Optional properties depending on other property

The most straightforward way to represent this is with a type alias instead of an interface:

type Sample = {
  key1: true,
  key2?: string,
  key3: number
} | {
  key1: false,
  key2?: string,
  key3?: never
}

In this case the type alias is the union of two types you’re describing. So a Sample should be either the first constituent (where key1 is true and key3 is required) or the second constituent (where key1 is false and key3 is absent).

Type aliases are similar to interfaces but they are not completely interchangeable. If using a type alias leads to some kind of error, please add more detail about your use case in the question.

Hope that helps. Good luck!