Typescript — The satisfies Operator

Hi all,
Yesterday I found a new way in Typescript to ensure the type-safety better in the code. Let’s check what is it and how to do it!

We are going to see new satisfies operator, released in TypeScript 4.9.

Let’s check the following code :

We have city, it can be eitherCityName orCityCoordinates

type City = CityName | CityCoordinates;

CityName is string which respresents any cities,

//another type CityName
type CityName = "Colombo" | "Jaffna" | "Galle";

//CityCoordinates represents coordinates of a city
type CityCoordinates = {
  x: number;
  y: number;
};

And the Person type has placeOfBirth and currentPlace, both of which are of type City

//the Person type has placeOfBirth and currentPlace, both of which are of type City:
type Person = {
  placeOfBirth: City;
  currentPlace: City;
};

Now we want to create a new variable person and use Person type.

const person:Person = {
  placeOfBirth: "Colombo",
  currentPlace: { x: 6, y: 3 },
} 

Here come the issue, let’s imagine that you have a new requirement to make the placeOfBirth to capitalise. So what we will do is, use toUpperCase() right? Ok let’s try that in the Typescript Playground.

toUpperCase()

error

What happened above ? placeOfBirth is a type of CityNamewhich can be either string or or object with numbers. Till Typescript 4.8 , Typescript cannot solve this issue.

HERE COMES SATISFIES KEYWORD

let’s use the satisfies keyword ,the syntax as follows:

const person1= {
  placeOfBirth: "Colombo",
  currentPlace: { x: 6, y: 3 },
} satisfies Person

let’s now try to console log the person1,

const person1= {
  placeOfBirth: "Colombo",
  currentPlace: { x: 6.2121, y: 3 },
} satisfies Person

console.log(person1.placeOfBirth.toUpperCase());
//OUTPUT-> COLOMBO
console.log(person1.currentPlace.x.toFixed(2));
//OUTPUT-> 6.21

In conclusion, with satisfies we can use in two different places. Type safety saves us from runtime errors and powerful type inference gives us most specific types for free.

Thank you for reading !