Why are the partial operator `?` and explicit `undefined` incompatible?

I define the type:

import { type } from 'arktype';

const checkType = type({
  'redirect?': 'string'
});

checkType({}); // pass
checkType({ redirect: undefined }); // ArkTypeError

import { type } from 'arktype';

const checkType = type({
  'redirect?': 'string'
});

checkType({}); // pass
checkType({ redirect: undefined }); // ArkTypeError

I don’t understand why it is designed like this, as far as I know, partial operator will accept explicit undefined in TypeScript. (or I’m wrong ?)🤨 #questions #typescript

9 Replies

LukeAbby•6d ago

I can only guess as to why ArkDavid decided to design it this way since I am not him. It's probably because of exactOptionalPropertyTypes

TSConfig Reference - Docs on every TSConfig option

From allowJs to useDefineForClassFields the TSConfig reference includes information about all of the active compiler flags setting up a TypeScript project.

LukeAbby•6d ago

There are a number of functions where passing { x: undefined } will cause an error but {} will not for example

function doesStuff(options: { x?: number }): void {
    const mergedOptions = {
        x: 1,
        ...options,
    }

    mergedOptions.x * 5;
}

function doesStuff(options: { x?: number }): void {
    const mergedOptions = {
        x: 1,
        ...options,
    }

    mergedOptions.x * 5;
}

Will error when called as doesStuff({ x: undefined })

Dimava•6d ago

ArkType explicitly requires you to use exactOptionalPropertyTypes If you don't, you should get a type error saying to add it to tsconfig

AsfamilybankOP•6d ago

~~So ArkType will read and use the configuration from tsconfig.json? Including when packing.~~ How to set the exactOptionalPropertyTypes in global? I try it:

import { configure } from 'arktype/config'

configure({
  exactOptionalPropertyTypes: false
})

import { configure } from 'arktype/config'

configure({
  exactOptionalPropertyTypes: false
})

// tsconfig.json
{
  "compilerOptions": {
    "exactOptionalPropertyTypes": false
  }
}

// tsconfig.json
{
  "compilerOptions": {
    "exactOptionalPropertyTypes": false
  }
}

None of them take effect

Dimava•6d ago

No, it's a requirement, not a flag Nvm is was strictNullChecks

Dimava•6d ago

Dimava•6d ago

Installation You'll also need... - TypeScript version >=5.1. - A package.json with "type": "module" (or an environment that supports ESM imports) - A tsconfig.json with... - strict or strictNullChecks (required) - skipLibCheck (strongly recommended, see FAQ) - exactOptionalPropertyTypes (recommended)

But how to use exactOptionalPropertyTypes=false 🤔 @Asfamilybank for now just be verbose and

import { type } from 'arktype';

const checkType = type({
  'redirect?': 'string | undefined'
});

checkType({}); // pass
checkType({ redirect: undefined }); // ArkTypeError

import { type } from 'arktype';

const checkType = type({
  'redirect?': 'string | undefined'
});

checkType({}); // pass
checkType({ redirect: undefined }); // ArkTypeError

I guess

Dimava•6d ago

It's planned https://github.com/arktypeio/arktype/issues/1191 but not implemented

GitHub

Add an option to allow undefined for optional properties · Issue ...

Request a feature Add a global option to allow undefined as a value for optional properties. configure({ optionalAllowsUndefined: true }); 🤷 Motivation const arkUser = type({ phoneNumber: 'stri...

AsfamilybankOP•6d ago

I don't want to change the existing ts configuration because that would require me to refactor the entire project's code. I think this plan is pretty good.

Gaming

Programming

Why are the partial operator `?` and explicit `undefined` incompatible?