user session tracking in javascript?

You can use HTML5 feature Local Storage

For eg;

When user logs in, set one variable as isAuthenticated in your local storage

By default, it will be 0, on successful login, set it to 1.

localStorage.isAuthenticated = 0; // default
OR
localStorage.setItem('isAuthenticated',0);

On Successful login

localStorage.isAuthenticated = 1; // default
OR
localStorage.setItem('isAuthenticated',1);

And whenever you want to check user's authenticity, simply compare value of isAuthenticated variable from Local Storage.

var isLoggedIn = localStorage.getItem('isAuthenticated');

if(isLoggedIn){
    //your code
} else {
    // invalid user code
}

UPDATE If you are storing javascript objects then

var myObject = { 'one': 1, 'two': 2, 'three': 3 };

// Put the object into storage
localStorage.setItem('myObject', JSON.stringify(myObject));

// Retrieve the object from storage
var retrievedObject = localStorage.getItem('myObject');
retrievedObject = JSON.parse(retrievedObject)

Convert your object into string using JSON.stringify and then store it into the local storage. While retrieving cast that string back into the javascript object using JSON.parse

Looking at the local storage documentation, the functionality seems to be limited to handle only string key/value pairs.

You can take a look at the documentation here

Apple

Mozilla

PS : Phonegap has nothing to do with working of local storage or any other HTML feature. Phonegap is just a middleware that facilitates communication between your device hardware and your javascript/html code.