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Creating Reusable Base Classes in TypeScript with a Real-Life Example

Hey CSS-Tricksters! Bryan Hughes was kind enough to take a concept from an existing post he published on converting to TypeScript and take it a few couple steps further in this post to elaborate on creating reusable base classes. While this post doesn’t require reading the other one, it’s certainly worth checking it out because it covers the difficult task of rewriting a codebase and writing unit tests to help the process.

Johnny-Five is an IoT and robotics library for Node.js. Johnny-Five makes it easy to interact with hardware by taking a similar approach that jQuery took to the web a decade ago: it normalizes the differences between various hardware platforms. Also like jQuery, Johnny-Five provides higher-level abstractions that make it easier to work with the platform.

Johnny-Five supports a wide array of platforms via IO Plugins, from the Arduino family, to the Tessel, to the Raspberry Pi, and many more. Each IO Plugin implements a standardized interface for interacting with lower-level hardware peripherals. Raspi IO, which I first created five years ago, is the IO plugin that implements support for the Raspberry Pi.

Any time there are multiple implementations that conform to one thing, there is a chance to share code. IO Plugins are no exception, however, we have not shared any code between IO Plugins to date. We recently decided, as a group, that we wanted to change this. The timing was fortuitous since I was planning on rewriting Raspi IO in TypeScript anyways, so I agreed to take on this task.

The goals of a base class

For those who many not be familiar with using class-based inheritance to enable code sharing, let’s do a quick walk-through of what I mean when I say “creating a base class to reuse code."

A base class provides structure and common code that other classes can extend. Let’s take a look at a simplified TypeScript example base class that we will extend later:

abstract class Automobile {
  private _speed: number = 0;
  private _direction: number = 0;
  public drive(speed: number): void {
    this._speed = speed;
  }
  
  public turn(direction: number): void {
    if (direction > 90 || direction < -90) {
      throw new Error(`Invalid direction "${direction}"`);
    }
    this._direction = direction;
  }
  
  public abstract getNumDoors(): number;
}

Here, we have created a base class that we call Automobile. This provides some basic functionality shared between all types of automobiles, whether it’s a car, a pickup, etc. Notice how this class is marked as abstract, and how there is one abstract method called getNumDoors. This method is dependent on the specific type of automobile. By defining it as an abstract class, we are saying that we require another class to extend this class and implement this method. We can do so with this code:

class Sedan extends Automobile {
  public getNumDoors(): number {
    return 4;
  }
}

const myCar = new Sedan();
myCar.getNumDoors(); // prints 4
myCar.drive(35);     // sets _speed to 35
myCar.turn(20);      // sets _direction to 20

We’re extending the Automobile class to create a four-door sedan class. We can now call all methods on both classes, as if it were a single class.

For this project, we’ll create a base class called AbstractIO that any IO plugin author can then extend to implement a platform-specific IO Plugin.

Creating the Abstract IO base class

Abstract IO is the base class I created to implement the IO Plugin spec for use by all IO Plugin authors, and is available today on npm.

Each author has their own unique style, some preferring TypeScript and others preferring vanilla JavaScript. This means that, first and foremost, this base class must conform to the intersection of TypeScript best practices and JavaScript best practices. Finding the intersection of best practices isn’t always obvious though. To illustrate, let’s consider two parts of the IO Plugin spec: the MODES property and the digitalWrite method, both of which are required in IO Plugins.

The MODES property is an object, with each key having a human readable name for a given mode type, such as INPUT, and the value being the numerical constant associated with the mode type, such as 0. These numerical values show up in other places in the API, such as the pinMode method. In TypeScript, we would want to use an enum to represent these values, but JavaScript doesn’t support it. In JavaScript, the best practice is to define a series of constants and put them into an object “container."

How do we resolve this apparent split between best practices? Create one using the other! We start by defining an enum in TypeScript and define each mode with an explicit initializer for each value:

export enum Mode {
 INPUT = 0,
 OUTPUT = 1,
 ANALOG = 2,
 PWM = 3,
 SERVO = 4,
 STEPPER = 5,
 UNKNOWN = 99
}

Each value is carefully chosen to map explicitly to the MODES property as defined by the spec, which is implemented as part of the Abstract IO class with the following code:

public get MODES() {
 return {
   INPUT: Mode.INPUT,
   OUTPUT: Mode.OUTPUT,
   ANALOG: Mode.ANALOG,
   PWM: Mode.PWM,
   SERVO: Mode.SERVO,
   UNKNOWN: Mode.UNKNOWN
 }
}

With this, we have nice enums when we’re in TypeScript-land and can ignore the numerical values entirely. When we’re in pure JavaScript-land, we still have the MODES property we can pass around in typical JavaScript fashion so we don’t have to use numerical values directly.

But what about methods in the base class that derived classes must override? In the TypeScript world, we would use an abstract class, as we did in the example above. Cool! But what about the JavaScript side? Abstract methods are compiled out and don’t even exist in the output JavaScript, so we have nothing to ensure methods are overridden there. Another contradiction!

Unfortunately, I couldn’t come up with a way to respect both languages, so I defaulted to using the JavaScript approach: create a method in the base class that throws an exception:

public digitalWrite(pin: string | number, value: number): void {
 throw new Error(`digitalWrite is not supported by ${this.name}`);
}

It’s not ideal because the TypeScript compiler doesn’t warn us if we don’t override the abstract base method. However, it does work in TypeScript (we still get the exception at runtime), as well as being a best practice for JavaScript.

Lessons learned

While working through the best way to design for both TypeScript and vanilla JavaScript, I determined that the best way to solve discrepancies is:

  1. Use a common best practice shared by each language
  2. If that doesn’t work, try to use the TypeScript best practice “internally" and map it to a separate vanilla JavaScript best practice “externally," like we did for the MODES property.
  3. And if that approach doesn’t work, default to using the vanilla JavaScript best practice, like we did for the digitalWrite method, and ignore the TypeScript best practice.
  4. These steps worked well for creating Abstract IO, but these are simply what I discovered. If you have any better ideas I’d love to hear them in the comments!

    I discovered that abstract classes are not useful in vanilla JavaScript projects since JavaScript doesn’t have such a concept. This means that they are an anti-pattern here, even though abstract classes are a TypeScript best practice. TypeScript also doesn’t provide mechanisms to ensure type-safety when overriding methods, such as the override keyword in C#. The lesson here is that you should pretend that abstract classes don’t exist when targeting vanilla JavaScript users as well.

    I also learned that it’s really important to have a single source of truth for interfaces shared across modules. TypeScript uses duck typing as its core type system pattern, and is very useful inside any given TypeScript project. However, synchronizing types across separate modules using duck typing turned out to be more of a maintenance headache than exporting an interface from one module and importing it into another. I had to keep publishing little changes to modules until I got them aligned on their types, leading to excess version number churn.

    The final lesson I learned isn’t a new one: get feedback throughout the design process from library consumers. Knowing this going in, I set up two primary rounds of feedback. The first round was in person at the Johnny-Five collaborator’s summit where we brainstormed as a group. The second round was a pull request I opened against myself, despite being the only collaborator on Abstract IO with permission to merge pull requests. The pull request proved invaluable as we sifted through the details as a group. The code at the beginning of the PR bared little resemblance to the code when it was merged, which is good!

    Wrapping up

    Producing base classes destined to be consumed by both TypeScript and vanilla JavaScript users is a mostly straightforward process, but there are some gotchas. Since TypeScript is a superset of JavaScript, the two mostly share the same best practices. When differences in best practices do arise, a little creativity is required to find the best compromise.

    I managed to achieve my goals with Abstract IO, and implemented a base class that serves both TypeScript and vanilla JavaScript IO Plugin authors well. Turns out, it’s (mostly) possible to have your cake and eat it too!

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