Why is TypeScript not restricting the type alias to adhere to its structure?

In the following TypeScript code:

type User = [number, string];
const newUser: User = [112, "[email protected]"];

newUser[1] = "hc.com";     // ✅ Allowed
newUser.push(true);        // ⚠️ No error?!

I expected TypeScript to prevent newUser.push(true) since User is defined as a tuple of [number, string]. However, TypeScript allows this due to the mutable nature of tuples.

What's going on?

Tuples in TypeScript are essentially special arrays. At runtime, there's no real distinction between an array and a tuple — both are JavaScript arrays. Unless specified otherwise, tuples are mutable, and methods like .push() are available.

So newUser.push(true) compiles because:

TypeScript treats the tuple as an array.

.push() exists on arrays.

TypeScript doesn't strictly enforce the tuple's length or element types for mutations unless stricter typing is applied.

How to enforce stricter tuple rules

1. Make the tuple readonly: This prevents any modification, including .push() and element reassignment.

type User = readonly [number, string];

2. Use as const for literals: This creates an immutable tuple from the start.

const newUser = [112, "[email protected]"] as const;

This will infer the type as readonly [112, "[email protected]"] and block any mutation attempts.