Quick Answer

Compile-time immutability enforcement

Understanding the Issue

The readonly modifier prevents assignment to properties outside the constructor. For arrays and objects, it prevents reassignment but not mutation of content.

The Problem

This code demonstrates the issue:

Typescript Error 
// Problem: Accidental reassignment
interface Point {
    x: number;
    y: number;
}
const p: Point = { x: 1, y: 2 };
p.x = 3; // Allowed

The Solution

Here's the corrected code:

Typescript Fixed 
// Solution 1: Readonly interface
interface Point {
    readonly x: number;
    readonly y: number;
}

// Solution 2: Readonly utility type
const p: Readonly<Point> = { x: 1, y: 2 };
p.x = 3; // Error

// For arrays:
const nums: readonly number[] = [1, 2, 3];
nums[0] = 4; // Error

Key Takeaways

Use readonly to enforce immutability at compile time.