All Articles

Const type parameters bring 'as const' to functions

Matt Pocock
Matt PocockMatt is a well-regarded TypeScript expert known for his ability to demystify complex TypeScript concepts.

I got a message from my friend Tom on Twitter the other day:

Hey @mattpocockuk this is really bugging me!

I'm sure there was a feature that let me do


and it'd be equivalent to

fn('hello' as const)

Can't find any docs about it tho, have I made this feature up?

Turns out he hadn't made it up.

TypeScript 5.0 introduced a brand-new piece of syntax to the language: const type parameters.

const myFunc = <const T>(input: T) => {
  return input;

To understand why it's useful, let's first take a look at a function that doesn't use a const type parameter:

const myFunc = <T>(input: T) => {
  return input

Let's say you call myFunc using an object:

const result = myFunc({foo: 'bar'})

The type of result will be { foo: string }. This is exactly the same as if you'd declared your object as a variable:

const myObj = {foo: 'bar'} // { foo: string }

If you hover over myObj in VS Code, you'll see that the type is the same as above - { foo: string }.

But what if we don't want to infer a string, but instead the literal bar?

On the variable, we can use as const to do this:

const myObj = {foo: 'bar'} as const // { readonly foo: "bar" }

But how do we handle it on the function? Enter the const type parameter:

const myFunc = <const T>(input: T) => {
  return input;

const result = myFunc({ foo: "bar" }); // { readonly foo: "bar" }

This is a really useful way of preserving the literal types of objects passed to your functions.

How are you planning on using const type parameters? Let me know on my Discord Server.

Matt's signature

Share this article with your friends

`any` Considered Harmful, Except For These Cases

Discover when it's appropriate to use TypeScript's any type despite its risks. Learn about legitimate cases where any is necessary.

Matt Pocock
Matt Pocock

No, TypeScript Types Don't Exist At Runtime

Learn why TypeScript's types don't exist at runtime. Discover how TypeScript compiles down to JavaScript and how it differs from other strongly-typed languages.

Matt Pocock
Matt Pocock

Deriving vs Decoupling: When NOT To Be A TypeScript Wizard

In this book teaser, we discuss deriving vs decoupling your types: when building relationships between your types or segregating them makes sense.

Matt Pocock
Matt Pocock

NoInfer: TypeScript 5.4's New Utility Type

Learn how TypeScript's new utility type, NoInfer, can improve inference behavior by controlling where types are inferred in generic functions.

Matt Pocock
Matt Pocock