Length of Tuple

Count tuple elements at compile time with TypeScript's type system! 📏

This easy-level challenge teaches you to create a generic Length<T> that returns the length of a tuple as a literal number type. You'll discover how TypeScript tuples have a special length property that contains the exact count of elements, unlike regular arrays.

This challenge is perfect for understanding the difference between arrays and tuples in TypeScript, and how to leverage tuple-specific properties in your type-level programming.

For this challenge, you will need to change the following code to make the tests pass (no type check errors).

type tesla = ['tesla', 'model 3', 'model X', 'model Y']
type spaceX = ['FALCON 9', 'FALCON HEAVY', 'DRAGON', 'STARSHIP', 'HUMAN SPACEFLIGHT']
 
type teslaLength = Length<tesla>  // expected 4
type spaceXLength = Length<spaceX> // expected 5

Detailed Explanation

The solution is elegantly simple, leveraging TypeScript's built-in tuple length property.

type Length<T extends readonly unknown[]> = T['length']

Why this works:

• Tuples in TypeScript have a literal length property
• T['length'] accesses this property at the type level
• For ['a', 'b', 'c'], the length is the literal type 3
• The readonly unknown[] constraint ensures T is an array-like type

This showcases a fundamental difference between tuples and arrays: tuples have a fixed, known length at compile time, while arrays have number as their length type.