Using TypeScript to define generic types for classes, method parameters, and method return types

I am facing an issue with this particular function

function getCollection<T>(collectionType: T): Collection<T> {
  return new Collection<T>()
}

In the Collection class, I have the following code snippet

export class Collection<T> {
  public add (item: T) {
    // .. logic
  }
}

Additionally, I have a user class named as follows

export class Student {

}

When trying to execute the following line of code

getCollection(Student).add(new Student());

I encounter an error message

TS2345: Argument of type 'Student' is not assignable to parameter of type 'typeof Student'.   Property 'prototype' is missing in type 'Student' but required in type 'typeof Student'.

However, the following code snippet works perfectly fine

new Collection<Student>().add( new Student());

Hence, what could be causing the issue when the function returns a generic collection?

Answer №1

T is identified as the type typeof Student. While Student represents an instance of a class, typeof Student refers to the constructor. To determine the instance type of a constructor, utilize the conveniently named InstanceType built-in function:

public getCollection<T>(collectionType: T): Collection<InstanceType<T>> {
  return new Collection<InstanceType<T>>("some-arg1", "some-arg2")
}

However, you will need to impose a constraint which should not pose too much difficulty:

public getCollection<T extends new (...args: any[]) => any>(...

This modification will yield:

public getCollection<T extends new (...args: any[]) => any>(collectionType: T): Collection<InstanceType<T>> {
  return new Collection<InstanceType<T>>("some-arg1", "some-arg2")
}

Answer №2

This issue arises because the generic type is inferred based on the parameter, resulting in T being not Student, but actually typeof Student. Therefore, return new Collection<T> does not function like

return new Collection<Student>
, rather it becomes
return new Collection<typeof Student>
.

To resolve this, a specific type needs to be assigned to the generic parameter:

getCollection<Student>(Student)

The above eliminates the need for the parameter and allows for refactoring of getCollection as follows:

getCollection<T>(): Collection<T> {
  return new Collection<T>("some-arg1", "some-arg2");
}

This can then be invoked as:

getCollection<Student>()

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