在TypeScript泛型中，类型声明不是必需的

Question

Short example of the issue I want to ask about in TypeScript:

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

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

Both "runTest" statements will execute normally. First will require you to pass a string (which is great), second - not really. But I want to make it mandatory for anyone using "runTest" to pass some type into T.

Can it be done? Can't figure out a way to do it via code. Also tried looking for TSLint rule that could help, but also found none.

Thanks

Answer 1

If you really want to get this done (I'm not going to worry about why you're doing this), you need to convince TypeScript to infer some bad type for T when you leave it out, so that it will fail to run the test. I can't figure out how to do it directly, but here's an indirect way:

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

So runTestIndirectly() produces a function which behaves like runTest . The type of T is inferred when you call runTestIndirectly() , which has no access to the type of object you pass to the function it produces. And if you have T default to never , you will get a failure when you neglect to specify T to something that works:

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

Hope that helps!