Example from https://reasonml.github.io/ as Typescript

schoolPerson.js

class Teacher {
}
class Director {
}
class Student {
    constructor(name) {
        this.name = name;
    }
}
function greeting(stranger) {
    if (stranger instanceof Teacher)
        return "Hey professor!";
    if (stranger instanceof Director)
        return "Hello director.";
    if (stranger instanceof Student) {
        if (stranger.name === "Richard")
            return "Still here Ricky?";
        else
            return "Hey, " + stranger.name + ".";
    }
}

schoolPerson.ts

class Teacher {}

class Director {}

class Student {
  constructor (public name: string) {}
}

type SchoolPerson = Teacher | Director | Student

function greeting (stranger: SchoolPerson): string | void {
  if (stranger instanceof Teacher) return "Hey professor!"
  if (stranger instanceof Director) return "Hello director."
  if (stranger instanceof Student) {
    if (stranger.name === "Richard") return "Still here Ricky?"
    else return "Hey, " + stranger.name + "."
  }
}