$(function(){

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

$("#dd").click(function(){

    $.each(JSON.parse(json), function(idx, obj) {//第一种方式

    alert(obj.tagName);

});

//or 

$.each($.parseJSON(json), function(idx, obj) {//第二种方式

    alert(obj.tagName);

});

});

});