as const: the most underrated TypeScript feature
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- Опубликовано: 28 июн 2024
- '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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You can also use `as const` on individual value literals or use it on template string literals to get all possible strings.
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)[]
The template literal trick is mind blowing!
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.
You can also use 'satisfies' with 'as const' to constraint the object typings, like 'as const satisfies Record', its super neat
Damn I needed this so many times and didn't know about it, thanks for sharing.
holy crap I needed this, tysm
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.
"const as const"
sounds really smart and not ridiculous at all
It makes perfect sense within the typescript syntax, and it's relationship with javascript syntax
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.
Awesome! Thanks!
works on arrays as well:
const abc = ['a', 'b', 'c'] as const;
type ABC = typeof abc[number];
After watching half of the video, I found an immediate use case in my TS project I'm working on. Great video!
I use this quite often, I don't think it's underrated, I think other things may just be rated too highly... LOL
PS: Where is the Top hat and Monocle?
@@ColinRichardsonPeople do wear two monocles to make a _spectacle_ for themselves, so now he's just missing the top hat, old chap 🎩
I love your typescript content, thanks
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.
That's absolutely brilliant! I wasn't super sure what "as const" was doing, thanks for clarifying that!
Amazing video.
Such really good pedagogy, loved it, even if I was already very familiar with as const and the other notions sumed up.
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.
Hey can you expand a little bit on that? Sounds interesting
@@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.
This is such a helpful feature. I'll be using this all the time now, thank you
Amazing! I needed this earlier today!
The best and simplest explanation of "as const"!
Just came to say this video finally helped make `as const` click for me. Thanks for the info, Matt!
Needed this a few months ago 😂 excellent content on this channel dude keep it up
i was in a live stream a few days ago on this,learned so much
Sometimes I utilize this tool to manage database names, collections, roles, etc., enabling me to infer values instead of strings :).
Was using an enum for a radio group in Zod and discovered this. Never went through the trouble of understanding it, though. Thanks 👍🏿
Very nice ticks! keep them comming sensei!
Videos are very useful and clear, thanks
Really great.
You are a great teacher 😊
Thx from France.
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
Thank you very much Mr Matt!
Finally... now I understand it. Thank you Matt.
That's worthy of a medium article mate. Very nice solution to a very popular recurring problem
I did not know as const prevented the object being changed - thanks!
Watched small video - huge power acquired. Thank you!
This channel is also underrated. awesome video! you just got a new subscriber here
Amazing so concise and clean.
That was a whole lot of fantastic stuff that will immediately be used
Nice video! Heads up on this format on mobile because there's no RUclips outro, the "suggested video" card appeared whilst you were still coding and blocked the code in video
Made me subscribe. Thank you very much. 👍
thank you, you are a life saver!
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.
This is so beautiful that i want to cry.
Simply a typescript magician
What do you prefer “as const” or enum in cases like in video?
"Object.values on a type level" trick is pretty neat. Thanks!
Wow great video especially as a new typescript developer, I subscribed
Awesome tip here. Thank you @mattpocockuk for sharing this.
Brilliant. Instant follow
Awesome sharing!
I love your videos. Very insightful, fun, and straight to the point
It's ridiculous that I can't expect const as constant value. Love your vids
You can, you just need to know what is constant about your value.
Great video, typescript is beautiful
Just wow. Thank you.
You just saved me wtf. I was looking for a solution like this yesterday
Can we use enum for store routes and avoid create additional types?
Niiiiice ! I always forget how to target the type of keys and values of objects 😅
That's brilliant!
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!
this video is amazing!
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.
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.
Excellent....pretty cool. Thanks....
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.
It is!
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
helpful video. thanks.
Oh cool! I knew it for array to union, but not for object to union. Thanks!
Nice!! thanks
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
Matt the goat!
Nice job 🎉
YOU are the most underrated RUclipsr
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 ?
4:05 me knowing new ts magic
Would an enum work the same?
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.
Beautiful
You didn't talk about: "as const satisfies Record "
I started using a lot of "as const satisfies typeX", I'm creating a lib and it's helping me a lot
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.
Great video and example.
Why not using enum for this case ?
Check my video on enums
@@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
Merci !
How can I type safe for api end point url for fetch ? Some of them will need query or other will need only body .
How about routes with matching variables like '/users/:id' that match /users/3 ?
But what benefit it adds in comparison to string enums?🥺
Wondering the same lol
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
Finally something from youtube I will actually use
this is beautiful
Fenomenal! 🎉
Anybody knows that translation plugin for error messages ?
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)
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",
}
/* 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);
@@programming5274 Yes!
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.
@@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
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 🤷
What's the different between this and using Enums?
Normally I would an enum in this case like "enum routes" and avoid declaring the type. What do you think?
this is awesome
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;
};
}
what is the VSCode extension you're using to get the human-readable ts error messages?
www.totaltypescript.com/vscode-extension
@@mattpocockuk Thanks so much!
Mind blowing...
what is that extension that translates ts errors i need it
probably Total Typescript
And this is because, I am assuming from the pop-up, enums should not be used?
never knew this 😂😂😂😂 GOAT🎉🎉
"as const or how to implement a real enum in typescript" 😂
yes!
I now understand Rich Harris. We are packing up and moving to Jsdoc
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?
Search for "ExclusifyUnion", there's a good stack overflow answer with an explanation
How to use for array
I may have agreed, but even better is the new const in generics feature.