javascript - 如何操作json中的某个元素-Questions et réponses sur le réseau chinois PHP

有这样一个json：

[ { groupname:'主要任务', index:0, tasks:[ { id:2, title:'完善通讯录aaweeeeg', urgency:'正常', remark:'some word here', tasker_main:'henry', tasker_other:['jim','john'], participators:['1','2','3'], publisher:'henry', ddl:'1288323623006', base_score:'5', upvoters:['jim','小华'], completed:true }, { id:3, title:'task2', urgency:'正常', remark:'', tasker_main:'12', tasker_other:['小李','小王'], participators:['1','2','3'], publisher:'小华', ddl:'1289355623006', base_score:3, upvoters:[], completed:true }, { id:4, title:'紧急会议', urgency:'紧急', remark:'some word here', tasker_main:'henry', tasker_other:['smith','harry'], participators:['1','2','3'], publisher:'小明', ddl:'1285978923006', base_score:2, upvoters:['henry'], completed:false } ] }, { groupname:'技术任务', index:1, tasks:[ { id:5, title:'task1', urgency:'火急', remark:'some word here', tasker_main:'henry', tasker_other:['jim','john'], participators:['1','2','3'], publisher:'mary', ddl:'1285978923006', base_score:'5', upvoters:['jim','小华'], completed:false } ] } ]

我想通过task的id这个属性，找到特定的task对象，然后把它的completed属性改成true。
请问该如何实现这个功能啊？

PS.这个json实际上是当对象来用的，不是字符串。。我是想用JavaScript，在用户单击按钮时，把相应的task标记成已完成。

谢谢

var data = [ { groupname:'主要任务 index:0, tasks:[ { id:2, title:'完善通讯录aaweeeeg', urgency:'正常', remark:'some word here', tasker_main:'henry', tasker_other:['jim','john'], participators:['1','2','3'], publisher:'henry', ddl:'1288323623006', base_score:'5', upvoters:['jim','小华'], completed:true }, { id:3, title:'task2', urgency:'正常', remark:'', tasker_main:'12', tasker_other:['小李','小王'], participators:['1','2','3'], publisher:'小华', ddl:'1289355623006', base_score:3, upvoters:[], completed:true }, { id:4, title:'紧急会议', urgency:'紧急', remark:'some word here', tasker_main:'henry', tasker_other:['smith','harry'], participators:['1','2','3'], publisher:'小明', ddl:'1285978923006', base_score:2, upvoters:['henry'], completed:false } ] }, { groupname:'技术任务', index:1, tasks { id:5, title:'task1', urgency:'火急', remark:'some word here', tasker_main:'henry', tasker_other:['jim','john'], participators:['1','2','3'], publisher:'mary', ddl:'1285978923006', base_score:'5', upvoters:['jim','小华'], completed:false } ] } ] var _id = 4; //把id是4的变成true for(var i=0;i