There is no requirement to declare types in TypeScript generics

Here is a brief example outlining the TypeScript issue I am seeking help with:

export class Test {
    public runTest<T>(param: T): T {
        return param;
    }
}

let test1: Test = new Test();
test1.runTest<string>("string1");
test1.runTest("5555");

Both calls to "runTest" will work without any errors. The first one expects a string as input (which is intended), but the second does not. I am looking for a solution to enforce passing a type into T whenever using the "runTest" method.

Is there a way to achieve this? I have tried various approaches in the code but haven't been successful. I also searched for a relevant TSLint rule but couldn't find one.

Thank you!

typescriptgenerics

Answer №1

In order to accomplish this task (regardless of the reason behind it), you must find a way to prompt TypeScript to infer an incorrect type for T when it is omitted, causing the test to fail. While I have not been able to determine a direct method for achieving this, here is an alternative approach:

export class Test {
    public runTestIndirectly<T = never>(): (t:T)=>T {
      return (t:T)=>t  
    }
}

As a result, calling runTestIndirectly() generates a function that mimics the behavior of runTest. The type of T is deduced when runTestIndirectly() is invoked, but without any access to the type of object passed to the function it produces. By defaulting T to never, attempting to leave T unspecified will lead to failure:

let test1: Test = new Test();        
test1.runTestIndirectly<string>()("string1"); // succeeds
test1.runTestIndirectly()("5555"); // fails:
// Argument of type '"5555"' is not assignable to parameter of type 'never'.

I hope this explanation proves useful!

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