My server will return the result as:

{
    code: number;
    info: string;
    data //only exists when code is 200
}

How do I define an type to say the data property only exists when code is 200?

So I write my code as

const res = await fetch(...)
const result:MyType = await res.json()
if (result.code != 200) {
   //process no data case 
} else {
  //process result.data
}