This is a very c# type of code:
var bks: Book[] = new Book[2];
In Javascript / Typescript you don't allocate memory up front like that, and that means something completely different. This is how you would do what you want to do:
var bks: Book[] = [];
bks.push(new Book());
bks[0].Author = "vamsee";
bks[0].BookId = 1;
return bks.length;
Now to explain what new Book[2]; would mean. This would actually mean that call the new operator on the value of Book[2]. e.g.:
Book[2] = function (){alert("hey");}
var foo = new Book[2]
and you should see hey. Try it