Typescript - object 或

Question

I'd like to have a type that says:

If there's property container present, also expect property a . If there's item property present, also expect property b . container and item cannot exist both at the same time.

The code I'd assume would look as follows, but it doesn't seem to do the above.

type A = { container: true; a: string } | { item: true; b: number };

How do I build such a type?

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null | string null | string seems to mean null OR string , but SomeObject | AnotherObject SomeObject | AnotherObject seems to mean all properties present BOTH in SomeObject and AnotherObject .

Answer 1

This may seem hacky, but basically creates an XOR. So if you were to add either item or b to the variable myVar , you would get a typescript error.

type ExampleType =
  | { container: true; a: string; item?: never; b?: never }
  | { item: true; b: number; container?: never; a?: never }

// Adding item or b to the below type, will cause a ts error
const myVar: ExampleType = { container: true, a: 'z' }

So say if the container key is provided, it will expect the key value for a to be present, and will error if item or b were passed.