Extending custom interface of array within other interface

Question

I keep getting an error during compilation: error TS2339 Property 'customMethod' does not exist on type 'Action[]'

These are my interfaces:

export interface Action {
    Name: string;
    someFunc(): void;
}

export interface ActionCollection {
    Actions: Action[];
}

Then in my code I'm trying to use a method that has not been described YET in the interface but it is available at runtime. This method is is available from the Actions array within ActionsCollection, just like a native .length property.

let myAC: ActionCollection = new ActionCollection( stuff );
myAC.Actions.customMethod(); // Note that it is attached to Actions

My question is how do I define it in the interfaces?

I've tried something like this, but I got errors:

export interface Action<> {
    customMethod(): any;
}

You shouldn't be able to do new ActionCollection as ActionCollection is an interface, you don't get an error there? — Nitzan Tomer
– Nitzan Tomer, Commented Jul 1, 2016 at 12:20

Nitzan Tomer · Accepted Answer · 2016-07-01 12:32:06Z

2

If you want your actions array to have this method then you either need to add it to the Array interface:

interface Array<T> {
    customMethod(): void;
}

And then all arrays will have it:

let a = [];
a.customMethod();

But that's probably not what you're after, instead just define your own array:

export interface Action {
    Name: string;
    someFunc(): void;
}

export interface ActionsArray extends Array<Action> {
    customMethod(): void;
}

export interface ActionCollection {
    Actions: ActionsArray;
}

You say that you already implemented the actual function, so this should be enough.

answered Jul 1, 2016 at 12:32

Nitzan Tomer

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