@@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 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.
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
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
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!
@@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 };
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.
Man, this video just put an end to all the madness I've been through. Thank you very very much.
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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.
Typescript docs recommend to use interfaces until you need types because interfaces have more readable errors. Would love a follow up with this considered
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...😅
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.
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.
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).
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.
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
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!
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
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.
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)
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 }
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!
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.
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.
"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.
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 😮😊
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
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.
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! 😂
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
Sounds like interface is better for long term project as it is scalable. For my personal project I would use type but a project other people actually use I would use interface because I’ll probably need to make changes in the future
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?
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
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
Does it really matter for an irrelevant TypeScript "newbie" such as myself? I simply use interface for objects, and type for anything else. That's it, no other logic behind it, and I don't think someone like me even needs any logic to it.
If I were teaching someone from scratch, I'd recommend they use types for everything until they need interfaces. I think the traditional framing of interfaces being the default isn't correct - you just won't need most features of interfaces.
there are definitely differences between types and interfaces, interface must have statically known properties while types don't, so you can have type A = { [k in K]: O[K] } you can't really define this type using interfaces, right? or am I missing something?
I think that's fine and it aligns with his advice. I think this video was for those ambiguous or subjective circumstances. In your case, it's absolutely clear you'd need an interface.
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
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 ;-)
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
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.
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)
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!
It's not even neccessary for declaring that a class implements a type or that an interface extends from a type that the type being extended from is an interface. As long as the result is a plain object type, it can be extended from.
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.
I had switched to types but then switched back to interfaces since I had a Type named User and a React Component also named User. Now I have an interface named IUser and a React component named User.
Type has four character, Interfaces has 10 character. what a obvoius choice! Type is always my first choice. I use interface when you cant use type. In my expirence in typescript I havnt encounter that situation.
I promote using types instead of interfaces to describe Props in React just because of merging, Props types are finite, like saled classes in other languages Performance suffers only if they are complex, interfaces are cached, types are not
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.
"Language-Feature-Oriented-Programming" -- it's funny how you've spent the whole runtime discussing which feature to use, types or interfaces, touching their characteristics and sub-features and not a single word about a use-case or what they actually help one do better and why... P.S. yes, you could say "that is obvious", and if so, if the use-cases for both are indeed obvious, the benefits are clear then what is the point of discussing the feature, unless the actual feature is more important than what you use it for =)
So, I agree, "and" I would likely go a bit farther and say it would be best if you can simply use "types" all the time. Why? Well, basically because those things you mentioned Interfaces have that types don't are things that, IMHO, are things that should be avoided in JS as much as possible (for those who are about to argue that this is TS, not JS, TS gets transpiled to JS, and that's important to realize). I feel like just about everything that has gone wrong with JS started the very minute they tried to make it look more like JAVA (and that goes all the way back to the constructor function that was added to original JS). In my JS experience (which is over 25 years), every time I have tried to implement common OOP techniques like polymorphism and inheritance, very, very bad things eventually happened. The keyword there is eventually. The problems don't manifest right away. The main things that manifest early is slower code, and I could almost always point out a JAVA developer who was hating on JS because of its "slowness" because I knew they were doing it wrong. When ECMA capitulated and added the "class" keyword I thought, "here we go". That practically gave a license to use JS wrong. In short, composition and factory functions continue to be easier to maintain in the long run than inheritance (in JS/TS) and that's basically because JS is not object oriented. it's prototypal and the difference doesn't seem big until it does, but by then, there's not much you can do about it without lots of expense. It's a rarity when entire teams are so disciplined that their OOP code of JS doesn't get out of control (same with non-oop, actually, but many oop folks seem to think that they are magically protected from being undisciplined). Also, there is one other benefit I've found to types that interfaces rarely have, and it has to do with the IDEs themselves. In my experience, many IDEs tend to give you more information about types than they do about interfaces when you hover over parameters, properties and variables that were declared with a type, vs an interface. Often, when it's an interface, all I will see is an interface name, but when it's a type, I'll often see the properties, restrictions, etc from that type. It's a subtle distinction and may depend on the IDE you use, but it makes a difference to me.
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'm using interfaces whenever there's a function or a class that needs parameters. Inside those interfaces I use types. But what really bothers me, is this ambiguity about what each is useful for. There should be more strict rules about this. I really dislike the idea that you should use one versus the other...
