Readonly Properties.md

"Readonly" Properties

Introduction

The readonly keyword is used to make class properties or object properties immutable after their initial assignment. Once a property is marked as readonly, its value cannot be changed. This ensures that the property remains constant throughout the object's or class instance's lifetime.

class Circle {
  // Readonly properties
  readonly radius: number;

  // Readonly properties can be initialized in the constructor
  constructor(radius: number) {
    this.radius = radius;
  }

  // Readonly properties cannot be reassigned
  updateRadius(newRadius: number): void {
    // Error: Cannot assign to 'radius' because it is a read-only property.
    // this.radius = newRadius;
  }

  // Readonly properties can be accessed
  calculateArea(): number {
    return Math.PI * this.radius * this.radius;
  }
}

// Creating an instance of the class
const myCircle = new Circle(5);

// Accessing readonly properties
console.log(myCircle.radius);  // Output: 5

// Readonly properties cannot be modified after initialization
// Error: Cannot assign to 'radius' because it is a read-only property.
// myCircle.radius = 10;

In this example:

• The Circle class has a readonly property radius.
• The readonly property is initialized in the constructor and cannot be reassigned afterward.
• An attempt to modify the readonly property after initialization results in a compilation error.

Readonly Modifiers with Parameters:

You can also use readonly in constructor parameters to automatically create and initialize readonly properties:

class Rectangle {
  // Automatically created and initialized readonly properties
  constructor(readonly width: number, readonly height: number) {}

  // Readonly properties can be accessed
  calculateArea(): number {
    return this.width * this.height;
  }
}

// Creating an instance of the class
const myRectangle = new Rectangle(4, 6);

// Accessing readonly properties
console.log(myRectangle.width);   // Output: 4
console.log(myRectangle.height);  // Output: 6

// Readonly properties cannot be modified after initialization
// Error: Cannot assign to 'width' because it is a read-only property.
// myRectangle.width = 8;

In this example, the width and height properties are automatically created and initialized as readonly based on the readonly modifier in the constructor parameters.

Use Cases:

1. Immutable Configuration:
   • When certain properties of an object or class should remain constant throughout its lifecycle.
2. Preventing Accidental Modifications:
   • To avoid unintended modifications to critical properties.
3. Documentation of Intent:
   • Clearly indicating that a property is intended to be read-only in the code, making the code more self-explanatory.
4. Constructor Initialization:
   • Automatically creating and initializing read-only properties based on constructor parameters.

Using readonly properties in TypeScript provides a way to enforce immutability for specific properties, promoting safer and more predictable code.