2022-11-15-typescript-series-5-record-and-the-satisfies-operator.markdown

layout	post
title	TypeScript - Record and the <code>satisfies</code> operator
comments	true
categories	typescript
date	2022-11-15 21:16:00 +0200
excerpt	TypeScript 4.9 introduces the <code>satisfies</code> operator. It can be used together with the <code>Record</code> type to solve a problem I posted about some years ago.

{% include typescript.html %}

In a previous entry I argued that [the use of the Record type is dangerous][previous-post] in some circumstances and suggested using a Dictionary type to solve that. The goal of the Dictionary type is to make sure the type of values from property access returns T | undefined as opposed to just T (which would suggest that we have a T for any K in the dictionary).

The release of TypeScript 4.9 includes a new operator called satisfies. It can be used to solve the situation I presented in that other post:

type Breed = string

interface Dog {
  name: string
}

const dogsByBreed: Record<Breed, Dog[]> = {
  akita: [{ name: 'Walter' }, { name: 'Gracie' }],
  dachshund: [{ name: 'Charlie' }],
}

dogsByBreed.poodle // Type is Dog[], value is undefined.

Instead of introducing a Dictionary type, you can now simply do this:

const dogsByBreed = {
  akita: [{ name: 'Walter' }, { name: 'Gracie' }],
  dachshund: [{ name: 'Charlie' }],
} satisfies Record<Breed, Dog[]>

dogsByBreed.poodle // Type error!

TypeScript gives us a static type-check with the message:

Property 'poodle' does not exist on type '{ akita: { name: string; }[]; dachshund: { name: string; }[]; }'.

Which is absolutely correct and gives us the best of both worlds:

• We made sure that dogsByBreed satisfies our Record<Breed, Dog[]> constraint.
• We lost no type information at all about the dogsByBreed variable, and TypeScript was able to tell us that poodle is not one of its keys.

I'm happy to be able to update the previous post to link to this one. Until next time!

[previous-post]: {% post_url 2019-01-30-typescript-series-1-record-is-usually-not-the-best-choice %}