Stefan Baumgartner

Web ops, performance and front-end

TypeScript: The constructor interface pattern

15 August 2019 by @ddprrt | Posted in: TypeScript

If you are doing traditional OOP with TypeScript, the structural features of TypeScript might sometimes get in your way. Look at the following class hierachy for instance:

abstract class FilterItem {
  constructor(private property: string) {}
  someFunction() { /* ... */ }
  abstract filter(): void;
}


class AFilter extends FilterItem {
  filter() { /* ... */ } 
}


class BFilter extends FilterItem {
  filter() { /* ... */ }
}

The FilterItem abstract class needs to be implemented by other classes. In this example by AFilter and BFilter. So far, so good. Classical typing works like you are used to from Java or C#:

const some: FilterItem = new AFilter('afilter'); // ✅

When we need the structural information, though, we leave the realms of traditional OOP. Let’s say we want to instantiate new filters based on some token we get from an AJAX call. To make it easier for us to select the filter, we store all possible filters in a map:

declare const filterMap: Map<string, typeof FilterItem>;

filterMap.set('number', AFilter)
filterMap.set('stuff', BFilter)

The map’s generics are set to a string (for the token from the backend), and everything that complements the type signature of FilterItem. We use the typeof keyword here to be able to add classes to the map, not objects. We want to instantiate them afterwards, after all.

So far everything works like you would expect. The problem occurs when you want to fetch a class from the map and create a new object with it.

let obj: FilterItem;
const ctor = filterMap.get('number');

if(typeof ctor !== 'undefined') {
  obj = new ctor(); // 💣 cannot create an object of an abstract class
}

What a problem! TypeScript only knows at this point that we get a FilterItem back, and we can’t instantiate FilterItem. Since abstract classes mix type information and actualy language (something that I try to avoid), a possible solution is to move to interfaces to define the actual type signature, and be able to create proper instances afterwards:

interface IFilter {
  new (property: string): IFilter;
  someFunction(): void;
  filter(): void;
}

declare const filterMap: Map<string, IFilter>;

Note the new keyword. This is a way for TypeScript to define the type signature of a constructor function.

Lots of 💣s start appearing now. No matter where you put the implements IFilter command, no implementation seems to satisfy our contract:

abstract class FilterItem implements IFilter { /* ... */ }
// 💣 Class 'FilterItem' incorrectly implements interface 'IFilter'.
// Type 'FilterItem' provides no match for the signature 
// 'new (property: string): IFilter'.

filterMap.set('number', AFilter)
// 💣Argument of type 'typeof AFilter' is not assignable 
// to parameter of type 'IFilter'. Type 'typeof AFilter' is missing 
// the following properties from type 'IFilter': someFunction, filter

What’s happening here? Seems like neither the implementation, nor the class itself seem to be able to get all the properties and functions we’ve defined in our interface declaration. Why?

JavaScript classes are special: They have not only one type we could easily define, but two types! The type of the static side, and the type of the instance side. It might get clearer if we transpile our class to what it was before ES6: a constructor function and a prototype:

function AFilter(property) { // this is part of the static side
  this.property = property;  // this is part of the instance side
}

// instance
AFilter.prototype.filter = function() {/* ... */} 

// not part of our example, but instance
Afilter.something = function () { /* ... */ }

One type to create the object. One type for the object itself. So let’s split it up and create two type declarations for it:

interface FilterConstructor {
  new (property: string): IFilter;
}

interface IFilter {
  someFunction(): void;
  filter(): void;
}

The first type FilterConstructor is the constructor interface. Here are all static properties, and the constructor function itself. The constructor function returns an instance: IFilter. IFilter contains type information of the instance side. All the functions we declare.

By splitting this up, our subsequent typings also become a lot clearer:

declare const filterMap: Map<string, FilterConstructor>; /* 1 */

filterMap.set('number', AFilter)
filterMap.set('stuff', BFilter)

let obj: IFilter;  /* 2 */
const ctor = filterMap.get('number')
if(typeof ctor !== 'undefined') {
  obj = new ctor('a');
}
  1. We add FilterConstructors to our map. This means we only can add classes that procude the desired objects.
  2. What we want in the end is an instance of IFilter. This is what the constructor function returns when being called with new.

Our code compiles again and we get all the auto completion and tooling we desire. Even better: We are not able to add abstract classes to the map. Because they don’t procude a valid instance:

// 💣 Cannot assign an abstract constructor 
//    type to a non-abstract constructor type.
filterMap.set('notworking', FilterItem)

Traditional OOP, weaved in into our lovely type system. ✅

Here’s a playground with the full code

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