TypeScript and dot-notation access to objects

Question

If TypeScript is a strict superset of JavaScript, why is dot notation on an arbitrary object erroneous? I have JS code that I want to convert over to TS for better type safety, but all access using dot notation (eg, myObj.thing) gives me the error Property 'thing' does not exist on type '{}'.. It works properly when I use bracket notation (eg, myObj['thing']).

Answer 1

I know you say this is odd, but this is one of the main reasons TypeScript exists. This error helps prevent accidentally setting or getting non-existent properties on an object.

Right now, as the compiler is telling you, the property bar does not exist on x because it has been implicitly typed to {} when writing var x = {};.

You can tell the compiler that x has more than zero properties by explicitly defining the type:

var x: { foo?: string; bar?: string; } = {}; 

Now you can get or set x.foo and x.bar without the compiler complaining. In most cases, you would move this into an interface like so:

interface IFooBar {     foo?: string;     bar?: string; }  var x: IFooBar = {};  x.foo = "asdf";  // ok x.test = "asdf"; // error, as it should be 

Some people are recommending you cast to any, but don't get lazy. You should make full use of the type system TypeScript provides. Doing so will most definitely save you time down the road as you maintain an application.

Answer 2

Because of the strongly-typed nature of Typescript Object, you can use "any" to make it untyped:

var x: any = {}; x.bar = "bar";   /// ok 

If what's you need is to define type of literal properties

var x: { [index: string]: TypeA } = {}; 

then x["bar"] can now only be instance of TypeA.