Variable merging

[ArtemAvramenko]

declare var test: (s: string) => void;

declare var test: {
    (s: string) => void;
    tests: any[];
}

At the moment TS compiler shows error: Subsequent variable declarations must have the same type.

[RyanCavanaugh]

What's the intent of this code?

[ArtemAvramenko]

First declaration is from nuget package. I do not want to modify it.

[danquirk]

We've had this request before. The problem is how do we support this dynamic, expando type of var usage but disallow meaningful errors that people generally associate with a good static checker. For example, you presumably would not have wanted to be allowed to declare var test: number; here, that would surely have been/caused a bug. But perhaps if the subsequent declaration is a subtype of the previous declaration we could allow it?

[ArtemAvramenko]

It would be nice if it will work the same way as in the case of interface merging. I.e. interpret a type of the variable, as a declaration of an anonymous interface:

declare interface _anonymous_interface_for_global_var_test: {
    (s: string) => void;
}

declare interface _anonymous_interface_for_global_var_test: {
    (s: string) => void;
    tests: any[];
}

[yahiko00]

If we allow this, it seems to imply we should allow extension of existing union types. But maybe I am wrong.

It might be useful to support this in the particular case of a property of type any. Often DefinitelyTyped declarations will declare something to be of type any when a particular user will know a better type.

// DefinitelyTyped
interface I { foo: any; }

// My code
interface I { foo: { x: number }; }

Currently we give an error because the types for foo conflict. We might just throw out any declarations typed as any when getting the type.

[OliverJAsh]

Just ran into this again trying to define the type of state in history's Location, which is defined as any in the official typings, and there's no provided means of overriding it: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/566e0394859fdc1dc893658ccec6b06372d56a91/types/history/index.d.ts#L46

DefinitelyTyped/DefinitelyTyped#27012