The latest version of Typescript, 2.4, is causing errors during compilation when using basic generics

I have been attempting to update a project from TypeScript 2.3 to 2.4, but the process has become quite frustrating and perplexing. I am encountering errors related to generics that are proving difficult to comprehend.

To simplify the issue, I have extracted a portion of the code:

interface Service {
    serviceName: string;
}

interface TableParams {
    selectionListLabelFn: <T>(item: T) => string;
}

const tableParameters: TableParams = {
    selectionListLabelFn: (service: Service) => service.serviceName
};

The snippet above results in the following error:

λ tsc test.ts
test.ts(9,7): error TS2322: Type '{ selectionListLabelFn: (service: Service) => string; }' is not assignable to type 'TableParams'.
  Types of property 'selectionListLabelFn' are incompatible.
    Type '(service: Service) => string' is not assignable to type '<T>(item: T) => string'.
      Types of parameters 'service' and 'item' are incompatible.
        Type 'T' is not assignable to type 'Service'.

What could be causing this issue? I find it completely nonsensical.

typescriptgenericstypescript2.4

Answer №1

It seems like the error is intentional and not a mistake in your code. The issue lies in the fact that selectionListLabelFn is defined as a generic delegate, requiring a generic delegate to be passed in instead of one with a Service parameter. To resolve this, you can create a generic arrow function like so:

const tableParams: TableParams = {
    selectionListLabelFn: <T>(service: T) => ""
};

If you want selectionListLabelFn to specifically reference Service, consider declaring it within the interface:

interface TableParams<T> {
    selectionListLabelFn: (item: T) => string;
}    
const tableParams: TableParams<Service> = {
    selectionListLabelFn: (service: Service) => service.serviceName
};

The error may be stricter in newer versions, possibly related to changes implemented in version 2.4 as seen on this link.

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