I'm completely new to conditional types, and I recently attempted a straightforward static approach with no success:
type NoUnion<Key> =
Key extends 'a' ? 'a' :
Key extends 'b' ? 'b' :
never;
type B = NoUnion<'a'|'b'>;
Despite my efforts, the type B remains a union. Can someone kindly provide me with some guidance or explanation on this matter?
For those who might be interested, here's a playground.
I'm not exactly sure what the intended use is for this, but it seems that we can set NoUnion to always be never if the input type is a union.
Some have pointed out that conditional types distribute over unions, which is referred to as distributive conditional types
Distributive conditional types occur when the checked type is a naked type parameter. These types are automatically distributed over union types during instantiation. For instance, if T extends U ? X : Y has an argument of A | B | C for T, it is resolved as (A extends U ? X : Y) | (B extends U ? X : Y) | (C extends U ? X : Y).
The key point here is 'naked type'; if the type is enclosed in a tuple type, the conditional type will no longer be distributive.
type UnionToIntersection<U> =
(U extends any ? (k: U)=>void : never) extends ((k: infer I)=>void) ? I : never
type NoUnion<Key> =
// If it's a basic type, UnionToIntersection<Key> remains the same, otherwise it creates an intersection of all types in the union and might not extend `Key`
[Key] extends [UnionToIntersection<Key>] ? Key : never;
type A = NoUnion<'a'|'b'>; // never
type B = NoUnion<'a'>; // a
type OtherUnion = NoUnion<string | number>; // never
type OtherType = NoUnion<number>; // number
type OtherBoolean = NoUnion<boolean>; // never since boolean is essentially true|false
In the last example, there's an issue because the compiler interprets boolean as true|false, resulting in NoUnion<boolean> being never. It may be solved by treating boolean as a special case:
type NoUnion<Key> =
[Key] extends [boolean] ? boolean :
[Key] extends [UnionToIntersection<Key>] ? Key : never;
Note: The UnionToIntersection function is sourced from here
Just wanted to share a slightly simpler version I came up with:
( Please note that the code below may not work in TypeScript versions after v3.3, as explained here:
// type NotAUnion<T> = [T] extends [infer U] ?
// U extends any ? [T] extends [U] ? T : never : never : never;
Instead of the previous code, you can use this one since defaults are instantiated before distribution, for now at least: )
type NotAUnion<T, U = T> =
U extends any ? [T] extends [U] ? T : never : never;
I believe this should work (please feel free to test it out; not sure why the original version in my previous answer was incorrect but it's been corrected now). This concept is similar to the UnionToIntersection: ensuring that each part of a type T is assignable to T when it's distributed. Generally, this is only true if T consists of a single constituent part (referred to as "not a union").
Of course, @TitianCernicovaDragomir's solution works perfectly fine too. Just thought I'd share this alternative version. Cheers!
Here is another option that can be utilized:
const NonUnionType<A, B = A> = A extends B ? [B] extends [A] ? A : never : never;
Is it feasible to retrieve the type of object property when that object is nested within a table structure? Take a look at this playground. function table<ROW extends object, K extends Extract<keyof ROW, string>>({ columns, data, }: { col ...
Imagine having a function like the one shown here: function addressCombo(street1:string, street2:string = "NA", street3?:string) { console.log("street1: " + street1); console.log("street1: " + street2); console.log("street2: " + street3); } I ...
I am working on a dropdown menu that contains numbers ranging from 1 to 10. Below is the HTML code for it: <div class="form-group"> <label>{{l("RoomNumber")}}</label> <p-dropdown [disab ...
Is it possible to achieve a similar outcome as shown below? type Info = { category: string } type Product = { code: string, ...Info } Resulting in the following structure for Product: type Product = { code: string, category : string } ...
I am working on a simple layout structure with 2 rows: Row 1 consists of 2 columns Row 2 consists of 1 column The goal is to have both rows expand in width and avoid vertical scrolling of the page. However, when resizing the screen, I want the columns of ...
I'm working with enums in TypeScript and I need to convert the enum value to a string for passing it to an API endpoint. Can someone please guide me on how to achieve this? Thanks. enum RecordStatus { CancelledClosed = 102830004, Completed = ...
Whenever my angular frontend sends a patch request to my express backend, all routes work smoothly except for the patch routes. The specific error message that pops up is: Error: No default engine was specified and no extension was provided. Should I be ...
Currently, I am developing a Next.js App (v13.2.3) using Typescript and have set up a path alias in the tsconfig.json. Does anyone know how I can configure the jest environment to recognize this path alias? // tsconfig.json { "compilerOptions": ...
While creating this: class AB<Initial, Current = Initial> { array: AB<unknown, unknown>[] = [] push<Next extends any>(ab: AB<Current, Next>) : AB<Initial, Next> { this.array.push(ab) return this as AB ...
I have a requirement to set up fixtures where the first fixture is always available (acting as a base class) and the second fixture will vary in different test files (like a derived class). I've implemented the following code which seems to be working ...
I am facing a situation where I have a package.json file in both the root directory and the /src folder within my projects. However, during the build process, the /src package.json is the one that gets copied to the /dist folder (and eventually published t ...
Hey there, I'm just starting to dive into Angular and TypeScript. My current challenge is figuring out how to check if a table is empty or not so that I can show or hide a specific div accordingly. I've attempted the following: var rows = docume ...
Encountering Typescript error while using Material UI component for date range picker Link - https://mui.com/x/react-date-pickers/date-range-picker/ Snippet of the code import * as React from 'react'; import { Dayjs } from 'dayjs'; im ...
I have encountered a challenge in my coding where we are dealing with quite large data objects. I am exploring the possibility of compressing these data objects to reduce payload size. For instance, if our input json size is 2 MB, can it be compressed to a ...
I am looking to convert my JavaScript files (*.js) to TypeScript files (*.ts) in my ASP.net MVC5 application (not an Asp.net Core app). I am using Visual Studio 2015. After downloading the TypeScript Definition Files into the Scripts\typings\ fol ...
Thank you in advance for your help. I am new to TypeScript and I have encountered an issue with a piece of code. I am attempting to wrap a union type into a Promise and return it, but I am unsure how to do it correctly. export interface Bar { foo: number ...
This situation is really puzzling to me. I wrote a function to calculate the number of documents in a collection getDocCount(): Promise<number> { return MyModel.countDocuments({}); } Everything seemed fine. However, when I removed async since I ...
Within our web app that utilizes TypeScript and express, we integrated the npm package express-request-id to enhance our HTTP Requests and responses by adding X-Request-Id headers. We crafted a middleware to assign the request's id to the HTTP respon ...
Imagine having a scenario with the following class: class Example { method1 = this.generateDynamicFunction((param: string) => { return "string" + param; }); method2 = this.generateDynamicFunction((param: number) => { return 1 + param; ...
I'm currently tackling the challenge named 'Merge Two Binary Tree' on leetcode with TypeScript. My solution is passing the tests successfully, but the compiler is throwing an error that says: https://i.sstatic.net/KZYmJ.png What's puz ...