Example
Array type with known and possibly different types:
let day: [number, string];
day = [0, 'Monday']; // valid
day = ['zero', 'Monday']; // invalid: 'zero' is not numeric
console.log(day[0]); // 0
console.log(day[1]); // Monday
day[2] = 'Saturday'; // valid: [0, 'Saturday']
day[3] = false; // invalid: must be union type of 'number | string'
