as const: the most underrated TypeScript feature

'as const' is an amazingly useful TypeScript feature that most people still don't know about.
00:00 SO underrated
00:12 Objects are mutable!
01:54 How as const works
02:38 Object.freeze
03:19 Deriving types from as const
04:30 keyof typeof
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[@TechnologicNick]

You can also use `as const` on individual value literals or use it on template string literals to get all possible strings.

[@ElektrykFlaaj]

or on array to get a tuple. For instance typeof (["a", 1] as const) is readonly ["a", 1], while typeof ["a", 1] is (string | number)[]

[@cool_scatter]

The template literal trick is mind blowing!

[@weirdwordcombo]

Metadata based programming can be very very powerful. It essentially allows you to expand an application by just adding metadata to an object, while also writing less code overall. I have found that old codebasis often have similar and/or duplicate code all over the place, which can be refactored with this metadata based driven approach. Cuts down on TONS of boilerplate.

[@leotravel85]

You can also use 'satisfies' with 'as const' to constraint the object typings, like 'as const satisfies Record', its super neat

[@Ma1ne2]

Damn I needed this so many times and didn't know about it, thanks for sharing.

[@willshadow]

holy crap I needed this, tysm

[@goncaloflorencio6529]

Thanks a lot for this longer explanation! I sometimes have a harder time catching up on the really short videos but here I was able to get everything immediately.

[@nadavrot]

Excellent video! You often make videos about bleeding edge TS features (that I can’t use yet) or incredibly complex topics (that make me scratch my head) but this video was super accessible and easy to follow! Here’s hoping you will make more mid-level TS concepts videos.

[@josephsliman3395]

I love your typescript content, thanks

[@DulithMutucumarana]

Awesome! Thanks! works on arrays as well: const abc = ['a', 'b', 'c'] as const; type ABC = typeof abc[number];

[@uicornerwithJ]

Thank you sir for this great explanation. I am glad that there are so passionate typescript writers out there like yourself. Just watching this video made me understand the keyof typeof stuff and as const much easier.

[@waterbreak]

After watching half of the video, I found an immediate use case in my TS project I'm working on. Great video!

[@apina2]

"const as const" sounds really smart and not ridiculous at all

[@zuma206]

It makes perfect sense within the typescript syntax, and it's relationship with javascript syntax

[@hatchibomb]

Amazing! I needed this earlier today!

[@AlessioMichelini]

That's absolutely brilliant! I wasn't super sure what "as const" was doing, thanks for clarifying that!

[@Gustavewpm]

Amazing video. Such really good pedagogy, loved it, even if I was already very familiar with as const and the other notions sumed up.

[@naraksama3903]

I love as const so much. It gives so much information of the code base you're trying to work with. It's makes localization strings global constants so much more useable.

[@scylk]

Hey can you expand a little bit on that? Sounds interesting

[@naraksama3903]

@@scylk It's very simple. You create an object that basically stores all of your strings and add an handler to dynamically change it based on the users OS language. When you mark those strings as const, TypeScript still sees the values of your default language and it's easier to know what string represents what text.

[@mattinpjs]

This is such a helpful feature. I'll be using this all the time now, thank you

[@adi96adi]

Needed this a few months ago 😂 excellent content on this channel dude keep it up

[@JDalmasca]

Just came to say this video finally helped make `as const` click for me. Thanks for the info, Matt!

[@DC-xt1ry]

Very nice ticks! keep them comming sensei!

[@nixoncode]

i was in a live stream a few days ago on this,learned so much

[@brad8961]

Videos are very useful and clear, thanks

The best and simplest explanation of "as const"!

[@Nabulio85]

Really great. You are a great teacher 😊 Thx from France.

[@ColinRichardson]

I use this quite often, I don't think it's underrated, I think other things may just be rated too highly... LOL

[@ColinRichardson]

PS: Where is the Top hat and Monocle?

[@SimGunther]

​@@ColinRichardsonPeople do wear two monocles to make a _spectacle_ for themselves, so now he's just missing the top hat, old chap 🎩

[@grandpowr]

Thank you very much Mr Matt!

[@giodefreitas]

This channel is also underrated. awesome video! you just got a new subscriber here

[@TheGrandChieftain]

Was using an enum for a radio group in Zod and discovered this. Never went through the trouble of understanding it, though. Thanks 👍🏿

[@aamiramin6112]

Amazing so concise and clean.

[@melvins126]

Made me subscribe. Thank you very much. 👍

[@AudaiAlFandi]

Finally... now I understand it. Thank you Matt.

[@rizkyagungprasetyo8293]

Sometimes I utilize this tool to manage database names, collections, roles, etc., enabling me to infer values instead of strings :).

[@williamlvea]

The biggest issue I've had with 'as const' is that it doesn't play well with libraries. Because of the readonly you'll get into cases where libraries are expected something like string[] and will require casting to mask the readonly. It also doesn't actually freeze the values, which can cause hard to debug bugs

[@yoni532s9M5w]

That's worthy of a medium article mate. Very nice solution to a very popular recurring problem

[@DoorThief]

As someone who doesn't know TS and just knows C#, I recoiled slightly when I saw the keyof typeof [] bit But then I remembered reflection is pretty much like that anyways in C#, accessing public (or private) members and the like.

[@somewonderfulguy]

Watched small video - huge power acquired. Thank you!

[@Rivernile345]

Wow great video especially as a new typescript developer, I subscribed

[@AbWischBar]

That was a whole lot of fantastic stuff that will immediately be used

[@leesasuki]

thank you, you are a life saver!

[@ginger-viking]

I did not know as const prevented the object being changed - thanks!

[@stephanfitzpatrick1769]

Brilliant. Instant follow

[@PraiseYeezus]

You guys are lucky I'm dumb, I asked the question that inspired this video :) Thanks for all you do Matt, I learned a whole lot.

[@irfanfauzi8704]

It's ridiculous that I can't expect const as constant value. Love your vids

[@wiktorchojnacki9746]

You can, you just need to know what is constant about your value.

[@christopheradolphe]

Awesome tip here. Thank you @mattpocockuk for sharing this.

[@Leon-xg7zj]

Awesome sharing!

[@Hadi-gd7ul]

This is so beautiful that i want to cry.

[@griffadev]

Nice video! Heads up on this format on mobile because there's no KZbin outro, the "suggested video" card appeared whilst you were still coding and blocked the code in video

[@robertmengual8357]

Great video, typescript is beautiful

[@Dmitri_Ivanovich]

"Object.values on a type level" trick is pretty neat. Thanks!

[@PapitaPure]

You just saved me wtf. I was looking for a solution like this yesterday

[@skandertebourbi357]

Simply a typescript magician

[@VolodymyrEisenhorn]

Just wow. Thank you.

[@MightyKingKala]

this video is amazing!

[@zb2747]

I love your videos. Very insightful, fun, and straight to the point

[@alexeypetrov8543]

That's brilliant!

[@maxwebstudio]

Niiiiice ! I always forget how to target the type of keys and values of objects 😅

[@RockTheCage55]

Excellent....pretty cool. Thanks....

[@thekaasim]

helpful video. thanks.

[@Mujahedeen_]

YOU are the most underrated KZbinr

[@user-hk6ns2eg1w]

Nice!! thanks

[@rockNbrain]

Nice job 🎉

[@wattsfield1889]

Nice. Theo made a similar video of why Enums are bad. He did something similar like this object as const, but his was a string array as const. And the type Route = typeof routeArray[number]. So the difference is whether or not you want a key name associated with the routes

[@olegsbk3562]

What do you prefer “as const” or enum in cases like in video?

[@dawid_dahl]

Awesome explanation, thank you! Do you know if TS typesystem is Turing complete? Like the programming you can do at the type level like that? I bet it is.

[@mattpocockuk]

It is!

[@aram5642]

Would you consider making a video/talk about the 'declare' keyword. While lurking into various .d.ts files I can spot all possible combinations of 'declare class', 'declare abstract class' (also: 'abstract class' alone). I have only used declare in typescript playground to mimic the existence of a function to make my types go trhough, but I can't get my head wrapped around real-life usage and how it is different from interfaces. Thanks!

[@osamanajjar1934]

Matt the goat!

[@lalumierehuguenote]

Oh cool! I knew it for array to union, but not for object to union. Thanks!

[@moneyfr]

You didn't talk about: "as const satisfies Record "

[@joaofrancisco5781]

I started using a lot of "as const satisfies typeX", I'm creating a lib and it's helping me a lot

[@lambgoat2421]

Finally something from youtube I will actually use

[@StephenRayner]

Beautiful

[@mbenlahrech]

Now using rust and coming from c this is really funny to me how JS devs (i am a 4 years js/ts dev) go nuts with typing systems

[@sc76399]

As const is really cool. Didn't know we could do that. Is there not an easier way to get values of an object as a union? Like we have keys of us there not values of

[@segen8324]

this is beautiful

[@Alec.Vision]

The thing about 'as const', when not used on type parameters, is that it lies. The runtime object is, in fact, not readonly. You can get the same effect, with added runtime correctness, from Object.freeze. I consider this an extension of the 'One Source of Truth' ethos. 'as const' is, essentially, a type assertion. But I'm %100 guilty of using it everywhere so 🤷

[@maxwebstudio]

Question : But when compiled, the 'as const' Object won't be frozen to JS i guess. So, if it's for heap memory optimization it is better to freeze the object a long side 'as const', isn't it ?

[@sergey_llal6897]

Can we use enum for store routes and avoid create additional types?

[@omkarbhale442]

Why didnt you use enums for the routes? It looks like you only need one of the three routes at any moment. This particular example doesn't show use of as const i think. (I'm new to typescript so maybe I'm wrong, please correct me)

[@programming5274]

Right, why prefer the code in the video in place of a string enum? String enums are made for this, and they generate faster and generally smaller JS output. const enum Routes { Home = "/", Admin = "/admin", Users = "/users", }

[@programming5274]

/* String enum */ const enum Routes { Home = "/", Admin = "/admin", Users = "/users", } const goToRoute = (route: Routes) => { console.log(route); } goToRoute(Routes.Home); console.log("----------------------"); /* as const */ const rts = { home: "/", admin: "/admin", user: "/users", } as const; type Rte = (typeof rts)[keyof typeof rts]; const goToRt = (route: Rte) => { console.log(route); } goToRt(rts.home);

[@omkarbhale442]

@@programming5274 Yes!

[@punsmith]

Because Matt has a hate boner for them and prefers magic variables. Sharing a codebase with him can be only the first step of hell.

[@oscargm1979]

@@programming5274 I totally agree yet I supose it is just to show that we have that option too. Imho u end with objects from other devs everytime so maybe u can just put 'as const' on them instead of refactoring to enums other people's job, idk

[@rhodler]

Merci !

[@shane3744]

Nice quick dive into using "as const" with objects! I know it's a useful TypeScript feature when I find myself using it intuitively as I'm coding.

[@alexking7684]

Fenomenal! 🎉

[@micelumail5829]

4:05 me knowing new ts magic

[@anasalhariri5474]

Good to know thanks for sharing :) Like and new Sub

[@Orenrs]

Great video and example. Why not using enum for this case ?

[@mattpocockuk]

Check my video on enums

[@3453wer]

@@mattpocockuk checked your vid about enums, so again - why dont just use enum for this case? cause enum with specified values transpiles to a simple obj as you had in your video

[@barneylaurance1865]

So if you want as const but you don't want the whole tamale const you can define the internal object first as standard mutable object, and then put it inside in the outer const object. I.e. if you have const deep = { whatever: "/deep/whatever", }; const routes = { admin: "/admin", home: "/", deep: deep, } as const; then routes will be inferred as of type: { readonly admin: "/admin"; readonly home: "/"; readonly deep: { whatever: string; }; }

[@AlfredLotsu]

I now understand Rich Harris. We are packing up and moving to Jsdoc

[@edgeeffect]

I first got interested in Typescript just because I thought I'd get type-checking... and weak typing was the 1990's biggest mistake. All of this auto-completion stuff in VSCode came along later as just a delightful extra little gift.

[@ThanHtutZaw3]

How can I type safe for api end point url for fetch ? Some of them will need query or other will need only body .

[@ra2enjoyer708]

Another trick is `as const` objects somewhat work with built-in methods while almost not losing all type info: ```typescript const ROUTES = { home: "/", admin: "/admin", users: "/users" } as const; // sadly not a tuple of literals, merely an array of literal type union const routeValues = Object.values(ROUTES).map((value) => value); export type IRoute = (typeof routeValues)[number]; export function validateRouteValue(inputValue: unknown): inputValue is IRoute { // have to coerse type there because typescript is iffy with `Array.includes()` // when it's array of string literals return routeValues.includes(inputValue as IRoute); } export function doSomethingFromValue(inputValue: IRoute): void { switch (inputValue) { case "/": case "/admin": case "/users": { break; } default: { throw new Error(`Invalid value "${inputValue satisfies never}"`) } } } ``` Achieves the same result but you also get a validation function which acts as a type predicate. The `doSomethingFromValue()` also shows how literal type unions can be paired with `satisfies` keyword to achieve exhaustive switch cases.

[@mattpocockuk]

You can use ts-reset to fix that Array.includes issue.

[@mananabanana]

"as const or how to implement a real enum in typescript" 😂

[@MrTrevorrrrrrrr]

I may have agreed, but even better is the new const in generics feature.

[@kelvindecosta5350]

Hey Matt, I've come across a pattern that I think deserves a shorthand: type X = { a: string } | { b : number } // Non discriminated union const x : X = { a: "hello"} const a = !!x && 'a' in x ? x.a : undefined // works fine, but is somewhat verbose. const a = x?.a // errors out because 'a' doesn't exist on type { b: number} Is there a simpler way of picking x.a?

[@forivall]

Search for "ExclusifyUnion", there's a good stack overflow answer with an explanation

[@user-qk2nl8lf9y]

what is the VSCode extension you're using to get the human-readable ts error messages?

[@mattpocockuk]

www.totaltypescript.com/vscode-extension

[@user-qk2nl8lf9y]

@@mattpocockuk Thanks so much!

[@josemfcheo]

this is awesome

[@prashantjoshi6703]

But what benefit it adds in comparison to string enums?🥺

[@scylk]

Wondering the same lol

[@am29d]

Mind blowing...

[@danielratiu4318]

How about routes with matching variables like '/users/:id' that match /users/3 ?

[@omarkraidie]

const obj = { username: 'abc', password: 'dce', email: 'fgh', } as const; let a: (typeof obj)[keyof typeof obj]; // Value of an object let b: Exclude; // Exclude a value from an object let c: keyof typeof obj; // Keys of an object let d: Exclude; // Exclude a key from an object

[@zuperxtreme]

Would an enum work the same?

[@punsmith]

It would, but Matt has an irrational hatred for them, as seen in one of his videos "Why you shouldn't use Enums". Watch him abuse the concept of Enums and instead using magic variables. Biggest tool I've seen in TS.

[@furkan7013]

Anybody knows that translation plugin for error messages ?

[@vrtech473]

never knew this 😂😂😂😂 GOAT🎉🎉

[@Cahnisama]

Great... now I have to clean my walls.... you HAD to blow my mind didn't you Matt? sigh... Feedback: at 5:20, you video recommendation thumbnail is covering the code you are still explaining, maybe make an outro?

[@lorenzorev]

Normally I would an enum in this case like "enum routes" and avoid declaring the type. What do you think?