檢視一個簡單的jQuery的例子來遍歷一個JavaScript陣列物件。

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

$.each(json, function(idx, obj) {
alert(obj.tagName);
});

上面的程式碼片斷工作正常，提示 “apple”, “orange” … 等，如預期一樣。
問題: JSON 字串

下面的例子中，聲明瞭一個JSON字串（隨附單或雙引號）直接地。

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

$.each(json, function(idx, obj) {
alert(obj.tagName);
});

在Chrome中，它顯示在控制檯下面的錯誤：

Uncaught TypeError: Cannot use 'in' operator to search for '156' 
in [{"id":"1","tagName":"apple"}...

解決方案：JSON字串轉換為JavaScript物件。
要修復它，通過標準JSON.parse()或jQuery 的 $.parseJSON 將其轉換為JavaScript物件。

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

$.each(JSON.parse(json), function(idx, obj) {
alert(obj.tagName);
});

//or 

$.each($.parseJSON(json), function(idx, obj) {
alert(obj.tagName);
});

參考：http://www.codes51.com/article/detail_641.html