The same as Primitive Types and Object Types, Hegel provides support for usage array literal as type, and this array literal types called Tuples. Tuple types allow you to define an array with a fixed number of elements with known types. The syntax fo tuple types is [type1, type2, type3].
const tuple: [string, number] = ["", 0];
Getting element from Tuple
Unlike Array Types Tuple will return strict type which is placed at position in tuple type definition and will show an error, if you try to access out of tuple range.
const tuple: [string, number] = ["", 0];// first type is "string"const first = tuple[0];// second type is "number"const second = tuple[1];// Error: Property "2" does not exist in "[string, number]"const third = tuple[2];
Also, unlike Array Types, Tuples have strict defined type for length and as result they cannot mutate "length" property in a tuple type.
const tuple: [string, number] = ["", 0];// length type is "2"const length = tuple.length;// Error: Type "3" is incompatible with type "2"tuple.length = 3;
Tuple methods and mutation
As you may understand, Hegel provides the same methods for Tuples Types as for Array Types, but with one exception. If you try to use any of methods from Array which will mutate source tuple (https://doesitmutate.xyz/) you will have an error.
const numbers: [number, number, number] = [1, 2, 3];const plusOne = numbers.map((a) => a + 1);const lessThan3 = numbers.filter((a) => a < 3);// Error: Property "reverse" does not exist in "[number, number, number]"const reversed = numbers.reverse();
Subtyping
The same as Array Types Tuple types have invariant nature, so you can't assign to one tuple another with wider element type or with greater or less elements count.
const source: [number, number] = [0, 0];// Error: Type "[number, number]" is incompatible with type "[number | string, number]"const widerByElementType: [number | string, number] = source;// Error: Type "[number, number]" is incompatible with type "[number, number, number]"const widerByElementsCount: [number, number, number] = source;// Error: Type "[number, number]" is incompatible with type "[number]"const narrowerByElementsCount: [number] = source;
And also tuples don’t match array types:
const source: [number, number] = [0, 0];// Error: Type "[number, number]" is incompatible with type "Array<number>"const array: Array<number> = source;