TypeScript: function that extends an object with a dynamic key

index.ts

// TS Playground link: https://tsplay.dev/wRJl5w

interface Props {
  baseKey: string
}

const withCustomKey = <K extends string>(customKey: K) => {
  return <P>(props: P): P & { [k in K]: boolean } => {
    return { ...props, [customKey]: true } as P & { [k in K]: boolean }
  }
}

const myProps: Props = { 'baseKey': 'some value' }
const myPropsWithCustomProperty = withCustomKey('customInjectedKey')(myProps)

console.log(myPropsWithCustomProperty.baseKey)
console.log(myPropsWithCustomProperty.customInjectedKey)