[TypeScript] Use the JavaScript “in” operator for automatic type inference in TypeScript

Sometimes we might want to make a function more generic by having it accept a union of different types as an argument. Using the JavaScript “in” operator, we can test for the presence of different properties on the argument object, and TypeScript will automatically infer the exact type of our object in that block. It does this by looking at all the possible types in the union and then keeping only the ones that have that specific property defined.

interface Admin {
  id: string;
  role: string:
}
interface User {
  email: string;
}

function redirect(usr: Admin | User) {
  if(/*user is admin*/) {
    routeToAdminPage(usr.role);
  } else {
    routeToHomePage(usr.email);
  }
}

So in the code above, what we can write into the if block to ensure that, it is admin type, so that IDE won't complain that, 'role' or 'email' may not be defined on user object?

Solution we can use is 'in' operator in Javascript:

function redirect(usr: Admin | User) {
  if("role" in usr) {
    routeToAdminPage(usr.role);
  } else {
    routeToHomePage(usr.email);
  }
}

'in' operator check whether one prop is existing on the object but also helps Typescript to narrow down the type, in this case, helps to choose from 'Admin' or 'User'.