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Generic type defaulting to {} instead of any #7505
Description
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 $http.get() is as follows
get<T>(url: string, config?: IRequestShortcutConfig): IHttpPromise<T>;so if you don't explicitly specify what T is you get an error when you try to access the response.
Activity
sandersn commentedon Mar 14, 2016
myFunc(and$http.get, it looks like) doesn't have any arguments to inferTfrom. 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 oftest, for example.robianmcd commentedon Mar 14, 2016
When you don't specify a type everywhere else in TypeScript the type defaults to
anyso this behavior seems counter intuitive to me. But yeah I can just specify the type to workaround it.mhegazy commentedon Mar 14, 2016
#6354 would be a better solution here.
mhegazy commentedon Mar 14, 2016
also, you can achieve the same result today by defining an overload.
sandersn commentedon Mar 14, 2016
Rob (@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 thanany.anywould be easier to use, though.robianmcd commentedon Mar 14, 2016
Mohamed Hegazy (@mhegazy) yeah the issue you linked to would be a good solution. Also I didn't think of overloading the function like that. thanks