Say I have a union that looks like this
type Colors = 'red' | 'blue' | 'pink'
Is it possible to type check an array against this union and make sure the array contains all of the types?
I.e.:
const colors: UnionToTuple<Colors> = ['red', 'blue'] // type error, missing 'pink'
const colors: UnionToTuple<Colors> = ['red', 'blue', 'pink'] // no error
Taking ideas from this answer, you can make a function that type-checks by using a type parameter of the passed array, T. Type it as T extends Colors[] to make sure every item of the array is in Colors, and also type it as [Colors] extends [T[number]] ? unknown : 'Invalid' to make sure every item of the Colors type is in the passed array:
type Colors = 'red' | 'blue' | 'pink';
const arrayOfAllColors = <T extends Colors[]>(
array: T & ([Colors] extends [T[number]] ? unknown : 'Invalid')
) => array;
const missingColors = arrayOfAllColors(['red', 'blue']); // error
const goodColors = arrayOfAllColors(['red', 'blue', 'pink']); // compiles
const extraColors = arrayOfAllColors(['red', 'blue', 'pink', 'bad']); // error
More generically, wrap it in another function so you can pass and use a type parameter of the union type:
type Colors = 'red' | 'blue' | 'pink';
const arrayOfAll = <T>() => <U extends T[]>(
array: U & ([T] extends [U[number]] ? unknown : 'Invalid')
) => array;
const arrayOfAllColors = arrayOfAll<Colors>();
const missingColors = arrayOfAllColors(['red', 'blue']); // error
const goodColors = arrayOfAllColors(['red', 'blue', 'pink']); // compiles
const extraColors = arrayOfAllColors(['red', 'blue', 'pink', 'bad']); // error
@CertainPerformance's solution does not reject empty arrays. Here is a fix:
const arrayOfAll = <T>() => <U extends T[]>(
array: U & ([T] extends [U[number]] ? unknown : 'Invalid') & { 0:T }
) => array;
If we have a value that has a union type, we can only access members that are common to all types in the union.What the example does show me are just pipe separatedstringvalues and not different types of color objects. I'm not even sure if this union definition of yours does make any sense.Colors[], and you can check an array against it with generics and a helper function.const colors = ['red', 'blue', 'pink'] as const; type Colors = typeof colors[number];