How do I accept either `href` or `onClick`, but reject objects that provide both?

The recipe

type Without<T, U> = {
  [K in Exclude<keyof T, keyof U>]?: never
}

type XOR<T, U> =
  | (T & Without<U, T>)
  | (U & Without<T, U>)

type LinkProps = XOR<
  { href: string },
  { onClick: () => void }
>

The build compiles this and checks each result below.

How it works

1. 01

   [K in Exclude<keyof T, keyof U>]?: never

   Unique keys from the opposite branch may be absent, but can never hold a value.

2. 02

   | (U & Without<T, U>)

   Each union member combines its required shape with a ban on the other one.

What you get

• { href: string } extends LinkProps ? true : false

  →true

• { onClick: () => void } extends LinkProps ? true : false

  →true

• { href: string; onClick: () => void } extends LinkProps ? true : false

  →false

Where it goes wrong

This is clearest for two small object shapes. Large overlapping types produce noisy diagnostics, and `exactOptionalPropertyTypes` changes whether an explicit `undefined` can satisfy optional `never`.