( 57 );
// returns Infinity

@example
var v = factorial2f( -10 );
// returns NaN