Typescript discriminator

Last updated: 2020-04-10 • 2 min read

Very often we have use-cases where we would like to use a single type Union Type but still be sure about the properties inside them.

interface Employee {
  name: string;
  salary: number;
}

interface HumanResource {
  name: string;
  power: boolean;
}

let person: Employee | HumanResource;

But under the hood Typescript union’s are more of “common” properties inside them instead of “having all the properties”.

So when you try and use it like:

let helloPerson = (person: Employee | HumanResource): void => {
   console.log(person.<only has properties common to Employee & HumanResource aka name>);
}

where as you would expect person to have {name, salary} or {name, power} and choose between them depending on what person is.

This is where discriminators come in power-play. Bear with my variable nomenclature:

Lets define an enum to contain both the use-cases of Person’s we have, calling it PersonType and then define a type combining HumanResource and Employee.

enum PersonType { "EMPLOYEE" , "HR" }

interface Employee {
  name: string;
  type: PersonType.EMPLOYEE
	salary: number;
}

interface HumanResource {
  name: string;
  type: PersonType.HR
  power: boolean;
}

type Person = HumanResource | Employee;

let person: Employee | HumanResource;

let helloPerson = (person: Person): void => {
  switch(person.type) {
    case PersonType.EMPLOYEE:
      // this gives you both name/salary since now TS understands
      // and discriminates between the two types you have
      console.log(person.{gives you name, salary});
  }
}