Typescript does not identify error when using generics

Question

When I use the following code, typescript does not complain that I try to pass IAction<Product> instead of the expected IAction<Customer>:

interface IAction<T> {
    canExecute<T>(input: T): void;
}

interface Product {
    name: string,
}

interface Customer {
    displayName?: string;
    description?: string;
}

class MyAction implements IAction<Product> {
    public canExecute<Product>(product: Product): void {
    }
}

class MyMenuItemAction {
        constructor(private action: IAction<Customer>) {
    }
}

function foo(action: IAction<Customer>): void {
}

const action: IAction<Product> = new MyAction();
const menuItem = new MyMenuItemAction(action);
foo(action);

But if I remove the <T> from the canExecute function in the IAction interface, it does complain about it:

interface IAction<T> {
    canExecute(input: T): void;
}

Is this a bug in typescript?

This is not a bug in Typescript. With that extra <T> you are actually declaring a new type parameter for the method, different than the one the interface declares. — Thiago Barcala
– Thiago Barcala, Commented May 26, 2020 at 6:09
But I'm still confused why I can pass IAction<Product> to a function which expects IAction<Customer>...? — Dikla
– Dikla, Commented May 26, 2020 at 6:38

satanTime · Accepted Answer · 2020-05-26 06:04:51Z

1

You need to change your interface to avoid override of a generic

interface IAction<T> {
    canExecute(input: T): void; // <- remove <T> from here.
    // canExecute<T> means there's another generic that has a conflict
    // with IAction<T>
}

Then update MyAction

class MyAction implements IAction<Product> {
    public canExecute(product: Product): void { // <- remove <Product>
    }
}

Profit: Playground

edited May 26, 2020 at 6:04

answered May 26, 2020 at 5:58

satanTime

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8 Comments

Dikla Over a year ago

Thanks! can you please clarify what you mean by: "canExecute<T> means there's another generic that has a conflict with IAction<T>"

satanTime Over a year ago

once you have <T> in definition - it means there's a generic. In your case you say IAction<T> has a generic T, then you say canExecute<T> has a generic T, but this T is an other T (name collision), and when you say IAction<Product> it doesn't set type to canExecute<T>. Therefore it's fine to have IAction<Product>.canExecute<Customer> and that's why you don't get an error.

satanTime Over a year ago

If it answers your question - might you mark it as a correct answer?

Dikla Over a year ago

this: IAction<Product>.canExecute<Customer> I understand. But I'm still confused why I can pass IAction<Product> to a function which expects IAction<Customer>...?

satanTime Over a year ago

because IAction<Product> doesn't narrow the 2nd canExecute<T> and it can be any type. It means it accepts a top level Product only and canExecute accepts anything as its T and anything fits Customer therefore you don't get an error.

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