$btn.click(function () {

        var

            x = parseFloat($('#x').val()),

            y = parseFloat($('#y').val()),

            r;

        try {

        if (isNaN(x) || isNaN(y)) {

            throw new Error('输入有误');

        }

        r = x + y;

        alert('计算结果：' + r);

        } catch (e) {

          alert('输入有误！');

        }

    });