Concat

Join arrays together at the type level with Concat! 🔗

In this easy-level challenge, you'll implement the JavaScript Array.concat function in TypeScript's type system. This type takes two tuple arguments and outputs a new tuple that includes all elements from both inputs in left-to-right order.

You'll learn about TypeScript's spread operator in tuple types, a powerful feature for manipulating arrays at the type level. This challenge is perfect for understanding how to work with variadic tuple types.

type Result = Concat<[1], [2]> // expected to be [1, 2]

Detailed Explanation

The Concat type uses TypeScript's spread operator to merge tuples at the type level.

type Concat<T extends readonly unknown[], U extends readonly unknown[]> = [
  ...T,
  ...U,
]

How it works:

• T extends readonly unknown[] and U extends readonly unknown[] ensure both inputs are array-like
• [...T, ...U] spreads both tuples into a new tuple
• The spread operator preserves the exact types and order of elements

This mirrors JavaScript's array spread syntax but operates entirely at the type level, demonstrating the elegance of TypeScript's type system in modeling runtime behaviors.