Add Includes type

[Yash-Singh1]

Resolves #216

The problem is that my solution to Includes works only with the PropertyKey type for Item, because it makes the values the key and the value a boolean, then checks if the value is true for the property with the key of Item. The other solution is:

type Includes<T extends readonly any[], U> = U extends T[number] ? true : false;

However as shown in type-challenges/type-challenges#1432, this solution does not work, because it uses extends allowing [{a: 5}] to include {}. Any ideas on supporting Item for types other than PropertyKey and at the same time not running into the extends problem (https://github.com/Yash-Singh1/type-fest/blob/main/test-d/includes.ts#L22)?

[sindresorhus]

How can we get this moving forward?

[Yash-Singh1]

I did find a solution but it depends on the Equal type presented here: microsoft/TypeScript#27024 (comment). Should I change this PR to add Includes and Equal or should I simply update the Includes type to use it inline? Here it is by the way:

export type Includes<Value extends any[], Item> =
  (<G>() => G extends Value[0] ? 1 : 2) extends
  (<G>() => G extends Item ? 1 : 2)
	? true
	: Value extends [Value[0], ...infer rest]
		? Includes<rest, Item>
		: false;

[sindresorhus]

Great. Make it a separate internal type called IsEqual, but put it inside the same file as Includes. And link to microsoft/TypeScript#27024 (comment) in a code comment for context.

[sindresorhus]

This should use the expectError assertion method instead.

[Yash-Singh1]

This should use the expectError assertion method instead.

When you say instead, do you mean without the // @ts-expect-error, because I couldn't figure out how to use the expectError assertion method works without it. Is this how I am supposed to do it?:

// @ts-expect-error
const oneArgIncludes: Includes<['my', 'array', 'has', 'stuff']> = false;
expectError<Includes<['my', 'array', 'has', 'stuff']>>(oneArgIncludes);

Removing the // @ts-expect-error, gives an error (as expected):

✖  34:22  Generic type Includes requires 2 type argument(s).

[sindresorhus]

When you say instead, do you mean without the // @ts-expect-error

Yes

[sindresorhus]

You have to put the type inside the method call to expectError. tsd only ignores TS error inside the ().

[Yash-Singh1]

a10c8b5