数组一些遍历方法

数组方法：

• forEach： array.forEach(callback,[ thisObject])
• map: array.map(callback,[ thisObject]);
• filter: array.filter(callback,[ thisObject]);
• reduce:array.reduce(callback[, initialValue])

• forEach

/*js使用方法*/
    var arr = ['独孤九剑','上官飞燕','欧阳春秋','皇甫云峰']
    arr.forEach( function(element, index) {
        console.log(element);   // 输出值
        console.log(index);     // 输出索引
    });

/*jQ使用方法*/
    $.each(arr, function(index, value, array) {
        console.log(index);     // 输出索引
        console.log(value);     // 输出值
});

• map

    var arr = [1,2,3]
    arr.map(function(index, elem) {
        return item*item;
    })

• filter

     var users = [
       {name: "南派三叔",  "email": "nan@email.com"},
       {name: "唐家三少",  "email": "tang@email.com"},
       {name: "天蚕土豆",  "email": "tian@email.com"}
     ];
     
     var emailsZhang = users
       // 获得邮件
       .map(function (user) { return user.email; })
       // 筛选出zhang开头的邮件
       .filter(function(email) {  return /^t/.test(email); });
     
     console.log(emailsZhang.join(", ")); 

• reduce

     // 迭代
     var sum = [1, 2, 3, 4]
     sum.reduce(function (previous, current, index, array) {
       return previous + current;
     });
     
     console.log(sum); // 10

     var matrix = [
       [1, 2],
       [3, 4],
       [5, 6]
     ];
     
     // 二维数组扁平化
     var flatten = matrix.reduce(function (previous, current) {
       return previous.concat(current);
     });
     
     console.log(flatten); // [1, 2, 3, 4, 5, 6]