tuple is sub-type of array
tuple is sub-type of array, see [[Type array in Typescript]]. They’re a special way to type arrays that have fixed lengths, where the values at each index have specific, known types.
we should explicitly declare tuple type
Unlike most other types, tuples have to be explicitly typed when we declare them. That’s because the Javascript syntax is the same for tuples and arrays(both []).
let a: [number] = 1;
let b: [string, string, number] = ['malcolm', 'gladwell', 1963];
b = ['queen', 'elizabeth', 'ii', 1926]; // Type 'string' is not assignable to type 'number'.
see [[Create a tuple constructor function in Typescript]].
tuple supports optional elements
// An array of train fares, which sometimes vary depending on direction
let trainFares: [number, number?][] = [
[3.75], [8.25, 7.70], [10.50]
];
tuple supports rest elements, which we can use to type tuples with minimum lengths.
// A list of strings with at least 1 element
let friends: [string, ...string[]] = ['Sara', 'Tali', 'Chloe', 'Claire'];
// A heterogenous list
let list: [number, boolean, ...string[]] = [1, false, 'a', 'b'];