adding property to javascript object in typescript without errors

Question

I found this article its quite interesting, but it does not help me when I try to extend a global variable like window.

Test.ts

window.test = {}; //ERROR: Property 'test' does not exist on type 'Window'.

(function (test)
{
//do stuff
} (window.test)); //Build: Property 'test' does not exist on type 'Window'

Error message:

ERROR: Property 'test' does not exist on type 'Window'.

How can I solve this ?

Nitzan Tomer · Accepted Answer · 2016-07-28 08:28:38Z

1

It's called Declaration Merging:

interface Window {
    test(): void;
}

window.test = function() {
    // do what ever
}

(code in playground)

As you can see you need to declare your new method in the Window interface and then the compiler won't complain when you add the actual implementation.

answered Jul 28, 2016 at 8:28

Nitzan Tomer

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11 Comments

Mahmoud Zakal Over a year ago

does it there's something like interface in JS or it's just in typescript ?

Nitzan Tomer Over a year ago

@MahmoudZakal interface is a typescript thing: typescriptlang.org/docs/handbook/interfaces.html, as there's no need to such a thing in javascript

Coderer Over a year ago

As of today there's a kinda-sorta need to do this in JS, if you're using the TS compiler to do type checking using the checkJs option. In that case, the compiler will complain about window.test = ... but is fine with window["test"] = ....

Nitzan Tomer Over a year ago

@Coderer Even if you add test to the window interface?

Coderer Over a year ago

Maybe I misunderstand but in vanilla Javascript there's no such thing as an interface. Mahmoud's question was how to tell the compiler that there should be a .test property on window in Javascript; that's what my comment is about.

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