Before going to answer read this Documentation once. Then you clearly understand the answer.

Try this It may work for you.

Object.keys(data.shareInfo[i]).length

This does the same think as Object.keys(obj).length, but without the browser compatibility issue (I think).

What about:

var obj = {
    yo: {
         a: 'foo',
         b: 'bar'
    }
    hi: {
        c: 'hello',
        d: 'world',
        e: 'json'
    }
}

var arr = [], len;

for(key in obj) {
    arr.push(key);
}

len = arr.length;

console.log(len) //2

OR

var arr = [], len;

for(key in obj.hi) {
    arr.push(key);
}

len = arr.length;

console.log(len) //3

OR, in your case

var arr = [], len;

for(key in studentMockBeanMap) {
    arr.push(key);
} 

len = arr.length;

console.log(len); //4

You can use something like this

var myObject = {'name':'Kasun', 'address':'columbo','age': '29'}

var count = Object.keys(myObject).length;
console.log(count);

The variable you are trying to store is already a json encoded string. You can not stringify it again.

You can drop those ' and reformat the json, then it would be a normal json, then you can call stringify on it.

var data = {"history":[{"keyword":key,"msg":msg,"ver":ver}]};
localStorage.setItem("history", JSON.stringify(data));

To get the length of the array after retrieve:

var historydata = JSON.parse(localStorage.getItem("history"));
console.log(historydata.history.length);

The jList property of the object is just a string, so you need to convert it to a Javascript object using JSON.parse().

// Dummy of your "data" variable
var data = {"jList":"[{\"added_by\":\"Ani\",\"description\":\"example description.\",\"start_date \":\"2014-10-10\",\"mark\":255,\"id\":975},{\"added_by\":\"Ani\",\"description \":\"example description..\",\"start_date\":\"2014-10-10\",\"mark\":255,\"id\":980 }]"};

var myList = JSON.parse(data.jList);
alert(myList.length); // Alerts "2"

Your JSON is not an array, but an object. If you want it to be an array, it should be something like this:

[
  "{\"start\":{\"lat\":22.9939202,\"lng\":72.50009499999999},\"end\":{\"lat\":23.0394491,\"lng\":72.51248850000002},\"waypoints\":[[23.0316834,72.4779436]]}",

  "{\"start\":{\"lat\":22.9999061,\"lng\":72.65318300000001},\"end\":{\"lat\":23.0420584,\"lng\":72.67145549999998},\"waypoints\":[[23.02237,72.6500747]]}",

  "{\"start\":{\"lat\":23.0394491,\"lng\":72.51248850000002},\"end\":{\"lat\":22.9999061,\"lng\":72.65318300000001},\"waypoints\":[[23.0016629,72.58898380000005]]}"

 ]

Then, you can get a javascript array as follows:

var array = JSON.parse(jax.responseText);

And access values as follows:

array[0]

array.length

EDIT: In order to have a real JSON array with the PHP json_encode method, see this related question.

With this modification you will be able to use all the possibilities of JS array without workaround.

your data is already json format:do like this

var App = [
           {
             "id": "123",
             "caption": "Test",
             "description": "Test Desc"
           },
           {
             "id": "345",
             "caption": "adsasdasd",
             "description": ""
           },
           {
             "id": "456",
             "caption": "adsasdasd",
             "description": ""
           },
           {
             "id": "578",
             "caption": "adsasdasd",
             "description": ""
           }
          ];

console.log(App);
console.log(App[0].length);// you can not get length from this because it is not array it's an object now.

var AppLen = App.length;
alert(AppLen);