as const: the most underrated TypeScript feature

Рет қаралды 111,554

Күн бұрын

'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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Пікірлер: 309

@TechnologicNick 10 ай бұрын

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

@ElektrykFlaaj 10 ай бұрын

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 8 ай бұрын

The template literal trick is mind blowing!

@weirdwordcombo 10 ай бұрын

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.

@apina2 10 ай бұрын

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

@zuma206 Ай бұрын

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

@leotravel85 10 ай бұрын

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

@Ma1ne2 10 ай бұрын

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

@willshadow 10 ай бұрын

holy crap I needed this, tysm

@goncaloflorencio6529 10 ай бұрын

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 10 ай бұрын

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.

@DulithMutucumarana 10 ай бұрын

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

@waterbreak 10 ай бұрын

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

@josephsliman3395 10 ай бұрын

I love your typescript content, thanks

@uicornerwithJ 9 ай бұрын

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.

@AlessioMichelini 10 ай бұрын

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

@Gustavewpm 2 ай бұрын

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

@JDalmasca 10 ай бұрын

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

@naraksama3903 10 ай бұрын

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 10 ай бұрын

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

@naraksama3903 10 ай бұрын

@@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 10 ай бұрын

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

@hatchibomb 10 ай бұрын

Amazing! I needed this earlier today!

10 ай бұрын

The best and simplest explanation of "as const"!

@ColinRichardson 10 ай бұрын

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

@ColinRichardson 10 ай бұрын

PS: Where is the Top hat and Monocle?

@SimGunther 10 ай бұрын

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

@nixoncode 10 ай бұрын

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

@adi96adi 5 ай бұрын

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

@DC-xt1ry 10 ай бұрын

Very nice ticks! keep them comming sensei!

@brad8961 10 ай бұрын

Videos are very useful and clear, thanks

@TheGrandChieftain 10 ай бұрын

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

@irfanfauzi8704 2 ай бұрын

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

@wiktorchojnacki9746 2 ай бұрын

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

@Nabulio85 10 ай бұрын

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

@grandpowr 10 ай бұрын

Thank you very much Mr Matt!

@yoni532s9M5w 10 ай бұрын

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

@AudaiAlFandi 10 ай бұрын

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

@giodefreitas 9 ай бұрын

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

@christopheradolphe 3 ай бұрын

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

@rizkyagungprasetyo8293 10 ай бұрын

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

@Leon-xg7zj 9 ай бұрын

Awesome sharing!

@aamiramin6112 10 ай бұрын

Amazing so concise and clean.

@melvins126 10 ай бұрын

Made me subscribe. Thank you very much. 👍

@leesasuki 4 ай бұрын

thank you, you are a life saver!

@somewonderfulguy 9 ай бұрын

Watched small video - huge power acquired. Thank you!

@stephanfitzpatrick1769 5 ай бұрын

Brilliant. Instant follow

@AbWischBar 10 ай бұрын

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

@ginger-viking 10 ай бұрын

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

@Rivernile345 10 ай бұрын

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

@williamlvea 10 ай бұрын

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

@Hadi-gd7ul 10 ай бұрын

This is so beautiful that i want to cry.

@PraiseYeezus 10 ай бұрын

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.

@skandertebourbi357 10 ай бұрын

Simply a typescript magician

@Dmitri_Ivanovich 10 ай бұрын

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

@robertmengual8357 10 ай бұрын

Great video, typescript is beautiful

@alexeypetrov8543 10 ай бұрын

That's brilliant!

@osamanajjar1934 10 ай бұрын

Matt the goat!

@VolodymyrEisenhorn 10 ай бұрын

Just wow. Thank you.

@PapitaPure Ай бұрын

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

@griffadev 10 ай бұрын

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

@Mujahedeen_ 10 ай бұрын

YOU are the most underrated KZfaqr

@zb2747 10 ай бұрын

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

@DoorThief 9 күн бұрын

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.

@anonymoususer520 2 күн бұрын

The difference is that reflection happens at runtime. This "as const" typing happens statically at compile time, so it's typesafe -- unlike reflection. On the other hand, typescript types are not reified, whereas .net types are. So reflection over typescript types isn't possible, because the type information is erased during compilation.

@MightyKingKala 10 ай бұрын

this video is amazing!

@RockTheCage55 10 ай бұрын

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

@olegsbk3562 10 ай бұрын

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

@maxwebstudio 10 ай бұрын

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

@lambgoat2421 5 ай бұрын

Finally something from youtube I will actually use

@user-hk6ns2eg1w 6 ай бұрын

Nice!! thanks

@thekaasim 10 ай бұрын

helpful video. thanks.

@rockNbrain 10 ай бұрын

Nice job 🎉

@moneyfr 10 ай бұрын

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

@joaofrancisco5781 10 ай бұрын

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

@wattsfield1889 10 ай бұрын

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

@micelumail5829 10 ай бұрын

4:05 me knowing new ts magic

@StephenRayner 10 ай бұрын

Beautiful

@segen8324 10 ай бұрын

this is beautiful

@aram5642 9 ай бұрын

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!

@alexking7684 10 ай бұрын

Fenomenal! 🎉

@lalumierehuguenote 10 ай бұрын

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

@sc76399 10 ай бұрын

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

@sergey_llal6897 9 ай бұрын

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

@Alec.Vision 10 ай бұрын

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 🤷

@mbenlahrech 6 ай бұрын

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

@mananabanana 10 ай бұрын

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

@omkarbhale442 10 ай бұрын

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 10 ай бұрын

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 10 ай бұрын

/* 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 10 ай бұрын

@@programming5274 Yes!

@punsmith 10 ай бұрын

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 5 ай бұрын

@@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

@dawid_dahl 10 ай бұрын

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 10 ай бұрын

It is!

@rhodler Ай бұрын

Merci !

@vrtech473 10 ай бұрын

never knew this 😂😂😂😂 GOAT🎉🎉

@shane3744 10 ай бұрын

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.

@heddshot87 6 ай бұрын

4:05 ❤

@maxwebstudio 10 ай бұрын

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 ?

@josemfcheo 5 ай бұрын

this is awesome

@AlfredLotsu 9 ай бұрын

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

@anasalhariri5474 6 ай бұрын

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

@RevHardt 10 ай бұрын

Can’t wait for languages to treat everything as const by default

@MrTrevorrrrrrrr 10 ай бұрын

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

@edgeeffect 10 ай бұрын

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.

@am29d 10 ай бұрын

Mind blowing...

@yes.daniel 10 ай бұрын

yes!

@barneylaurance1865 10 ай бұрын

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; }; }

@rutabega306 10 ай бұрын

Almost as good as 'as any'!

@zuperxtreme 10 ай бұрын

Would an enum work the same?

@punsmith 10 ай бұрын

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.

@patsy02 10 ай бұрын

KZfaq's suggested video box is hiding the end of your video

@Orenrs 10 ай бұрын

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

@mattpocockuk 10 ай бұрын

Check my video on enums

@3453wer 6 ай бұрын

@@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