Define a class within a factory function or closure that is being exported

In my TypeScript code, I have the following scenario:

export const makeMyClass = function(a: string, b: boolean){

   const MyClass = function(){

   };

   MyClass.prototype.foo = function(){};

   MyClass.prototype.bar = function(){};

   return MyClass;

}

I am struggling to translate the code inside the exported factory function into TypeScript.

When attempting this approach:

 export const makeMyClass = function(a: string, b: boolean): MyClass {

       class MyClass {

       }

      // ...

      return MyClass;


  }

TypeScript gives an error stating that it cannot locate the name 'MyClass'. Let's assume that I need to utilize the exported closure makeMyClass for this question's purpose.

Answer №1

When naming the return value type of the function makeMyClass() and making it visible outside of the function, TypeScript requires you to define its structure as an interface in a higher scope. This is how class expressions are implemented in TypeScript, as explained in this source.

For your specific case, you need to define both the instance type (describing instance properties and methods) and the constructor type (defining the constructor and any static methods):

export interface MyClassInstance {
  foo(): void;
  bar(): void; 
  // add other public interface definitions here
}
export interface MyClassConstructor {
  new(): MyClassInstance;
  readonly prototype: MyClassInstance;
  // include any static methods here
}

With these interfaces in place, you can specify that makeMyClass() returns the constructor type:

export const makeMyClass = function(a: string, b: boolean): MyClassConstructor {

  // implement MyClassInstance/MyClassConstructor interfaces
  class MyClass {
    foo() {
      // ...
    }
    bar() {
      // ...
    }
  }

  return MyClass;
}

It's worth noting that there is some redundancy involved, as you define the class structure within and outside the function. Any changes to the interfaces will require corresponding updates in the class implementation. Unfortunately, exporting types from within functions is not supported.

Hopefully, this explanation proves helpful to you. Best of luck!

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