Arrays in type script

Question

I am finding difficulty declaring array in typescript and accessing it.

below is the code working for me

class Book {     public BookId: number;     public Title: string;     public Author: string;     public Price: number;     public Description: string; }  class dataservice {     getproducts() {         var bk = new Book();         bk.Author = "vamsee";         bk.BookId = 1;         var bks: Book[] = [bk,bk];          return bks.length;     } }  var ds = new dataservice(); var button = document.createElement('button');  button.onclick = function () {          alert(ds.getproducts().toString()); }  document.body.appendChild(button); 

When I change my code as below it fails when trying to assign value to array item.

var bks: Book[] = new Book[2]; bks[0].Author = "vamsee"; bks[0].BookId = 1; return bks.length; 

For me to add object in a loop I have to do it the second way.

Answer 1

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

Answer 2

You can also do this as well (shorter cut) instead of having to do instance declaration. You do this in JSON instead.

class Book {     public BookId: number;     public Title: string;     public Author: string;     public Price: number;     public Description: string; }  var bks: Book[] = [];   bks.push({BookId: 1, Title:"foo", Author:"foo", Price: 5, Description: "foo"});   //This is all done in JSON.