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Create Factory Functions in TypeScript for Creating Multiple Classes

In TypeScript, you can define a function that, when passed a set of parameters, creates and returns a correctly configured object. One of the parameters passed to the function must be the class of the object being created (I'll call that the "class parameter"). Normally, that means your factory function can only create a single kind of class but, by leveraging generics and interfaces, you can create functions that will create and configure a variety of classes.

Your first step is to define an interface that specifies the properties that your factory function will configure. Here's an example of an interface that specifies two properties:

interface ICustomer
  name: string;
  age: number;

With that interface in place you can create a generic factory function that works with any class that implements that interface. First, when defining your factory function, follow the factory function's name with a data type marker and specify that the marker extends the interface. Second, in the factory function's parameter list, use TypeScript's new keyword along with the generic data type marker when defining the function's class parameter (you're actually specifying the type of the class' constructor).

The following code shows the declaration of a generic factory function that leverages my ICustomer interface. I first provide a type marker (c) that, using the extend keyword, ties the function to classes that implement the ICustomer interface. I then use that type marker with the new keyword to specify the return type of the function's class parameter (called cust in my example):

function CreateCustomer<c extends ICustomer>(cust:{new(): c;}, 
                                             name: string, age: number): c

Now, inside my function, I use the class parameter to instantiate the class passed in the class parameter. Once I've done that, I can configure the object by setting the properties specified in the interface:

    var newCust: c;
    newCust = new cust();
    newCust.name = name;
    newCust.age = age;
    return newCust;

To use this factory, I pass the class I want created (which must implement the ICustomer interface) along with the rest of the parameters required by my factory function. This example:

var cust: Customer;
cust = CreateCustomer(Customer, "Peter", 62);

uses my function to create an instance of a class called Customer.

Posted by Peter Vogel on 12/28/2015 at 11:35 AM

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