typescript complex example from twitter question

hello-properties-complex.ts

//From: https://www.typescriptlang.org/play?#code/C4TwDgpgBAEhA28D2AeACgJyZDwCWEAzlBAB7AQB2AJsRhAIbVKXwhSHAZ6UDmA2gF0AfFAC8UAN4AoKFDBYc+IgC4ombBFwFC0gL5QAZFABKEAMZIM1dIq3LC-SgFcAtgCMtggDRQXHrWFpaQB6EKhgAAtofgA6eI0lHUEoHgAzLWI8YCgGYgYI5zB4CGk050pzfBYoc3oGCjhEVET7HRJyKlooeuZWdk5uPiFhAApo5rUm5BQ4hLttIhEASilZHohgZwxKKAnkfWC6xkaEZFGZOQVNRcI1fgBySgZXCAffB4ZeN59159e1ABGABMAGZvOsvhA1AAWACsADZ9MtpEA
type Hello<Properties extends readonly string[]> = {
    properties: Properties
  } & Record<Properties[number], number>
  
  // the [...Properties] infers it as a tuple
  function createHello<Properties extends readonly string[]>(hello: Hello<[...Properties]>) {
    return hello
  }
  
  createHello({
    properties: ['name', 'age'],
    name: 123,
    age: 456
  })