function isfinite( x ) {
	return Boolean( addon( x ) );
}
Tests if a double-precision floating-point numeric value is finite.

@private
@param {number} x - value to test
@returns {boolean} boolean indicating whether the value is finite

@example
var bool = isfinite( 5.0 );
// returns true

@example
var bool = isfinite( -2.0e64 );
// returns true

@example
var bool = isfinite( Infinity );
// returns false

@example
var bool = isfinite( -Infinity );
// returns false