Your JSON should look like this:

let json = [{
    "id" : "1", 
    "msg"   : "hi",
    "tid" : "2013-05-05 23:35",
    "fromWho": ""
},
{
    "id" : "2", 
    "msg"   : "there",
    "tid" : "2013-05-05 23:45",
    "fromWho": ""
}];

You can loop over the Array like this:

for(let i = 0; i < json.length; i++) {
    let obj = json[i];

    console.log(obj.id);
}

Or like this (suggested from Eric) be careful with IE support

json.forEach(function(obj) { console.log(obj.id); });

There's a few problems in your code, first your json must look like :

var json = [{
"id" : "1", 
"msg"   : "hi",
"tid" : "2013-05-05 23:35",
"fromWho": ""
},
{
"id" : "2", 
"msg"   : "there",
"tid" : "2013-05-05 23:45",
"fromWho": ""
}];

Next, you can iterate like this :

for (var key in json) {
if (json.hasOwnProperty(key)) {
  alert(json[key].id);
  alert(json[key].msg);
}
}

And it gives perfect result.

See the fiddle here :

try this

var json = [{
    "id" : "1", 
    "msg"   : "hi",
    "tid" : "2013-05-05 23:35",
    "fromWho": ""
},
{
    "id" : "2", 
    "msg"   : "there",
    "tid" : "2013-05-05 23:45",
    "fromWho": ""
}];

json.forEach((item) => {
  console.log('ID: ' + item.id);
  console.log('MSG: ' + item.msg);
  console.log('TID: ' + item.tid);
  console.log('FROMWHO: ' + item.fromWho);
});

var arr = [
  {
  "id": "1",
  "msg": "hi",
  "tid": "2013-05-05 23:35",
  "fromWho": ""
  }, {
  "id": "2",
  "msg": "there",
  "tid": "2013-05-05 23:45",
  "fromWho": ""
  }
];

forEach method for easy implementation.

arr.forEach(function(item){
  console.log('ID: ' + item.id);
  console.log('MSG: ' + item.msg);
  console.log('TID: ' + item.tid);
  console.log('FROMWHO: ' + item.fromWho);
});

Since i already started looking into it:

var data = [{
    "id": "1",
    "msg": "hi",
    "tid": "2013-05-05 23:35",
    "fromWho": ""
}, {
    "id": "2",
    "msg": "there",
    "tid": "2013-05-05 23:45",
    "fromWho": ""
}]

And this function

var iterateData =function(data){   for (var key in data) {
       if (data.hasOwnProperty(key)) {
          console.log(data[key].id);
       }
    }};

You can call it like this

iterateData(data); // write 1 and 2 to the console

Update after Erics comment

As eric pointed out a for in loop for an array can have unexpected results. The referenced question has a lengthy discussion about pros and cons.