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JavaScript

Posted by Phillip Kazanjian
Phillip Kazanjian
6,273 Points

Why are instance variables not required in JS?

I'm used to Typescript, where instance variables must be defined in a class. In Typescript, the code below would throw an exception saying Property 'name' does not exist on type 'Pet'.

class Pet {
  constructor(name) {
    this.name = name;
  }
}

But this code works fine in Javascript. I can even retrieve this.name with a function. Is an instance variable implicitly created for every variable I declare inside a constructor?

Thank you!

Brandon Leichty
.a{fill-rule:evenodd;}techdegree seal-36
Brandon Leichty
Full Stack JavaScript Techdegree Graduate 35,193 Points

Just for clarification,

are you wondering why you don't have to declare the type of the variable like you do in TypeScript? Like:

class Pet {
  name: string;
  constructor(name) {
    this.name = name;
  }
}

As from what I understand, you define Classes in Javascript in almost the exact same way you would in TypeScript..

Phillip Kazanjian
Phillip Kazanjian
6,273 Points

Hi, Brandon:

No, my question wasn't about types.

Typescript requires an explicit declaration of instance variables like this...

class Pet {
 name; <--- this guy
  constructor(name) {
    this.name = name;
  }
}

Javascript doesn't require it.