Enable `--exactOptionalPropertyTypes`

[sindresorhus]

I don't want to do it yet as it will cause some friction. We can try to enable it in 2023.

https://devblogs.microsoft.com/typescript/announcing-typescript-4-4/#exact-optional-property-types

[kidonng]

Almost 3 months into 2023, is it time yet?

[fregante]

I think this will make conditional properties annoying to set:

const a: A = {
	name: 'Fregante',
	email: user.hasEmail ? user.email : undefined
}

Would need to be:

const a: A = {
	name: 'Fregante',
	...(user.hasEmail ? {email: user.email} : undefined)
}

Or worse yet split with an if.

JavaScript badly needs a new nullish value 🤭

const a: A = {
	name: 'Fregante',
	email: user.hasEmail ? user.email : reallyundefined
}

[kidonng]

TS ESlint has issues with exactOptionalPropertyTypes: typescript-eslint/typescript-eslint#5344

[voxpelli]

@fregante It only makes it annoying to set if the type hasn't been explicitly set to accept undefined, though it makes other cases less annoying, like:

type A = {
  email?: string
}

const foo: A = {
  email: 'foo@example.com'
}

if ('email' in foo && foo.email.length > 1) {
  console.log('This only works with exactOptionalPropertyTypes');
}

When it comes to module development its good to have exactOptionalPropertyTypes on as it will ensure that one specifies types that are not annoying to use with exactOptionalPropertyTypes, though its also dangerous as the above example would throw if someone were to send in an explicit undefined because they lacked exactOptionalPropertyTypes.

I personally have exactOptionalPropertyTypes in all my projects, including my modules.