I will share with us some tips that improved my skill in Typescript !
Typeguard
Typeguard allow you to validate the type of an object within a conditional block.
interface Fish {
swim: () => void
}
interface Bird {
fly: () => void
}
function isFish(pet: Fish | Bird): pet is Fish {
return (pet as Fish).swim !== undefined
}
We can be sure thanks to the condition, that pet object is a Fish
.
Why and where use this ?
It's very useful when you need to check the type of an object among other type. In the example above, the typeguard isFish
can be use like that.
function toto(pet: Fish | Bird) {
if (isFish(pet)) {
pet.swim() // At this moment, TS know that pet is `Fish` and no a `Bird`
}
}
function isFish(pet: Fish | Bird): pet is Fish {
return (pet as Fish).swim !== undefined
}
Type this
as argument in function
A small tips, you can type this
object in a function like this
function toto(this: { a: string }, arg: number) {
console.log(this.a, arg) // "toto", 55
}
toto.bind({ a: 'toto' })(55) // Don't forget to bind `this`
Utility Types
Typescript allow us to use utility type, it's a very usefull feature ! you have the full list at https://www.typescriptlang.org/docs/handbook/utility-types.html
I will show you the common utility type that I used !
Partial :
Constructs a type with all properties of Type set to optional.
interface Toto { a: string, b: string }
// Partial type equal to -> interface Partial<Toto> { a?: string, b?: string }
const partialToto: Partial<Toto> = { a: 'a' }
Pick & Omit :
Pick
is used to extract some keys from a type in order to create a new type.
interface Toto { a: string, b: string }
// Pick type equal to -> interface Pick<Toto, 'a'> { a: string }
const pickToto: Pick<Toto, 'a'> = { a: 'a' }
Omit
is used to remove some keys from a type in order to create a new type.
interface Toto { a: string, b: string }
// Pick type equal to -> interface Omit<Toto, 'a'> { a: string }
const omitToto: Omit<Toto, 'a'> = { b: 'b' }
With three utility types, you can create new very smart type ! And very useful to understand for other developers.
Record :
You can construct an object with typed keys and type and make useful type like this
type TotoKeys = 'a' | 'b' | 'c'
interface Toto { name: string, age: number }
const toto: Record<TotoKeys, Toto> = {
a: { name: 'a', age: 55 },
b: { name: 'b', age: 44 },
c: { name: 'c', age: 33 },
}
I love records since you can use enum for typing keys !
enum TotoEnum {
A = 'A',
B = 'B',
C = 'C'
}
interface Toto { name: string, age: number }
const toto: Record<TotoEnum, Toto> = {
[TotoEnum.A]: { name: 'a', age: 55 },
[TotoEnum.B]: { name: 'b', age: 44 },
[TotoEnum.C]: { name: 'c', age: 33 },
}
I hope you learn and improves your skills during this article !
If you have other suggestions or questions, don't hesitate to put it in comment bellow !
I hope you like this reading!
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Top comments (13)
Hi CodeOzz, I might be totally wrong here, because I'm a Typescript noob, but your typeguard example doesn't look like the "Typescript way" to me. Today a friend of mine twittered some Typescript wisdom so now I want to propose the following typeguard:
As I understand it my (type based) version does type-checking at compile time while your (interface based) version does type checking only at runtime? Oh, and as you can see I also changed the return type of isFish to Boolean, because I don't understand the "pet is Fish" return type.
So what do you think?
Hi Thorsten how are you ! Thank you for your comment !
I will try to explain you what is the
pet is Fish
. This none common synthax is calledType predicate
in TS, it allow us to tell to TS that your param is typed as aFish
only if your function return true.In my case I check if my object has swim methods, if it's the case I tell to TS ->
Typescript, trust me, this pet is a Fish, not a Bird
.In you code, you put an interest thing, you create a new property in order to determine the type of your object. It should be a great idea but in your case you should delete this since you have already a way to determine the type of your object, thanks to the methods
fly
orswin
, so you don't really need to add this new property !About your function, it's not a really
typeguard
, and why? it's because your are correctly checking the type but thetypescript
checker is again doubting about the type of this object since you are not usingpredicated type
!Why it can be not really usefull to use the function like this? I will show you why of course !
So if you want to use pet in the if block, you cannot really use Fish property since for TS,
pet is again a Fish or a Bird
.With the
Type predicate
, you don't have this issue:If you have any question ask me ! ;D
But when you add new type Duck you will not be able to differentiate pets and fishes from Duck just checking available methods because a Duck can both swim and fly. In that case you will have to turn to Thorsten's explicit kind of an animal.
In the case of your interface growth you will need to use another way to put in the type guard yes. But in this exemple it's overkill to use more properties
Thank you. This predicate thingy seems pretty cool indeed. 👍
I find your example of Typeguard a little problematic precisely because it is subject to fail in the case of "interface growth". Determining type based on the presence of a method looks really flaky to me. As already suggested a duck would be identified as a fish. In your example code it makes much more sense to simply check for the presence of the swim method (i.e.
canSwim()
); rather than checking against the type*.I appreciate it's sometimes hard to find simple real-world examples; but in this case you either need to find a better one or add a health warning: your
isFish
method is very likely to introduce hard to trace bugs in future and defeats the very purpose of using TS.Apart from that I found the article useful ;)
* In the same way you should prefer 'feature detection' over 'browser detection'.
Hello ! As I said in another comment the aim of this example is to understand what is a Typeguard ! My example doesn't reflect the reality since as you said, Duck can be identificated as Fish !
In a real project you should use another way to make the difference between each class :)
Thank you for your comment a lot !
Indeed; that was precisely my point: it's an ill-conceived example and demonstrates how not to use Typescript. There's little point demonstrating that functionality exists if you don't also demonstrate the thought processes required to use it properly.
This is awesome!! Thank you so much I've always wondered what some of this does in other peoples code. Now I can use it myself!
Thank you Richard ! I appreciate your comment ! If you have any question about some TS code that you don't understand, you can ask me on my twitter :)
Thank you so much!!
Thank you a lot Anjan, it's very motivating to see comment like this :)
Good article.