var _welcomeMessage="Welcome to WebTrainingRoom";
In above example, we declared a variable called _welcomeMessage, then assigned a string value within "".
To declare a variable use var then name of the variable
Once you assign a value to that variable, that become the data type of variable, in our case above variable become string type after value assigned
You cannot change the data type of any variable within the program scope, suppose you have written var x=10;, now within the program you cannot write var x="Hello World";