function area_of_circle(r, pi) {

            var s = 0;

            if (pi == null) {

                pi = 3.14;

                s = r * r * pi;

                alert(r)

                alert(pi);

            } else {

                s = r * r * pi;

                alert(r);

            }

            return s;

        }

        // 测试:

        if (area_of_circle(2) === 12.56 && area_of_circle(2, 3.1416) === 12.5664) {

            console.log('测试通过');

        } else {

            console.log('测试失败');

        }