抖动函数的封装

function shake ( obj, attr, endFn ) {
    var pos = parseInt( getStyle(obj, attr) );
    var arr = [];            // 20, -20, 18, -18 ..... 0
    var num = 0;34         
    for ( var i=20; i>0; i-=2 ) {
        arr.push( i, -i );
    }
    arr.push(0);
        
    clearInterval( obj.shake );
    obj.shake = setInterval(function (){
        obj.style[attr] = pos + arr[num] + 'px';
        num++;
        if ( num === arr.length ) {
            clearInterval( obj.shake );
            endFn && endFn();
        }
    }, 50);
}