Required key in interface with dependency on flexible generic

Is it possible in Typescript to enforce a required key in an interface only when a generic is used?

I am exploring ways to define type restrictions for keys in interfaces specifically when utilizing generics.

For example:

interface IExample {
  foo: string
}
​
/* Note that you cannot have two interfaces with the same name, this is just to illustrate the desired structure */
interface IExample<T> {
  foo: string,
  bar: T
}
​
/* Allowed */
const withoutBar: IExample {
  foo: 'some string'
}
​
/* Not allowed because a generic for Bar has been provided */
const withoutBar: IExample<number> {
  foo: 'some string'
}
​
/* Allowed */
const withBar: IExample<number> {
  foo: 'some string',
  bar: 1
};
​
/* Not allowed since no generic for Bar was passed in */
const withBar: IExample {
  foo: 'some string',
  bar: 1 // Error should occur for "bar" as no generic was used
};

Answer №1

You have the option to create a type alias with a conditional type.

type IExample<T = void> = T extends void ?  {
  foo: string
} : {
  foo: string,
  bar: T
}
​​
/* Allowed */
const withoutBar: IExample = {
  foo: 'some string'
}
​
/* Not allowed, as a generic for Bar has been passed in */
const withoutBar: IExample<number> = {
  foo: 'some string'
}
​
/* Allowed */
const withBar: IExample<number> = {
  foo: 'some string',
  bar: 1
};
​
/* Not allowed since a generic for Bar is missing */
const withBar: IExample = {
  foo: 'some string',
  bar: 1 // Error should occur on "bar" due to missing generic
};

Try it out here

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