JavaScript对象遍历

一、对象是数组:

var arr = [9, 8, 7, 6, 5, 4, 3, 2, 1]; 

//each遍历：
$.each(arr,function (index,value) {
  alert(index)
})                   
//0 1 2 3 4 5 6 7 8


$.each(arr,function (index,value) {
  alert(value)
})                  
//9 8 7 6 5  4 3 2 1     


$.each(arr,function () {
  alert(this)
})                                     
//9 8 7 6 5  4 3 2 1     


---------------------------------------------------------


//for遍历：
for (var i in arr) {                //不推荐for in 遍历数组
  console.log(arr[i]);
}                                   //9 8 7 6 5  4 3 2 1     
 

for (var i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}                                   //9 8 7 6 5  4 3 2 1

var arr = [{"id":"1","tagName":"apple"},{"id":"2","tagName":"orange"},{"id":"3","tagName":"banana"},{"id":"4","tagName":"watermelon"},{"id":"5","tagName":"pineapple"}]; 

$.each(arr, function(index, obj) {
  alert(index);          // 0,1,2,3,4 
  alert(obj);            //{"id":"1","tagName":"apple"},{"id":"2","tagName":"orange"}...
  alert(obj.id);         // 1,2,3,4,5
  alert(obj.tagName);    // apple, orange, banana, watermelon, pineapple
});                       

$.each(arr, function(index, obj) {
  alert("第"+ index +"项的tagName是"+obj.tagName);
});                         
//第0项的tagName是apple  第1项的tagName是orange..................

二、对象是json:

var json  =  { "jim": "11", "tom": "12", "lilei": "13" }; 

//jquery获取
$.each(json , function(key, value) { 
  alert(key);                  //jim  tom  lilei      
  alert(value);                //11  12  13     
}); 

//原生JS获取
for(var i in json){
  alert(i);                   //jim  tom  lilei
  alert(json[i]);             //11  12  13
};

三、json是字符串:（解决方案：JSON字符串转换为JavaScript对象。）

var json = '[{"id":"1","tagName":"apple"},{"id":"2","tagName":"orange"},{"id":"3","tagName":"banana"},{"id":"4","tagName":"watermelon"},{"id":"5","tagName":"pineapple"}]';

//用$.parseJSON(json)或者JSON.parse(json)将其转换为JavaScript对象   字符串转json对象
$.each($.parseJSON(json), function(idx, obj) {        
  alert(obj.tagName);          //apple, orange, banana, watermelon, pineapple
});

json的解析方法共有两种：eval() 和 JSON.parse()，使用方法如下：

var jsonData = '{"data1":"Hello", "data2":"world"}';

eval("("+jsonData+")");             //Object {data1: "Hello", data2: "world"}                      
JSON.parse(jsonData);               //Object {data1: "Hello", data2: "world"}

这样就把jsonData这个json格式的字符串转换成了JSON对象。

eval("("+data+")");                 //转换为json对象

eval在解析字符串时，会执行该字符串中的代码（这样的后果是相当恶劣的）

var obj= [{"id":"1","tagName":"apple"},{"id":"2","tagName":"orange"},{"id":"3","tagName":"banana"},{"id":"4","tagName":"watermelon"},{"id":"5","tagName":"pineapple"}];

JSON.stringify(obj)                //json对象转字符串  与JSON.parse()相反

"[{"id":"1","tagName":"apple"},{"id":"2","tagName":"orange"},{"id":"3","tagName":"banana"},{"id":"4","tagName":"watermelon"},{"id":"5","tagName":"pineapple"}]"