Typescript, using classes without constructor

Question

While working with the "Tour of Heroes" tutorial on the Angular website I found the following syntax (shortly):

class Hero {
  id: number,
  name: string,
}

const aHero: Hero = {
  id: 1,
  name: 'Superman'
}

console.log(aHero instanceof Hero); //false

What would be the point in doing this? when if I check the type of "aHero", it is only a common object and not a "Hero" type. Would it be better just initializing an object with a constructor?:

class Hero {
  constructor(id: number, name: string) {}
}

Good question. The answer is the tutorial promotes a bad practice. Hero should be an interface. — Aluan Haddad
– Aluan Haddad, Commented Dec 27, 2017 at 17:41
Todd Motto has a great article explaining the difference and when/why to use interface vs class — mhodges
– mhodges, Commented Dec 27, 2017 at 17:43
IMO this is a really bad example for newcomers, already reported on github github.com/angular/angular/issues/21186 — Jota.Toledo
– Jota.Toledo, Commented Dec 27, 2017 at 18:24
good spot! that's a bad practice - one should be using interfaces in those cases. — tlt
– tlt, Commented Dec 27, 2017 at 19:01

Community · Accepted Answer · 2019-04-09 11:46:49Z

23

You can use class as a type which is the way you're using it. So, whether Hero is an interface or class it doesn't matter because you're using it as a type.

class Hero { id: number; name: string }

or

interface Hero { id: number; name: string }

The following doesn't concern whether Hero is a class or interface

let hero: Hero = { id: 1, name: 'me' }

Where interface and class differentiate is interface is only for you, it doesn't get transpiled into javascript. A class does and you cannot new an interface.

Constructor or no Constructor, if you new it then it is an instanceof. Try it again with this

let hero = new Hero();

Your instanceof log will be true because you did create an instance of Hero with the key word new.

edited Apr 9, 2019 at 11:46

CommunityBot

11 silver badge

answered Dec 29, 2017 at 2:30

lock42

8452 gold badges7 silver badges16 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Shane Thorndike · Accepted Answer · 2018-11-06 06:31:26Z

-3

I know this question has been answered before but as far as object oriented code goes it was always like this.

I believe you can do this without a constructor:

let hero: Hero = { ID: 1, Name: 'goku'};

but if you want to instantiate with the constructor you can do this in the class

class Hero
{
    //notice optional parameters. You have more flexibility this way.
    constructor(id?: number, name?: string)
    {  
      this.ID = id;
      this.Name = name;
    }

    ID: number;
    Name: string
}

And do this with the implementation

let hero: Hero = new Hero(1,'goku');
let hero2: Hero = { ID: 2, Name: 'superman' }

edited Nov 6, 2018 at 6:31

answered Nov 6, 2018 at 6:21

Shane Thorndike

353 bronze badges

1 Comment

Cleanbeans Over a year ago

The above is incorrect, as the above answers hero = new Hero , creates an instance of the class Hero, where hero2: hero is an object with a type of Hero.

Collectives™ on Stack Overflow

Typescript, using classes without constructor

2 Answers 2

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related