JavaScript排序 — sort()方法
——解决null、undefined、0之间的排序(混乱)问题
一、普通的数组排序
JavaScript中用方法sort()为数组排序。sort()方法有一个可选参数,是用来确定元素顺序的函数。如果这个参数被省略,那么数组中的元素将按照ASCII字符顺序进行排序。
-
数组元素是字符串
//定义新字符串元素数组
var arr = ["a", "b", "A", "B"];
//sort()方法排序
arr.sort();
console.log(arr);//["A", "B", "a", "b"]
备注:因为字母A、B的ASCII值分别为65、66,而a、b的值分别为97、98,所以上面输出的结果是 ["A", "B", "a", "b"]
。
-
数组元素是数字
//定义新数字元素数组
var arr = [15, 8, 25, 3];
//sort()方法排序
arr.sort();
console.log(arr);//[15, 25, 3, 8]
结果并不是我们想要的顺序(理想顺序:按数字大小排序)!原因如下:
sort()方法会对数组的每一项执行toString()方法,然后再对得到的字符串进行排序。所以之所以25比3大,但却排在3之前,是因为比较的是字符串25和字符串3。如果要得到我们理想的数据,则使用比较函数(通俗的说就是sort()方法内的参数)
-
比较函数的使用
//定义新数字元素数组
var arr = [15, 8, 25, 3];
//sort()方法排序
arr.sort((a,b)=>{
//对数组进行遍历,通过比较a与b的差值大小,来排序
return a-b;
});
console.log(arr);//[3, 8, 15, 25]
二、数组对象排序
其实和上文基本一致,只不过比较对象加了属性值。
//定义数组对象
var arr=[{"age":24,name:'zs'},{"age":0,name:'ls'},{"age":7,name:'pl'}]
//sort()方法排序
arr.sort((a,b)=>{
//对数组进行遍历,
//通过对a对象的age属性与b对象的age属性
//进行取差,根据插值大小,进而排序
console.log(a['age']-b['age']);//-14 7 -17 7
return a['age']-b['age'];
});
console.log(arr);//[{age: 0, name: 'ls'},{age: 7, name: 'pl'},{age: 24, name: 'zs'}]
对比较函数进行提取:
//定义数组对象
var arr=[{"age":24,name:'zs'},{"age":0,name:'ls'},{"age":7,name:'pl'}]
//将比较方法进行提取
var compare = function (obj1, obj2) {
var val1 = obj1.age;
var val2 = obj2.age;
return val1 - val2;
}
//[{age: 0, name: 'ls'},{age: 7, name: 'pl'},{age: 24, name: 'zs'}]
console.log(arr.sort(compare));
对比较函数再进行改造,使得参数的输入更加灵活!
var arr=[{"age":24,name:'zs'},{"age":0,name:'ls'},{"age":7,name:'pl'}]
var compare = function (prop) {
return function (obj1, obj2) {
var val1 = obj1[prop];
var val2 = obj2[prop];
return val1 - val2
}
}
//[{age: 0, name: 'ls'},{age: 7, name: 'pl'},{age: 24, name: 'zs'}]
console.log(arr.sort(compare("age")));
重点:如果数组内某一对象为空值,排序是怎样的呢?(显然并不是按年龄顺序排序的,因为存在null和undefined)
var arr=[{"age":24,name:'zs'},{"age":0,name:'ls'},{"age":0,name:'gr'},
{"age":null,name:'yo'},{"age":7,name:'pl'},{"age":undefined,name:'tt'},
{"age":null,name:'jz'},{"age":0,name:'mn'},{"age":undefined,name:'we'}]
//[{"age": 0,"name": "ls"},{"age": 0,"name": "gr"},{"age": null,"name": "yo"},
//{"age": null,"name": "jz"},{"age": 0,"name": "mn"},{"age": 7,"name": "pl"},
//{"age": 24,"name": "zs"},{"age":undefined,"name": "tt"},
//{"age":undefined,"name": "we"}]
console.log(arr.sort(compare("age")));
我们可以知道上述比较函数是对比两个值的插值(可以是数字的插值,也可以是ASCII的插值),同时我们又知道:当值为null、undefined、0时,这三者任意方式搭配相减,其结果都为0!,并且使用if判断都为false!
解决办法:可以制造特殊值之间与特殊值与正常值之间的差足够大!
理解:
- undefined - 所有值 = -9999 (也就是说undefined与其他值差的最多,排最小)
- null - 所有值 = -999 (也就是说undefined与其他值差的第二多,排最第二小)
- undefined - undefined = 0
- null - null = 0
- undefined - null = 0
- 上述说明是等式a - b 那么相反b - a规则也适用;也就是下文第二个if的返回值999 9999
- -999和-9999、999,9999是用来区别其他差值的,也可根据实际情况进行差值设定
var arr=[{"age":24,name:'zs'},{"age":0,name:'ls'},{"age":0,name:'gr'},
{"age":null,name:'yo'},{"age":7,name:'pl'},{"age":undefined,name:'tt'},
{"age":null,name:'jz'},{"age":0,name:'mn'},{"age":undefined,name:'we'}]
var compare = function (prop) {
return function (obj1, obj2) {
var val1 = obj1[prop];
var val2 = obj2[prop];
if(!val1 && val1 != 0 && val2){
//定义undefined特殊值的差值
if(val1 === undefined) return -9999
//定义null特殊值的差值
return -999
}
if(!val2 && val2!= 0 && val1){
if(val2 === undefined) return 9999
return 999
}
return val1 - val2
}
}
//[{age: undefined, name: 'we'},{age: undefined, name: 'tt'},{age: null, name: 'jz'},
//{age: null, name: 'yo'},{age: 0, name: 'ls'},{age: 0, name: 'gr'},
//{age: 0, name:'mn'},{age: 7, name: 'pl'},{age: 24, name: 'zs'}]
console.log(arr.sort(compare("age")));
标签:sort,arr,null,undefined,age,排序,name
From: https://www.cnblogs.com/gby-web/p/16743564.html