Typescript override generic property

Question

I have the following code:

class Base<T> {}

class Test {
  prop: Base<any>;

  createProp<T>() {
    this.prop = new Base<T>();
  }
}

const test = new Test();

test.createProp<{ a: number }>();

test.prop // Base<any> expected Base<{ a: number }>

When I call createProp with the generic, I want it to be the new type of the prop property. Currently, after running this method, the type is still - Base<any>. There is a way to do it with TS?

MTCoster · Accepted Answer · 2019-03-25 20:23:59Z

1

prop is defined as Base<any> - it’ll never be a Base<T>, even if one is assigned to it.

In order to achieve this behaviour, you’ll have to make your class generic instead of just the method:

class Base<T> {}

class Test<T> {
  prop: Base<T>;

  createProp() {
    this.prop = new Base<T>();
  }
}

const test = new Test<{ a: number }>();

test.createProp();

test.prop

Alternatively, if you’re really against using a generic class, you could use a type assertion:

class Base<T> {}

class Test {
  prop: Base<any>;

  createProp<T>() {
    this.prop = new Base<T>();
  }
}

const test = new Test();

test.createProp<{ a: number }>();

test.prop as Base<{ a: number }>
// --- or ---
<Base<{ a: number }>> test.prop

edited Mar 25, 2019 at 20:23

answered Mar 25, 2019 at 19:40

MTCoster

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2 Comments

undefined Over a year ago

Thanks, I know that I can do it like that, but I thought there be might a way to do it as I want.

MTCoster Over a year ago

Ah fair enough. You could cast the returned value, since you know its type from the generic on createProp. It’s a little messy but it would save you from creating a generic class if you’re trying to avoid it

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