Declaring a boolean in JavaScript using just var

Types are dependent to your initialization:

var IsLoggedIn1 = "true"; //string
var IsLoggedIn2 = 1; //integer
var IsLoggedIn3 = true; //bool

But take a look at this example:

var IsLoggedIn1 = "true"; //string
IsLoggedIn1 = true; //now your variable is a boolean

Your variables' type depends on the assigned value in JavaScript.

No it is not safe. You could later do var IsLoggedIn = "Foo"; and JavaScript will not throw an error.

It is possible to do

var IsLoggedIn = new Boolean(false);
var IsLoggedIn = new Boolean(true);

You can also pass the non boolean variable into the new Boolean() and it will make IsLoggedIn boolean.

var IsLoggedIn = new Boolean(0); // false
var IsLoggedIn = new Boolean(NaN); // false
var IsLoggedIn = new Boolean("Foo"); // true
var IsLoggedIn = new Boolean(1); // true

As this very useful tutorial says:

var age = 0;

// bad
var hasAge = new Boolean(age);

// good
var hasAge = Boolean(age);

// good
var hasAge = !!age;