javascript 数组排序

var arr=[1,2,3,5,10,4,2,19,2,0];
    alert(arr);//[1,2,3,5,10,4,2,19,2,0]
    arr.sort(function (a, b) {//升序
        return a-b;
    });
    alert(arr);//[0,1,2,2,3,4,5,10,19]

    var arr2=[{a:1},{a:4},{a:0}];
    alert(arr2.map(function (value, index, array) {//[1,4,0]
        return value.a;
    }));
    arr2.sort(function (a, b) {//降序
        return b.a-a.a;
    });
    alert(arr2.map(function (value, index, array) {//[4,1,0]
        return value.a;
    }));