$('#demo').animate({
        width: 'toggle',
        height: 'toggle'
    }, {
    duration: 5000,
    specialEasing: {
        width: 'linear',
        height: 'elasticBoth' // 直接使用扩展的平滑函数名就好
    },
    complete: function() {
        $(this).after('<div>Animation complete.</div>');
    }
});