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TypeScript: Should you use Types or Interfaces?
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- เผยแพร่เมื่อ 4 ธ.ค. 2022
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This is my preferred approach. Use types by default, and interfaces only when necessary. I find type intersections and unions to be very useful
But when is it necessary?
@@happyfase I think when you need to extend global types, such as the Window type. Or extending a library (or creating a library which should be possible to extend) I suppose.
@@happyfase Tell me you haven't watched the video without telling me that you haven't watched the video.
I find interfaces to be much cleaner when reading errors. Especially when there is a lot of properties.
@@happyfase use interface if you think the implementation type will have multiple version. Like general type animal thing. Like walk, fly and swim. Walk speed each animal can be differend. not all animal can fly. not all animal can swim too deep.
Finally someone actually highlighted that Interfaces have different features from types. Good bless this man.
I'm just learning TS and pretty much most basic tutorials or articles I've found just say "types and interfaces do the same thing", so this video definitely caught me by surprise
Interfaces really only have 1 feature, interface merging. Types can do everything else and much more
I think its varied a lot over the development of Typescript what they can do, which probably adds to the confusion. Personally I think it was a mistake in the language design to have two things for this.
@@benjidaniel5595 Not only 1, but they also handle conflicts much sooner than what you would get with types. But as a general rule I use types most of the time, I like to think of types with unions and intersections much more than with interfaces.
Bytegrad does too sorta. Mostly he just discusses how to achieve the same results though. His take away is that it's best to be consistent so chose one vs the other. Gets interesting when you use shadcn though since all of its boilerplate is extended with interface
Amazing deep dive into something that one would never even thing of existing in the first place! Bravo, Matt 🍻
Man, this video just put an end to all the madness I've been through. Thank you very very much.
When the video is the normal distribution meme with the newbie and the guru saying "use types by default"
btw great content as always, love that you took on this journalistic effort, it's perfect to keep the pulse on TS world. Thank you!
Loved hearing your journey with this decision! It was very relatable, because I feel like I flip flop like this with everything in my life, not just technology...😅
I'm a noob so of course at first (last week), I was using interfaces for anything and everything more than a one line type definition. I've been coming around to your way of thinking more over the past few days and I feel better after hearing what you have to say about it. Thanks
I also went back and forth between both for a while, but ultimately settled for something like you did. Use types when I need their features, use interfaces when I need their features. And, if I absolutely don’t need any of their features, that actually raise the question why and we try to figure out if we truly need that new type/interface in the way we see it. Oftentimes we realise that processing an existing type trough a generic or a mapped type.
This is 100% my philosophy for interacting with types and interfaces. Awesome Video!
this video is very instructive for someone that doesn't went deep into types but still appreciate how is the common sense related to this way of typing things
first time viewer. good content and also pleasant to watch. Thank you
Appreciate the candor, Matt!
Love it. Been using exclusively types for awhile mostly because of the consistency. Had no idea that interfaces even had these issues.
I use interfaces for classes and types for everything else... so yeah, I never use interfaces 😉
Well that's just lovely
I was like this too, hating on the classes. But they are actually powerful if you are willing to learn how to use them properly
What are classes?
I use interfaces for typing React component props, since I can extend the base HTML element props
@@producdevity☠️
Thank you! Wanted to know more about this and you explained it well! 🎉
This is just what I was looking for! Thank you
Small note: `types` unwraps to the underlying structures, `interfaces` preserve the "box" name throughout
I’d like to understand your comment better. Can you give an example please?
@@karmasakshi What he means is that type hints are not expanded when using interfaces. Although they are also not expanded when you intersect or union types together:
interface SomeInterface {
foo: number;
bar?: string;
baz: number | undefined;
}
type ExampleInterface: SomeInterface; // => SomeInterface (not expanded)
type A = { a: number };
type B = { b: string };
type SomeIntersection = A & B; // => A & B (not expanded)
To get around this you can convert interface into a type, as well as simplify the intersection types by looping over them to combine them:
type Simplify = { [K in keyof T]: T[K] };
type SimplifiedInterface: Simplify = literal; // => { foo: number, bar?: string; baz: number | undefined; }
const SimplfiedIntersection: Simplify { a: number, b: string };
For me is weird using types for everything because I always though interfaces are the entrypoints for connecting things, the shape of the IO of a thing, and types were entities. But I always ended up using types to avoid the augmentation, because that is tricky to visualize and handle sometimes (at least for me).
I use interfaces only in a 'classic' OO sense: describe abstractions which multiple classes can inherit and provide the functionality described by the interface. These are usually 'classic' OO objects with private data, getters/setter and member functions ... no 'weird' JS stuff :).
For all other purposes I find types to be superior (mainly due to discriminating unions). I also find describing a function via interface weird personally so I would always do it via type alias.
Same here!
Great explanation. Thank you, Matt. I've always been on the side of "use types for everything except if you want to use "class implements interface" feature. My reasoning was simply I don't have enough reasons to use interfaces for anything else.
KISS FTW.
Great video, thanks a lot for sharing, I really love the way you're explaining things :)
Great typescript video!
Many thanks for content and insides!
very good clarification thank you
I always did this without even thinking about it, it's awesome to know the Typescript guru also do the same.
Typescript docs recommend to use interfaces until you need types because interfaces have more readable errors. Would love a follow up with this considered
Or, they could at least implement syntax highlighting and properly formatted error messages
Those issues are stale for over a year now lol
This was my approach for quite some time, the risk of working with types is when you extend an object. with type you can override any property of the extended object, while interface will not let you do this, so I'm using interface unless I know I want to override one of the extended object properties (or do something else that only type can do)
There is also a difference in error handling. If you have 2 interfaces with property with the same name but different type, you will get a compile error. If you do the same with types, you will get a never type.
Which you'll later spend 15 minutes on debugging/tracking down just to find out about this small little detail you've missed...
Kind of reminds me of tracking down silly typos in plain JS... Reference errors all over the place due to fast typing, and always in the exact wrong moments.
Great piece of advice !!!
Excellent video!
Thanks Matt!
I'm so glad you made this
I ran into this last year and kinda used both in the same project
I couldn't figure out what then difference was
But...
Everywhere you look says use an interface
Now I get it use Types in JavaScript / react the majority of times for safety reasons
Then use interfaces pretty much in every other language 😮😊
TypeScript hero ❤️ .Thankyou so much legend
I just found this amazing blog, so happy with its valuable content. I was wondering would u mind to prepare one about Hexagonal(Dependency Inversion) in TS? I come from OOP and I find TS really powerful but at times I miss the stability that brings Hexagonal and Service contracts in the form of Interfaces
Thanks for the video! I’ve been searching about types x interfaces for a quite but you were the first who actually pointed a real use case of both. That being said, is there a “semantic” (in terms of good practices or convention)use case for types/interfaces apart from its inner functionalities?
In objecty-oriented languages interfaces are meant to be used to describe what a certain thing (class) can do. It doesn't describe structure but behavior. Thus, using it to describe fields in the object is not a good practice - types suit it better.
Ok, now thanks to you, I'll have to convert all my interfaces to types again. Kiding 😄, good stuff!
phase 3 is where it's at, imo you should default to types and use interfaces when you have a specific need for them. note also that a class can implement a `type`, and sometimes you want to do this - in my experience you usually want an interface when doing classical OOP (i.e. not implementing arbitrary types) or when augmenting a module to merge an interface declaration. it's also worth noting that all object types (whether declared via `type`, `interface`, or inline) are open to extension, meaning that they can always contain extra properties that are not declared on the type. this is a feature of the language and it's the reason why Object.hasOwnProperty doesn't return a type predicate and Object.keys doesn't have a narrower return type.
i like your conclusion
good point!
Thank goodness! :)
Module augmentation can be used or abused. Good example is MUI Material library, where you can extend a component's prop interface without the need of reexporting the component or the interface, so you can use those props in styling
types work better with Visual Studio intellisense so when you hover over something that has a type it shows you the type definition, whereas with an interface you have to navigate to the interface to see it’s definition
Because typs is just an alias and interface is ….. whatever i dont even onow
This. I hate to only see the name when I'm hover a type name and it just gives me. I'm mean, thanks captain 🙄
if you hold `command` (mac) or `ctrl` (windows) before you hover it will show you the interface definition
I just found out that if we hover over the type itself, yes we get the whole type definition, but if we hover over the variable defined with the type, it just shows the name of the type, unless we press Ctrl before hovering.
I learned this shortcut from @Ben Haynes in this comment! Thanks!
I'm also using the 3rd option and it's probably best practice how using types in typescript but for typing API json objects i'm also using interfaces.
I personally use interfaces for everything unless I need types (Intersections, Unions, Type aliases, inferences, e.t.c)
Same here.
Same here. While I do think it's very important to be consistent in your codebase, I will not prefer types over interfaces simply because they're more powerful as I see them convey 2 different ideas. Types are just shorthands for whatever you define them to be and interfaces define a forced / non-dynamic structure for an object. Therefore I use them as such.
thank you :)
Nailed it.
I've had some niche little quirks where using an object intersection would break a type guard function, but rewriting those same types as interfaces with inheritance worked just fine.
I have no idea why it behaved that way, but interfaces fixed it so those will forever be interfaces.
I changed my mind three times just watching this video.
Yes, haha I love the word interface, and that's why I use interfaces over types. Thanks for your video though, I wasn't even aware of these differences between them
I've almost exclusively used Types for a looong time.
Yeah I've used interfaces indirectly and also in the VERY seldom situation where I needed to eg extend a global interface, but otherwise it's all been types.
And for my part a type union is what interface extension should always have been, and with its connected operations is just better.
The ONE thing where I sometimes miss interfaces is that they can be self-referential, and most of the times that probably should be avoided anyways.
But if you've got an interface that has a few fields that use similar types or something like that, you can use this.field to refer to the type of 'field'. That probably is possible due to the same mechanic as interface extension now that I think about it.
Anyways, that has been somewhat fascinating for me.
But it's not neccesarily all that predictable and in many instances you'll find other ways to express that stuff that are just a bit more usual and predictable.
Thanks
Yup, that’s what I thought and use for years. Thanks for the video though!
"If you want something predictable that's never going to act strange or do wierd things in certain situation then use types"
I'm going to counter that by saying that if you don't want wierd and strange behaviour then avoid Javascript entirely lol.
One thing that interface is better than type (arguably) is when you want to hide the complexity of the types in IDE.
type is type alias and they will expand by default when you hover over the variable (in normal cases, there are exceptions).
for interface, it will show the name of the interface only.
It's a "it depends" case really, because in function arguments, types is typically better because you can see what you need inside the variable.
On the other hand, when reading a variable interface can be cleaner.
However, I still stick with what I recommend. Use type over interface.
One could argue that it's desired to simplify the type because it improves type hints, which otherwise would require you to poke into the type before you know its shape. One problem you can end up in when using interfaces, though, is when you try to pass a value that is an interface type into an wider type, i.e. a Record in a function argument, even though they may have identical shapes:
interface SomeInterface {
foo: number;
bar?: string;
baz: number | undefined;
}
const literal = {foo: 123, bar: 'hello', baz: 456};
const someInterface: SomeInterface = literal;
function fn(object: Record): void {}
fn(someInterface); // Error: Index signature for type 'string' is missing in type 'someInterface'. Because `interface` can be re-opened
To get around this you can simplify the type by converting the interface to a type:
type Simplify = { [K in keyof T]: T[K] };
fn(someInterface as Simplify); // Transform an `interface` into a `type`, resulting in hapiness
On top of this is can be used to simplify the types for intersection and union types, which would otherwise also hide the shape:
type A = { a: number };
type B = { b: string };
type AB = A & B; // => A & B
type ABSimplified = Simplify // => { a: number, b: string }
This is exactly why we use types over interfaces
Could you please share the details of the lamp in the background?
So far I've only once had to use an interface... Types, and especially unions, are my favorite thing about TS.
I tend to rarely use interfaces even though the extension syntax for interfaces is a bit cleaner, I usually just use the intersection operator. I've resolved to only use interfaces for implementing classes which I rarely do because I don't write a lot of classes (oop is the devil (class is still a good tool, just don't orient your entire program with them)).
I started to use types more when I found out that the tooltips on vs code for types are much better.
Haha you just convinced me
I assume interfaces getting merged can be useful in places where npm packages can extend functionality for some lib without re publishing the original lib, as some sort of plugin system that can be made a bit more broadly version compatible.
There's a canonical blog post on this 5 years ago. Good to see that advice hasn't changed
This has been my approach for the last 2 years, only use Interfaces when you need them specifically for one of their unique features. Beforehand I was captain interface for everything! 😂
If anything the example given at the end makes me appreciate interfaces more, since to me it points out the flaw in the example. If I already know the data's structure well enough to be typing it (such as MyData in the example), then I'm more likely just going to modify the JSONValue type and the handle function to be generics that can take which exact set of keys I'll be expecting rather than just "[key: string]" to get better type-safety.
If you only need the function to handle those 4 options then you don't even run into the problem.
Personally and the main reason I 100% prefer interfaces is I think interfaces visually are much cleaner, and even eye-catching in comparison to the rest of the code. "type FooBar = {}" and "const fooBar = {}" are SOOOO similar, that it visually homogenises the code just that tiiiiny bit more than I'd like. Taking that one step further, extending interfaces is also more visually distinct and IMO less cluttery than "&" characters slotted between types.
Even separating those concerns into their own files can sometimes create a different type of problem where if you're moving very quickly through your code (jumping through go-tos, cursor back-forths) that when everything has the same structure and (depending on your editor) the same colours you can't identify certain things as quickly.
In the end mainly due to the very particular and petty reasons above I'm in the "use whatever!" camp. Unless you have some very specific use-cases, then using whatever is perfectly fine!
BTW you can extend a type in a class definition
I have always preferred types because of the intersections.
so many beautiful things to do in life and you decided to benchmark thousands of types with interfaces
I've wasted my precious hours
@@mattpocockuk just kidding bro, thanks a lot :) was helpful
I share your phase 3 view. I only use interfaces in very specific cases and types as default
I generally end up using types for functional programming and interfaces for OOP. Or is it also better to use types when using classes?
I was having an anxiety attack waiting for the final answer and just praying that it was *types.*
Phew, I can breathe easier now.
I use Type only. I used to use interfaces because I came from c#. But types as you said are predictable and just work nicely.
Types "intersections" is exactly the same as interface inheritance. The only things interfaces can do and types can't is declaration merging.
You can even "implement" a type in a class. Which really makes interfaces redundant.
I like that you said "If you like the word interface, use interface". More people should acknowledge your preference when giving advice.
I uses interface by default and types if I need a certain feature. Interfaces has a show reference feature which is useful to me
I use types for most things because they are strict, which tends to keep objects cleaner and enables many useful utility functions that rely on knowing exactly which properties exist and don't exist on an object.
Interfaces are best used when you explicitly don't want control over the properties of an object. If it doesn't matter I'll default to types.
2:34 Is ^? only a visual mark or it has some documentation meaning? I just can not find and I am super curious about it.
Matt, what are your thoughts on organizing types and interfaces in a project and naming? Separate /types and /interfaces folders? What about file naming? use name.ts or name.interface.ts / name.type.ts?
types.ts
Jon, please don't put all your types in a single file (unless they can all fit on the screen without scrolling while in an editor). that's a bad practice and terrible to see in enterprise code
regarding folders and naming, it really depends on the project, how many files there are, and the team
- if they're models, just put them in a /models folder like `/models/name.ts`
- if the models folder grows to like 10+ files, then consider reorganizing the files
I'm new to TS. Went with `type` as a default because it looks closer to vanilla JS than interfaces.
When your course will be available?
Hello and thanks.
This is why I use types
type x = { firstName: string}
And if I change my mind to do
type x = string
Only types can do that
Plus the type utilities that I use a lot
I put together some bullet notes on this video: docs.google.com/document/d/e/2PACX-1vQ6KyeWu-2MiHPe_9187HsXP7xdLyD3mRPjaWG70kJSXBHXKAXfWnnUCZnOHwLQSdePjGTlnGHwP9xH/pub
Should you use Types or Interfaces?
===============================================
By: Matt Pocock
Notes recorded on: 2023-05-05
* Matt has changed his mind on the Types or Interfaces discussion.
* He’s been through three phases.
* Phase 1: Interfaces are the bomb.
* Phase 2: It doesn’t matter which one you use as long as you’re consistent.
* Phase 3: Use types unless you need a specific feature of interfaces.
Phase 1: Interfaces are the bomb.
---------------------------------
* Phase 1 was endorsed because they optimized performance of the typechecker. At some point in TypeScript, if you had a ton of types, your typechecker could run very slowly.
* Matt ran a benchmark, which he admits was problematic, but he discovers that the conjecture of phase 1 was no longer true.
* Matt says the TypeScript team said that this advice is no longer relevant.
Phase 2: It doesn’t matter which one you use as long as you’re consistent.
--------------------------------------------------------------------------
* Every object should be typed as an interface.
* Everything else should be typed as a type.
* Or you can use types for everything.
* Matt mentions that interfaces have more features than types.
* One interface can easily inherit from another interface.
* Types can do this, but require the overhead of intersections.
* Matt frames the feature set of interfaces by saying that “interfaces come bundled with a bunch of features that you probably don’t care about.”
* He cites declaration merging as an example.
* This feature allows TypeScript to support the use case of appending properties to an existing object, such as the browser’s window object.
* The inventor of Angular asked Matt a TypeScript puzzle that arose because interfaces can be appended to.
* “You don’t actually know what’s going to be in an interface when you use it.”
* This problem introduces phase 3.
Phase 3: Use types unless you need a specific feature of interfaces.
--------------------------------------------------------------------
* If you need a type that extends another type, use an interface.
* If you need a class that extends an interface, use an interface.
* If you want something stabler, then use types.
* Don’t worry about the performance difference.
* Matt will be using types to declare most things.
I need to assignate myself the task of looking into the difference between assignment and assignation.
Assignate isn't a word, for starters
I’m in the “I like the word interface” category
interface just sounds oo and just has a better vibes
I agree with your statements in this video, though i'd probably add, I usually use interfaces more if i'm using es6 class syntax, because they just generally go well together feature wise.
Welcome to the party, pal.
Another reason why I’m starting to use types by default is:
interface SomeObject {
someProperty: string
}
is not assignable to Record
But the type version is:
type SomeObject = {
someProperty: string
}
Indeed. It's very annoying. But if you for some reason have to consume an interface, you can convert it into a type by simplifying it:
type Simplify = { [K in keyof T]: T[K] };
function fn(object: Record): void {}
fn(someInterface as Simplified); // Now this works!
Does this discussion with the Typescript team happened to be somewhere on the GitHub or it was private? Asking because it would be interesting to read the whole thread
My thought is that types should be preferred unless you need to do abstraction/contract where you could have potentially multiple implementations of the same contract. Caring about performance is bad for code readability here. Even if it does differ. We trade readability for runtime performance even... type is just keyword for any TYPE. if we had normal complex runtime types in js, there would be no such thing as declaring specific object shape type and using it as contract for something. That would be just abstract class type of things (from oop world). Interfaces is more semantic thing than technical. It means abstract stuff out. In c++ they even do keyword redifinition, so class and interface is completely same thing. Differs only on semantic level, on technical side it's just pure abstract class.
From this perspective i prefer types for declaring props in react, yes it's objects all the time, but they are not abstractions - they are concrete specific types (it's more like DTO objects for passing arguments)
I prefer to choose interface for anything that can be thought as abstact API to an object and types for anything else.
Seems we both came to the same conclusion. I used to use interfaces but I ran into werid instances where types just worked better and now I use types for everything unless I need a specific feature of interfaces.
How do you use a type to define an object shape, like you would with an interface? Example?
type Example = {}
How to choose between intersect two types or extends an interface? I still confuse on that
You can intersect types that have the same field and it will become union type. You cannot redeclare field while extending interface. Right?
you can do something like this though:
type Case = Base & {
title: string;
synopsis: string;
i use interfaces for objects/component props and types for anything else
I use types because they work with the type union operator.
Overloading interfaces with function IO seems to preserve types better than alternative methods. At least where enums are used.
Used to use interfaces all the time, but few months ago moved to types, because they do not merge each other. And backend data are better with types. You cannot reuse the same name and Union of types are better to work with.
Hm, never thought so much about it, but I never considered one to be better or worse in general. Just like in your conclusion, I use what's best suited for the purpose.
Besides that, the very frequent zooming makes me a bit seasick, but I am also old ;-)
Would it be helpful(and solve the problem mentioned in the video) if we had some keyword to disallow interface extension? Something like "final" in Java? (Not meaning that you can't extend from the interface), but that you can't amend properties to existing interface.
It's pretty tough, because ALL projects have some kind of declaration merging. It's present in lib.dom.d.ts, and libraries which depend on it. It's a critical part of library code.
It's also pretty hard to lint for outside of TS, too.
So it's a tough problem to try and restrict it. I'd argue that a type _is_ a 'final' interface.
yeah, thinking about it there is probably a very minimal amount of places where this would be really helpful, and for all others just using Type would kind of do it.
Why would you want this though? Declaration merging is a feature of the language - you can do it with interfaces, functions, and probably other things as well. If you want to disallow declaration merging for a type then you can declare it as a `type`. If you're writing a library, please don't declare everything as `type` just to prevent declaration merging unless you're sure it's the right thing to do. I've used too many libraries that expose an `any` where they shouldn't, and there's little I can do about it downstream because they've opted for `type` everywhere.
I just started today, have you changed again so I can get the latest recommended 😅