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Typescript Interface vs Class With Practical Examples

Raj Sekhar on August 05, 2021

Typescript, oh I love it. Take stackoverflow survey, or ask any developer, most of them do. All major UI libraries/frameworks are (following Angula...
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peerreynders • • Edited

In this context class is a waste i.e. it isn't carrying its weight. On a personal level I'm not fond of classes as they tend to conflate type space and value space (just like TypeScript does with regular function declarations and expressions)

On introduction I'd go with

// Type definition in the type (TypeScript) space
type Pizza = {
    variant: string;
    size: string;
    price: number;
    extraCheese: boolean;
    takeAway: boolean;
};

// Value creation in value (JavaScript) space
const myPizza: Pizza = {
    variant: "Maxican green wave",
    size: "medium",
    price: 550,
    extraCheese: true,
    takeAway: false
};
console.log(myPizza);
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By going with an object literal TypeScript can verify that myPizza conforms with the Pizza type and as a bonus the object properties act as "named parameters". For example variant and size are both string but there is no confusing them as you have to refer to them by name.

Creating Pizza's all over the place I'd move to:

// Type definition in the type (TypeScript) space
type Pizza = {
  variant: string;
  size: string;
  price: number;
  extraCheese: boolean;
  takeAway: boolean;
};

type MakePizza = (
  variant: string,
  size: string,
  price: number,
  extraCheese: boolean,
  takeAway: boolean
) => Pizza;

// Value creation in value (JavaScript) space
const makePizza: MakePizza = (
  variant = '',
  size = '',
  price = 0,
  extraCheese = false,
  takeAway = false
) => {
  return {
    variant,
    size,
    price,
    extraCheese,
    takeAway,
  };
};

const myPizza = makePizza('Mexican green wave', 'medium', 550, true, false);
console.log(myPizza);
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Now I can mass produce Pizza objects - yet don't have to bother with either new or this (not that they'll slow me down if it came to it).

However there is a trade off by going with a factory function (or a for that matter a constructor) - TypeScript won't be able to detect whether you swapped variant with size or extraCheese with takeAway.

Bottom line: I don't go "class-oriented" until there is a clear benefit - until then I'll stay function-oriented.

As to the issue of not using parameter properties - one could simply write:

class Pizza {
  constructor(
    public variant: string = '',
    public size: string = '',
    public price: number = 0,
    public extraCheese: boolean = false,
    public takeAway: boolean = false
  ) {}
}

const myPizza = new Pizza('Mexican green wave', 'medium', 550, true, false);
console.log(myPizza);
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Is it convenient? Perhaps.
Is it correct? Not in my book.

A type is a specification that a value has to conform to, to be considered a member of that type. It's a "target" that should be spelled out in explicit detail separately and prominently - not be buried inside the code that is responsible for creating the value.

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jwp profile image
John Peters •

For all your examples, does intellisense work on each?

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peerreynders profile image
peerreynders •

Are you familiar with Does Visual Studio Rot the Mind?:

I don’t need to remember anything any more. IntelliSense will remember it for me. Besides, I justify to myself, I may not want those 60,000 methods and properties cluttering up my mind. My overall mental health will undoubtedly be better without them, but at the same time I’m prevented from ever achieving a fluid coding style because the coding is not coming entirely from my head. My coding has become a constant dialog with IntelliSense.

So I don’t think IntelliSense is helping us become better programmers. The real objective is for us to become faster programmers, which also means that it’s cheapening our labor.

So if IntelliSense can't keep up - that's its problem . And no, I don't use Visual Code (The IDE Divide).

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peerreynders •

I personally stick to type aliases. interface comes in handy when it is necessary to augment existing classes with an interface merge:

class MyClass {
  readonly #value: number;

  constructor(value: number) {
    this.#value = value;
  }

  get value(): number {
    return this.#value;
  }
}

interface MyClass {
  square(): number;
}

function square(this: MyClass): number {
  const value = this.value;
  return value * value;
}

MyClass.prototype.square = square;

const container = new MyClass(42);
console.assert(container.square() === 1764, "Square isn't 1764");
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Raj Sekhar •

Type actually looks better than interface.. so for api response and request object structure, I'll be using type. And interface only to implement any class. Thanks for your time and explaining everything.

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yossarian •

You can use public keyword in your constructor to avoid boilerplate code

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Scott Simontis •

Have you considered dependency injection as a scenario? For example, what if I have a service class I depend on? If I inject that into my class constructor, I can easily test it.

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LAlves91 •

Hi! I believe this scenario doesn't work anymore. If I'm not mistaken, since Angular 9 or 10, you can't use Angular features (like DI, Inputs/Outputs and Lifecycle hooks) in non-annotated classes.

So, for models, I believe interfaces are the best way to go, unless you have custom behaviour/properties (kinda like transient properties in Java entities, which are calculated from base properties).

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Scott Simontis •

I apologize, I am not familiar with Angular and meant my comment as a general note for ES6/TypeScript. I appreciate your insight on Angular mechanics!

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LAlves91 • • Edited

Heey, no need to apologize! It's all about sharing knowledge! Cheers!

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Ben Taliadoros •

There is no need to add the properties on a ts class if they match the constructor. Simply add an access modifier (public for example):
"constructor(public xyz:string) {}"
And it will be automatically added as a property of the class and assigned the value.

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Raj Sekhar •

Actually I did not define any target in tsconfig and by default it compiled to ES5 probably.
Let me check this one again. Thanks for pointing out.

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John Peters •

Good point, and my thoughts too.