CODEWITHSUNDEEP
typescriptclassvariablesinheritanceoverriding
Bhavik Singh
Bhavik Singh

Reputation: 145

Re-define type of instance variable in an extended class

I have a question about inheritance and types for instance variables in typescript.

Let's imagine I have a simple class as follows:

class Person {
  instanceVariable: Object;
  age: Number;

  constructor(instanceVariable: Object, age: Number) {
    this.instanceVariable = instanceVariable;
    this.age = age;
  } 
};

let alice = new Person({}, 50);  //This will work
let sarah = new Person({}, "fifty"); //Typescript will complain here..

Typescript does what you'd expect it to here. It will complain when you try to create sarah :). But now let's imagine that you have another class that extends this, and you want to make sure this class still has the instance variable but overrides the type.

//Create a simple class that extends and overrides the type
class Student extends Person {
  instanceVariable: String 
}

let bvk = new Student("test", 50);  //This should work
let katie = new Student(50, 50);  //This shouldn't work

This unfortunately doesn't really work as expected though. Typescript complains: "Property 'instanceVariable' has no initializer and is not definitely assigned in the constructor."

If you try to add the constructor, I'm confused about how to make it work, because I want to basically call "super" to set the data. This doesn't work either though, typescript has the same complaint!

class Student extends Person {
  instanceVariable: String;
  constructor(instanceVariable: String, age: Number){
    super(instanceVariable, age)
  }
}

Anyway, I'm probably thinking about this all wrong, but am really curious how to best think about it.

Upvotes: 1

Views: 1162

Answers (1)

gurisko
gurisko

Reputation: 1182

You could use a generic type for your Person class:

class Person<T = object> {
  instanceVariable: T;

  constructor(instanceVariable: T, age: number) {
    // ...
  } 
}

Your Student then can extend Person as follows:

class Student extends Person<string> {}

PS: Have a look at the very first point at Do's and Don'ts

Upvotes: 2

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