IsTuple

Problem Link

Problem

Implement a type IsTuple, which takes an input type T and returns whether T is tuple type.

type case1 = IsTuple<[number]>      // true
type case2 = IsTuple<readonly [number]> // true
type case3 = IsTuple<number[]>      // false

Solution

Approach: Check for Finite Length

A tuple has a fixed, finite length (a numeric literal), while an array has number as its length.

type IsTuple<T> =
  T extends readonly unknown[]
    ? number extends T['length']
      ? false
      : true
    : false

How it works:

1. First confirm T is array-like (readonly unknown[]).
2. Check T['length']: for arrays it's number, for tuples it's a numeric literal like 0 | 1 | 2.
3. number extends T['length'] is true only when T['length'] is exactly number — i.e., it's an array, not a tuple.

Key Takeaways

• The key insight: number extends 3 is false, but number extends number is true.
• readonly unknown[] is the widest array type and accepts both mutable and readonly tuples/arrays.
• This pattern — checking whether a length is a literal vs. number — appears in many tuple-detection scenarios.