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.

basarat · Accepted Answer · 2013-04-26 10:01:52Z

68

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

edited Apr 26, 2013 at 10:01

answered Apr 26, 2013 at 9:56

basarat

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1 Comment

basarat Over a year ago

Update: The 'new Book[2]' will give an error in TS 0.9 : No longer accept "new number[]" : typescript.codeplex.com/… so the compiler will tell you that it is wrong :)

fletchsod · Accepted Answer · 2016-04-25 16:37:56Z

16

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.

answered Apr 25, 2016 at 16:37

fletchsod

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Comments

Serj Sagan · Accepted Answer · 2016-02-22 00:09:05Z

-1

A cleaner way to do this:

class Book {
    public Title: string;
    public Price: number;
    public Description: string;

    constructor(public BookId: number, public Author: string){}
}

Then

var bks: Book[] = [
    new Book(1, "vamsee")
];

answered Feb 22, 2016 at 0:09

Serj Sagan

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2 Comments

Ujjwal Kumar Gupta Over a year ago

But you are not initializing the data in constructor, will it initialized automatically?

Serj Sagan Over a year ago

Yes, it does. See the "Parameter properties" section of typescriptlang.org/docs/handbook/classes.html

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