What does the “-?” syntax mean in TypeScript mapped types?

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP



What does the “-?” syntax mean in TypeScript mapped types?



Can anybody please explain what that "-?" in the following TypeScript type declarations
means, compared to just using a "?" there?


type ValidationMap<T> = [K in keyof T]-?: Validator<T[K]>




1 Answer
1



It's not a wildcard. The -? notation was added in TypeScript 2.8 as a way of removing the optional modifier from mapped types.


-?



Basically, it means that ValidationMap<T> has the same keys as T, but none of the properties of ValidationMap<T> is optional, even if the corresponding property of T is. For example:


ValidationMap<T>


T


ValidationMap<T>


T


type Example = ValidationMap<a: string, b?: number >;
// type Example = a: Validator<string>, b: Validator<number



Here, Example has a required b property even though the type it's mapped from has an optional b property.


Example


b


b



(Note that if you change -? to just ? or to +?, it becomes the opposite... you would be adding the optional modifier. The ? notation was added in TypeScript 2.1 as part of the mapped types feature, while the +? notation was introduced in TypeScript 2.8 along with -?.)


-?


?


+?


?


+?


-?



Hope that helps. Good luck!






By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

-

Popular posts from this blog

Firebase Auth - with Email and Password - Check user already registered

Image
Clash Royale CLAN TAG #URR8PPP Firebase Auth - with Email and Password - Check user already registered I want to check when a user attempts to signup with createUserWithEmailAndPassword() in Firebase user Authentication method, this user is already registered with my app. createUserWithEmailAndPassword() 8 Answers 8 To detect whether a user with that email address already exists, you can detect when the call to createUserWithEmailAndPassword () fails with auth/email-already-in-use . I see that @Srinivasan just posted an answer for this. createUserWithEmailAndPassword () auth/email-already-in-use Alternatively, you can detect that an email address is already used by calling fetchSignInMethodsForEmail() (previous called fetchProvidersForEmail() ). fetchSignInMethodsForEmail() fetchProvidersForEmail() When the user trying to create an user with same email address, the task response will be "Response: The email address is already in use by another account." mFirebas
Read more

Dynamically update html content plain JS

Image
Clash Royale CLAN TAG #URR8PPP Dynamically update html content plain JS So I have a script bound in my index.html, which literally has to fetch an array of jsons from an api, then dynamically update my page when get response. But as a result I'm getting an as the only update of my page. here is my script fetch(url) .then((resp) => resp.json()) .then((resp) => resp.results.forEach(item => fetch(item.url) .then((response)=> response.json()) .then((response) => pokeArr.push(response)) .catch(console.log) ) ).catch(console.log) const filler = () => if(!pokeArr) return 0 pokeArr.map((i,j)=> return `<tr>$i.name</tr>` ) const pokeindex = () => document.getElementById('a').appendChild(document.createElement(filler())) pokeindex() When I'm consoling it, I can see in the console all the responses I get, so I'm at least doing the fetching part right. What's parberakan ? What's pokeArr ? – T.J. Crowder Aug 8 at 12
Read more

How to determine optimal route across keyboard

Image
Clash Royale CLAN TAG #URR8PPP How to determine optimal route across keyboard I'm trying to automate the typing process in a video game. Players are given an on-screen QWERTY keyboard layout that they can navigate with the control stick (up, down, left, right) in order to select a letter to type. I was wondering if there was a module for Python (or some other resource I could explore) that would help my program find a path across the keyboard as it types each letter in a given message. In other words, if the first letter typed was "A" and the next letter was "B", the module would know that the program should move 1 space down on the keyboard and 4 spaces to the right. Currently, I have the program resetting to the "G" key after each letter is typed (allowing me to use fixed-directions as the program parses each character in the desired message). As you can imagine, this is quite inefficient and defeats the purpose of automating the typing in the fir
Read more