Understanding the 'readonly' Keyword in TypeScript

The correct answer to the question "What is the purpose of the 'readonly' keyword in TypeScript?" is: to make a variable immutable. Essentially, the 'readonly' keyword is used in the TypeScript programming language to make the properties of an object or class immutable.

Immutable Variables in TypeScript

In programming, immutability refers to a state where an object or variable cannot be modified after it's created. When you assign a variable as 'readonly' in TypeScript, it signifies that the variable cannot be changed once it's been initialized. That means the variable's value, once set, remains the same and cannot be altered anywhere in the program.

readonly variableName: type = value;

This concept is important in many aspects of programming, especially in functional programming where the ability to change state can lead to bugs and difficult-to-trace behavior.

Practical Example in TypeScript

Let's see the 'readonly' keyword in action with a simple TypeScript example:

class Car {
  readonly make: string;
  readonly model: string;

  constructor(make: string, model: string){
    this.make = make;
    this.model = model;
  }
}

let myCar = new Car("Toyota", "Corolla");
myCar.make = "Honda"; // Error: cannot reassign a readonly property

In the above code, the properties 'make' and 'model' are marked as 'readonly.' This means that once an instance of the Car class is created, these property values cannot be changed.

Additional Insights

It's important to note that the 'readonly' keyword in TypeScript is different from the 'const' keyword. While both provide ways of creating a variable that cannot be reassigned, they behave differently. A 'const' variable must be initialized immediately with a value and cannot be declared without being assigned, while a 'readonly' variable can be initialized later such as from a constructor in a class.

The 'readonly' keyword is widely beneficial for enforcing immutability and can lead to safer, more predictable code. By understanding how it works, a programmer can make effective use of it in their TypeScript programs.