Generic type defaulting to {} instead of any

[robianmcd]

TypeScript Version:
1.7.3

Code

window['test'] = {a:1, b:2};

function myFunc<T>(): T
{
    return window['test'];
}

let test = myFunc();
console.log(test.a);

Expected behavior:
The type of test is any so there is no compiler error.

Actual behavior:
I get the following error message
myFile.ts(9,18): error TS2339: Property 'a' does not exist on type '{}'.

Use Case
I run into this issue whenever I'm working on an Angular project and using the $http service. The type definition for $http.get() is as follows

get<T>(url: string, config?: IRequestShortcutConfig): IHttpPromise<T>;

so if you don't explicitly specify what T is you get an error when you try to access the response.

[sandersn]

myFunc (and $http.get, it looks like) doesn't have any arguments to infer T from. So type inference will end up assigning the type {} if it's not specified. This behaviour is by design.

I don't think there's any alternative to specifying the type you're getting from $http.get. TypeScript doesn't infer types from later usage of test, for example.

[robianmcd]

When you don't specify a type everywhere else in TypeScript the type defaults to any so this behavior seems counter intuitive to me. But yeah I can just specify the type to workaround it.

[mhegazy]

#6354 would be a better solution here.

[mhegazy]

also, you can achieve the same result today by defining an overload.

function myFunc(): any;
function myFunc<T>(): T;
function myFunc(): any {
    return window['test'];
}

[sandersn]

@robianmcd I think it's because "T" inside of a function acts like {} -- you can't see any properties on it, for example. So if you return T, and the compiler never inferred any actual type for it, it sort of makes sense that it comes out as {}.

Also, unlike the other cases where there is no type specified, there is a type variable here -- T -- which the compiler infers a type for. {} indicates failure rather than any. any would be easier to use, though.

[robianmcd]

@mhegazy yeah the issue you linked to would be a good solution. Also I didn't think of overloading the function like that. thanks