3

Let's say I have an interface like the below:

interface IProps<T> {
   property: T
}

How can extend the above interface, something like below,

interface IPropChild<U> extends IProps {
       propertyChild: U
   }

I know that the above syntax is wrong. Can you provide the right way to accomplish it. Thanks in advance.

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1
  • Can you explain exactly what you want to accomplish? Is U completely independent of T? So then does interface PropChild<T, U> extends IProps<T> {propertyChild: U} meet your needs? If not, then please elaborate on what's missing. Commented Jan 19, 2022 at 15:56

1 Answer 1

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4

You can accept two generic type parameters and forward one to the interface that you extend:

interface Props<T> {
  property: T;
}

declare const props1: Props<string>;
props1.property; // string

// Forward the generic parameter T to the generic interface that it extends:
interface Extended<T, U> extends Props<T> {
  another: U;
}

declare const props2: Extended<string, number>;
props2.property; // string
props2.another; // number

Or, make the base interface generic type fixed when using the extending interface:

// Only use one generic parameter and decide in advance that
// this interface will always use `string` for the base interface:
interface ExtendedAndPropertyIsString<U> extends Props<string> {
  another: U;
}

declare const props3: ExtendedAndPropertyIsString<number>;
props2.property; // string
props2.another; // number

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