I want to write a function which accepts a class as an argument, but only classes which implement a specific interface.
I know that I can use any, but it would be nice to rely on the type system to enforce this.
Some pseudocode to express what I mean...
interface MyInterface { ... }
class Class1 implements MyInterface { ... }
class Class2 { ... }
function doStuff(classParameter: Class implements MyInterface) { ... }
doStuff(Class1); // OK
doStuff(Class2); // Error
You can use a constructor signature to enforce the fact that the constructor will return an instance compatible with the interface. You can use it either as a generic parameter constraint or directly as a parameter depending on what you need to do with the class:
interface MyInterface { foo: number }
class Class1 implements MyInterface { foo: number }
class Class2 { bar: number }
function doStuff0(classParameter: new (...args: any[]) => MyInterface) { }
doStuff0(Class1); // OK
doStuff0(Class2); // Error
function doStuff1<T extends new (...args: any[]) => MyInterface>(classParameter: T) { }
doStuff1(Class1); // OK
doStuff1(Class2); // Error
Note I added members to the example, don't forget Typescript uses a structural type system, so compatibility is determined by members not by implements MyInterface declaration, so any class would be compatible if MyInterface were empty, and a class would be compatible if it had a foo member even if it didn't explicitly declare implements MyInterface
new(...) => MyInterface directly as a type in places where I referenced the class that was accepted as a function argument. Thanks!
class Class2 { foo: number } still pass the test ?Maybe you can use generics to achieve what you want to do. Here's a workaround :
function doStuff<T extends myInterface>(classParameter: T) {
...
}
So here, T is a dynamic type which necessary implements myInterface (yeah, keyword extends while talking about interfaces is strange here).
You can check the documentation below, part "Generic constraints" https://www.typescriptlang.org/docs/handbook/generics.html
MyInterface (which would be answered with just function doStuff(x: MyInterface)), but about how to refer to the type of a constructor that produces a MyInterface. See @TitialCernicova-Dragomir's answer.
doStuff(Class1) not doStuff(new Class1()). This could be a mistake, but I don't think so, she wants to pass in the class