数组常用函数解析

sort: 用in-place 算法对数组的元素进行排序，并返回数组

语法: arr.sort(comparefunction)
comparefunction 可选，如果省略，元素根据字符串unicode码点进行默认排序
comparefunction(a, b) 小于 0 ，那么 a 会被排列到 b 之前
comparefunction(a, b) 等于 0 ， a 和 b 的相对位置不变(不保证所有顺序一定这样)
comparefunction(a, b) 大于 0 ， b 会被排列到 a 之前

码点参考：https://www.ruanyifeng.com/blog/2014/12/unicode.html

(1)比较数字

var number = [21,37,45,-12,0,37];
// number.sort((a,b) => a - b); //升序排列
number.sort((a,b) => b - a); //降序排列
console.log(number); //[45, 37, 37, 21, 0, -12]

(2) 比较对象

 var items = [
       { name: 'edward', value: 21 },
       { name: 'sharpe', value: 37 },
       { name: 'and', value: 45 },
       { name: 'the', value: -12 }, 
       { name: 'magnetic',value: 0 },
       { name: 'zeros', value: 37 } 
];
items.sort( (a, b) => a.value - b.value);

map

语法：array.map(function(value, index, array) {// ...});

(1) 原数组被“映射”成对应新数组

var data = [1, 2, 3, 4];
var arrayofsquares = data.map(function (item) {
    return item * item;
});
console.log(arrayofsquares); // 1, 4, 9, 16

(2) 获得对象数组中的特定属性值

var users = [
    {name: "张一", "email": "[e-mail]"},
    {name: "江二",   "email": "[e-mail]"},
    {name: "李三",  "email": "[e-mail]"}
];
var emails = users.map(function (user) { return user.email; });
console.log(emails.join(", ")); // [e-mail], [e-mail], [e-mail] 

(3)向对象数组新增属性

 var users = [
    {name: "张一", "email": "[e-mail]"},
    {name: "江二",   "email": "[e-mail]"},
    {name: "李三",  "email": "[e-mail]"}
 ];
 users  = users.map(function (user,index) { 
       user.index = index;
       return user; 
 });
 console.log(json.stringify(users)); // [{"name":"张一","email":"[e-mail]","index":0},{"name":"江二","email":"[e-mail]","index":1},{"name":"李三","email":"[e-mail]","index":2}]